We are pleased to announce the 15th workshop on breakdown science and high gradient technology, HG2023, that will be held in person in Italy at the Frascati National Labs of INFN - Via Enrico Fermi, 60 - Frascati (Rome) from 16 to 20 October 2023.
In order to allow a remote participation to the event a zoom link has been set up. Look for the new item on the left (Videoconference). An INFN indico account is requested to login in and join the meeting (all instructions are available), if you haven't it please create a new one. Without the log in, the incomplete zoom link will ask for a pass code.
In case of problems send an e-mail to: hg2023@lists.lnf.infn.it
The demand for pushing the RF structures operation to higher and higher accelerating gradients has been historically driven by the High Energy Physics community need of expanding the energy frontier of the discovery machines. This demand is still far to be exhausted, as clearly stated in the Accelerator R&D Roadmap for the European HEP Strategy released in late 2021. But high gradients are also extremely beneficial for accelerators destinated to different fields, such as compact light sources, medical and industrial applications, that are growing more and more as new drivers of the HG technology.
The HG workshop covers revisitations of basics and fundamentals concepts, recent theoretical and experimental achievements, advancements in modelling and understanding of the breakdown phenomena, new design concepts and innovation in manufacturing techniques, ideas and experimental activities towards more powerful and efficient RF power sources, reports on the existing test benches and plans for new ones, and much more! It is a unique opportunity to sketch a comprehensive landscape of a truly global activity, with many pulsing research sites spread all over the continents.
And, finally, it is the first opportunity to gather our community face to face since 2019, after the pandemic hard times. For all these reasons we look forward to seeing you at Frascati HG2023!
Due to the ongoing conflict in Ukraine, the HG2023 meeting is unable to accept any registrations from any organisation based in Russia or Belarus.
Participants in the Workshop are invited to register via web using the online registration form (handled via an INFN indico page - an INFN indico account required). The registration fee of 500,00 Euros (tax included), as contribution to the Workshop organisation, includes lunches, coffee breaks, cheese and wine during the poster sessions, a welcome cocktail, a social dinner + bus to/from the restaurant and meeting materials.
For the participation on site it is mandatory to be registered to the event and to show the badge.
Industrial exhibitors are invited to register via web through the registration form. The registration fee of 1000,00 Euros (tax included) includes a dedicated space in the foyer venue of Bldg. 36 and one representative registration fee, the logo on the workshop website linked to the commercial page on display and in the conference folder.
Students can apply for a student grant. Please follow all the instructions on the dedicated web page.
Those who need to apply for a VISA to enter Italy are kindly requested to ask for a formal invitation letter by e-mail to the Workshop Secretariat.
CANCELLATION AND REFUNDS
All refund requests must be in writing by mail to the Workshop Organisers as soon as possible
Registration fee less 50,00 € as administrative fee, will be refunded if a request is made in writing to the HG 2023 Secretariat (hg2023@lists.lnf.infn.it) and received prior August 31st, 2023.
Cancellations after this date will be non-refundable, also for "STORNI INFN" and no show.
Registrations may be transferred to substitute delegates without incurring a fine.
A broad overview is given of our current understanding of the effects that limit gradient in copper structures. Both experiments and theory from published literature are reviewed.
The linac energy booster of the EuPRAXIA@SPARC_LAB project foresees the use of 16, 1 m long, travelling wave X-Band structures, with a field phase advance per cell of 2π/3 and a repetition rate of 100 Hz. To reach the 1 GeV required by the linac these structures have been designed to work at 60 MV/m. An optimization of the electromagnetic and mechanical design has been done to simplify the fabrication and to reduce their cost. An intense prototyping activity was carried out to optimize the construction and the brazing process of the structure. In the presentation we review the whole design of the structure, the RF module layout and the prototyping activity status with the first low power RF test results on the few cells RF prototype.
The Argonne Wakefield Accelerator (AWA) facility supports an extensive research portfolio along three themes: electron beam production, electron beam manipulation, and electron beam-driven structure wakefield acceleration (SFWA). This talk presents efforts at AWA to explore and exploit short RF pulses (sub-10 ns) to drive accelerating structures to higher gradients. The talk begins with an overview of the AWA facility and a motivation for the short-pulse approach. Research highlights presented include:
These results demonstrate the potential of short-pulse RF to achieve high-gradient acceleration at AWA. The talk will conclude with future directions for short-pulse RF research at AWA.
In the past ten years, the high gradient technolog team of SSRF has been working on the research of high gradient RF structures. Here will gives some results of the recent RF activities, including dual-mode variable polarization deflector, cold electron guns and accelerarting structures. In the mean time, the X-band deflecting structures that have been successfully used on SXFEL will also continue to be applied to SHINE, and some of them have been manufactured and tuned in preparation for high power testing in the near future.
FERMI is the seeded Free Electron Laser (FEL) user facility at Elettra laboratory in Trieste, operating in the VUV to soft X-rays spectral range. In order to extend the FEL spectral range to shorter wavelengths, an upgrade plan for increasing the Linac energy from 1.5 GeV to 2.0 GeV is actually going on. After the successful testing of the short prototype of the new high gradient (HG) S-band accelerating structure up to an accelerating gradient of 40 MV/m, two full-length 3.0 m HG structures have been built and installed at the FERMI linac. In this paper, we report the low power measurement, conditioning results, and commissioning with the beam of the first HG module.
We will present the design of a compact, highly efficient, pulse-to-pulse energy-tunable 9.3 GHz linac to generate up to 500 W of 10 MeV electron beam power for medical and security applications. This linac will employ a patented travelling wave accelerating structure which combines the advantages of high efficiency with energy tunability of traveling wave cavities. One of the advantages of this accelerating structure over the standing wave structures commonly used in industrial linacs is that the proposed structure has little power reflected back to the RF signal source, eliminating the need for a heavy, lossy waveguide insulator. In contrast to the side-coupled cavity designs, the proposed structure is symmetrical and therefore it does not have deflecting axial fields that impair the beam transport. The high shunt impedance will allow the linac to achieve an output energy of up to 10 MeV when powered by a compact commercial 9.3 GHz 1.7 MW magnetron. For pulse-to-pulse tuning of the beam output energy we will change the beam-loaded gradient by varying the linac’s triode gun current.
This contribution summarizes some results obtained so far during the set-up of the two X-band TDSs with variable polarization (PolariX) installed downstream the Athos undulators in SwisFEL. The method used to avoid a beam kick in the plane orthogonal to the streaking plane and a study of the dependence of the deflecting voltage on the shape of the klystron pulse are described in more detail.
In this paper we will discuss the challenges that are open today in getting high gradients in nomral conductive cavities.
Particular enphasis will be devoted to the Muon Collider case.
New proposed and under development facilites will be presented and discussed.
The presentation starts with an overview of the solutions adopted by different labs around the world to develop LLRF systems driving a X-band power station.
Then, specifications for a user facility linac will be pointed out, especially referring to modern machines designed for plasma acceleration R&D that have demanding requests on amplitude and phase jitter of the accelerating fields.
The last part is focused on the technological challenges and possible synergies to design LLRF modules for a user facility linac from scratch or to upgrade/adapt existing devices to meet such specifications.
Currently SSRF/SXFEL is ongoing to develop the advanced transverse deflecting structure TTDS(Two-mode operation transverse deflecting structure) to carry out variable polarization based on the design of dual-mode RF structure. Driven by two different RF power sources, this novel TDS can work on both HEM11 and HEM12 modes in vertical and horizontal deflecting directions simultaneously, and consequently it is ultra-fast to fix and change linear polarization, in particular circular and elliptic polarizations as well by flexible vector combination of these two modes, actually in engineering which is very practically realized by amplitude and phase modulation from LLRF. The work presented in this paper is focused on analysis and design of the variable polarization TDS, consisting of dual-mode cells and two dual-mode couplers. Considering the complexity of dual-mode design and optimization, the advanced optimization procedure based on both neural network and multi-objective algorithms, is successfully developed to improve the accuracy and efficiency of the RF structure design. After many iterative optimizations, the dual-mode cells in the final design can work on high impedance and similar RF performance for both HEM11 and HEM12, in particular the two couplers for RF power input and output are also approached to the ideal design as well. Based on the optimized design and RF sensitivity analysis, the engineering design is finally completed and ready for manufacture.
The Compact Linear Collider (CLIC) beam-based acceleration baseline uses high-gradient traveling wave accelerating structures at a frequency of 12 GHz. To demonstrate the performance of these structures at high peak power and short pulse width RF, two klystron-based test facilities will be put into operation this year.
The X-band Laboratory for Accelerators and Beams (X-LAB) at the University of Melbourne will operate half of the CERN X-band test stand system, known as XBOX3. XBOX3 employs a novel approach that combines relatively low peak power (6 MW) but high average power klystron units. The power is steered to feed two testing slots with RF, delivering the required power at a repetition rate of up to 400 Hz. Additionally, the repetition rate, peak power, pulse length, and pulse shape can be customized to meet specific test requirements.
After a long recovery process from the fire in 20219, we have recently re-started high-power testing at the X-band test stand Nextef2. We will report the status and update.
The TEX facility has been commissioned for high-power testing to characterize radio frequency structures and components and to validate them for operation on future particle accelerators for medical, industrial, and research applications. To this end, TEX is directly involved in the LNF’s leading project EuPRAXIA@SPARC_Lab. A brief description of the facility commissioning, its status, and prospects will be provided.
RadiaBeam has developed C-band testing infrastructure to support a diverse array of structure and beam testing programs. The core capabilities revolve around a pair of E37212 Canon klystrons driven by K300 Scandinova modulators delivering 1.2us, 100Hz operation with peak RF powers of 25MW and 50MW. The infrastructure includes multiple phase shifter power splitters and high directivity directional couplers developed by SLAC, a fully evacuated waveguide network, a highly flexible LLRF system with pulse shaping capabilities and two large concrete test cells. Commissioning results along with examples of devices we have tested with the system will be presented.
The baseline design of the compact linear collider (CLIC) requires high gradient accelerating structures operating at 12 GHz with accelerating gradients of 100 MV/m. To demonstrate the feasibility and optimise the performance of these structures several x-band test facilities, x-Box 1, 2 and 3 have been operating at CERN.
A new test facility will come online at the x-band laboratory for accelerators and beams (x-LAB) at the University of Melbourne later this year. As part of the commissioning process, diagnostic, radiation monitoring and safety, and interlock systems are being designed installed and tested. This report presents an overview of these systems and their current status. As well as future plans for how these systems might evolve as the x-LAB develops.
A summary of the relevant activities in the CERN X-band RF facilities related to high gradient and high efficiency RF
The design and manufacturing of X-band, high-gradient structures have steadily developed in recent times beyond their original use in high-energy physics. After years of development and collaborations, the initial phase of validation regarding the viability of the machines and their operation has moved to focus on a more practical area. This new area focuses on manufacturing techniques, materials and components and their applications in and within the industry. In this presentation, we will have an overview of three distinct projects: Smartcell AS, Halves AS, EU i.Fast 7.5 task. Among others, they all contribute to advancements in X-band accelerating structures, their fabrication methods and components. Collectively, all projects highlight the current traction of the X-band technology, gathering many aspects like fabrication challenges, brazing techniques, UP machining, and material analysis.
750 MHz inter-digital H-mode drift tube linac (IH-DTL) with the capability to accelerate protons from 3 to 10 MeV was proposed for the compact therpy linac that now under development in the institute of Modern Physics, Chinese Academy of Science (IMP, CAS). Four drift tube sections were housed in a single vacuum chamber and coupled with three large drift tubes which housing focusing triplet lens inside. In each drift tube section, there were 9 to 10 drift tubes, supported by the separated ridges. This cavity will be powered by a 1 MW klystron at 0.1% duty cycle, the kp factor is about 1.7 at the operation power level. The overall cavity design is presented in this paper.
Linear Accelerators (LINACs) technology is evolving towards compactness and high intensity. As a consequence, these devices are becoming suitable for several industrial and medical applications, such as the upgrade of light sources to the so-called 4-th generation or the design of high-dose rate radio-therapy facilities. In such scenarios, intensity-dependent collective effects can arise and compromise the LINAC performance.
This work focuses on the Beam Loading (BL) effect, which induces a reduction of the available accelerating gradient of the structure as a consequence of the interaction of the beam with the cavity. A power-diffusive model for the BL effect has been derived and implemented into the tracking code RF-Track. With this, transient scenarios in both standing-wave and travelling-wave LINACs have been studied. Good agreement has been found with experimental measurements carried out in the CLEAR facility at CERN.
The Iranian Light Source Facility (ILSF) is a 3 GeV low-emittance synchrotron light source laboratory for scientific research with a very bright X-ray source in various applications. ILSF’s pre-injection system consists of a 2~3 MeV thermionic RF gun with an alpha compressor magnet and six constant gradient TW linac tubes. Each linac tube is 3 meters long with a minimum field gradient of 10 MV/m. To evaluate the design method of the constant gradient linac tubes, a smaller section of such tubes is designed and tuned to the desired operating frequency. The design of this short tube is presented in this report.
To reduce energy consumption and cost has become a prime objective of the development and operation of high energy particle accelerators. RF sources are major energy consumption components of any RF system for the particle colliders. The 30 GeV injector linear accelerator (LINAC) for the CEPC (Circular Electron and Positron Collider) requires S-band klystrons with higher efficiency to reduce energy consumption and cost. In this paper, two novel bunching method, including COM (Core Oscillation Method) and BAC (Bunching Alignment and Collecting), are applied to the BEPCII (Beijing Electron Positron Collider) S-band klystron. These methods increase the efficiency of klystron from 45% to 55%, therefore increasing the output power to 80 MW with the same operation voltage. The preliminary optimization design is completed with 1-D disk model based AJDISK code and further checked by 2-D code EMSYS. The density modulation of electron injection is improved by selecting a suitable cavity string structure and thereby to optimize electron bunching. Further improvement in the RF conversion efficiency of the klystron is also planned and is in line with a requirement to go green.
Nowadays, production of ultrashort and high-current electron bunches of few fs to hundred fs with several pc charge is a very attractive subject in the context of many recent applications, specially, in light sources, free electron lasers and plasma wake field accelerators. In the light sources and free electron lasers, the radiation gain is directly proportional to the beam pick current and so obtaining very short bunches on the order of fs plays an essential role for high gain machines.
Consequently, recent approaches for production of such electron bunches are based on usage of photo injectors in combination with a chain of bunch compressors. In the Iranian light source project attempts are directed toward the designing a novel compact soft X-ray synchrotron beyond the state of the art, using the latest advanced studies worldwide. The project will be started form a novel photoinjector which can provide very high quality, and in the meantime, ultra-short electron bunches in a compact structure. Moreover, finding the best structures for the rf components, specifically, the rf gun and its associated buncher cavity has a crucial effect on the function of the whole light source in near future. In this talk I will show some of the photocathode RF gun designing and compare the results
Dielectric Assist Accelerating (DAA) structures based on ultralow-loss ceramic are being studied as an alternative to conventional disk-loaded copper cavities. This accelerating structure consists of dielectric disks with irises arranged periodically in metallic structures working under the TM02$-\pi$ mode.
Here, the numerical design of an S-band DAA structure for low beta particles, such as protons or carbon ions used for hadrontherapy treatments, is shown. Four dielectrics with different permittivity and loss tangent are studied as well as different particle velocities depending on the energy range.
Through optimization, most of the RF power is stored in the vacuum space near the beam axis, leading to a significant reduction of power loss on the metallic walls. This allows to fabricate cavities with extremely high quality factor over 100 000 and shunt impedance over 300 M$\Omega$/m at room temperature.
During the numerical study, the design optimization has been improved by adjusting some of the cell parameters in order to both increase the shunt impedance and reduce the peak electric field in certain locations of the cavity, which can lead to instabilities in its normal operation. In addition, first multipactor simulations are being carried out, using several coatings to reduce SEY, which has also been taken into account in the electromagnetic result. Finally, thermal and mechanical analysis has been performed in order to estimate cavity performance and cooling.
The THz-frequency range could provide the accelerating gradients needed for next generation accelerators with compact, GV/m-scale devices. One of the most promising THz generation techniques for accelerator applications is optical rectification in lithium niobate using the tilted pulse front method. Current THz accelerators are limited by significant losses during transport of THz radiation from the THz source to the acceleration structure. In addition, the broadband spectral properties of high-field laser-driven THz sources make it difficult to couple THz radiation into accelerating structures. We demonstrate a fast and efficient technique for spatial and temporal characterization of single cycle strong field THz pulses in the near-field of a lithium niobate source using electro-optic sampling. Using this technique, we have reconstructed the full temporal 3D THz near-field close to the lithium niobate emission face and shown that the near-field can be controlled by manipulating the generation setup. Analysis of these results will allow for evaluation of the applicability of the THz near field to electron beam acceleration and manipulation and the development of improved THz coupling schemes.
After years of development, the phase-locked loop technology has gradually tended to maturate. The photoelectric phase-locked loop based on an optical-microwave phase detector can meet the high-precision synchronization requirements of scientific facilities such as UED, XFEL. Whereas, several kinds of noise introduced in the phase detection process will reduce the integrity of the original signal. In this work, we apply electro-optic sampling timing detection technology to designing an all-fiber optical-microwave phase detector based on a Sagnac interferometer, which is effective to be used in fully suppression of the amplitude-to-phase conversion noise introduced by beam direction and amplitude fluctuation, the thermal noise caused by the thermal disturbance of resistance components, and the shot noise caused by the random fluctuation of incident photons. The laser-microwave synchronization based on this phase detector enables the ultrahigh laser stability of the fiber mode-locked laser to be coherently transferred to the L-band, S-band or C-band microwave signal sources, which is of great significance for the improvement and application of RF synchronization in accelerator equipments.
A Dielectric Disk Accelerator (DDA) is an accelerating structure designed to be loaded with dielectric disks to increase its shunt impedance. These structures use short RF pulses of ~9 ns to achieve accelerating gradients of more than 100 MV/m. A single cell and a multicell clamped structure have been designed and high power tested at the Argonne Wakefield Accelerator. During testing, the single cell clamped DDA structure achieved an accelerating gradient of 102 MV/m with no visible damage in the rf volume region. The minimal damage that was seen outside the RF volume was likely due to uneven clamping during assembly. Based on the success of that experiment, a clamped multicell DDA structure has been designed and tested at high power. Simulation results for this new structure show a 108 MV/m accelerating gradient with 400 MW of input power with high shunt impedance and group velocity. Engineering designs were improved from the single cell structure to ensure consistent clamping over the entire structure. The results of the high power test will be presented. After this test an additional multicell structure will be designed and beam tested to measure the energy gain from the structure.
The Large Electrode System (LES) is a vacuum high-voltage experimental setup consisting of an anode and cathode separated using a ceramic spacer. By applying a dc high-voltage, numerous aspects of breakdown including field emission, location, conditioning, ultimate gradient and statistics can be studied. The system can operate in a voltage range up to 7000 V, pulse lengths from 1 $\mu$s to 1000 $\mu$s and repetition rates up to 2000 Hz, as well as static dc signals. Not only does the system monitor the applied voltage, current drawn by the electrodes and the pressure, but window openings on the side of the chamber enables breakdown localization and field spectra investigation. Recent results obtained using the system are described including the latest conditioing results, including additively manufactured electrodes and irradiated electrodes. Optical field taken during breakdown or constant field emission are also presented.
High gradient RF cavities, especially those at frequencies above S-band, are critical in several concepts for future electron linacs such as the ultra-compact X-ray free electron laser (UCXFEL) and the Cool Copper Collider ($C^3$). These two designs in particular rely on the cryogenic operation of RF cavities, taking advantage of several complementary physical effects. It is then advantageous to develop in greater detail an understanding of the complex surface and material physics associated with high gradient RF fields in normal conducting cavities. We present here measurements made at the CYBORG (CrYogenic Brightness-Optimized Radiofrequency Gun) beamline at UCLA. In this first phase of study we will relate our temperature dependence measurements of dark current and other RF cavity figures of merit to the general material science research goals via a modified theory of anomalous skin effect regime. The theory will also be presented with additional results from low level RF cavity testing. Future directions will be presented including the status of the second phase of the beamline in which photoemission of novel cathodes in extreme conditions will be studied.
Based on the high efficiency klystron scheme of circular electron positron collider (CEPC), the depressed collector design is proposed to improve the overall efficiency of RF power source. The difficulty of the research is that the velocity of electrons entering the klystron collector is scattered, and it is difficult to use the depressed collector to sort the velocity of electrons. Based on the CEPC high efficiency klystron design, this paper will carry out a detailed theoretical analysis of the depressed collecter and determine the basic design scheme. In order to verify the klystron energy recovery scheme, an energy recovery verification device is designed.
The CLARA accelerator (Compact Linear Accelerator for Research and Applications) located at Daresbury Laboratory in the UK has a new 1.5 cell S-band photoinjector designed by ASTeC (Accelerator Science and Technology Centre). The photoinjector underwent RF conditioning between October last year and March this year to obtain a peak E-field of 70MV/m and a pulse length of 2µs. This was carried out by a mixture of expert manual and automated conditioning using ASTeC’s NO-ARC (No Operator Automated RF Conditioner) conditioning script which allowed for conditioning at higher repetition rates and 24 hour operation. Here we detail the highlights of the RF conditioning period including some anomalous breakdown traces and behaviour where the cavity would suffer breakdowns below that of the frontier target powers. The causes of this unusual conduct are still being actively investigated.
One of the key aspects to provide on chip acceleration in Dielectric Laser Accelerators (DLA) from tens of keV up to MeV energies is the phase velocity tapering.
In this paper, we present simulated performance of sub-relativistic structures, based on tapered slot waveguides and tapered electromagnetic band bap (EBG) waveguides, where we engineered channel/defect modification in order to obtain a variable phase velocity matched to the increasing velocity of the accelerated particles.
In DLA structures co-propagating schemes are employed for higher efficiency and smaller footprint compared to the cross-propagating schemes. In this respect, we envisage tapered continuous co-propagating structures that simultaneously allow wave launching/coupling, beam acceleration and transverse focusing. The main figures of merit, such as the accelerating gradient, the total energy gain and the transverse focusing/defocusing forces, are evaluated and used to guide the
optimization of the channel/defect modification.
Index terms: Dielectric Laser Accelerators (DLA), Photonic Crystal, Dielectric Waveguides
Conditioning of a metal surface in a high-voltage system is the progressive development of resistance to vacuum arcing over the operational life of the system. This is relevant for accelerator cavities where high level of performance is only achievable after long conditioning period. Beyond the accelerator research field, this is an important topic for any technology where breakdowns can cause device failure, either by directly disrupting device operation or by causing cumulative hardware damage.
The cryogenic HV pulsing system in FREIA laboratory is used to study surface conditioning with high-repetition rate DC pulses, in function of a wide range of temperatures, down to 4K. It has been shown that the vacuum breakdowns are initiated by activities on the cathode, always accompanied by the field emission. To better understand this process, we are currently measuring field emission currents during the conditioning process of electrodes made out of different metals: copper, niobium and titanium.
We are also developing a new characterization method, consisting in measuring the surface resistivity of the metal surface that is being conditioned by inducing, in addition to DC pulses, a high frequency (GHz) radio-frequency current in the parallel-plate electrode system. If the system can function as a resonant cavity, the surface resistivity data would be encoded in its quality factor (Q-factor). The changes in the Q-factor measured in cryogenic conditions could indicate a formation of dislocations under the surface, something that has been speculated as an important process behind the conditioning, even more important than the changes on the surface.
We will present the results of the conditioning process and of the field emission measurements. Additionally, we will also present our plans for the surface resistivity measurements, the modified design of the electrode system, based on the choke cavity design of Shintake, and experimental data regarding the characterization of this resonant system. The results are relevant and bridge a knowledge gap between warm and cold, normal conducting, high gradient, accelerator projects.
With high shunt impedance and expected gradient, cryogenic structure has the ability to reduce accelerators' length and has the potential to operate at a higher repetition rate. In SSRF/SXFEL, the scheme and preliminary experimental studies about the cryogenic RF structure have been developed. Foremost, theoretical and experimental studies of the RF characteristics in cryogenic environments are investigated. Then we designed a cryogenic C-band standing wave bi-periodic accelerating structure. It is a 17-cell structure consisting of 9 accelerating cavities and 8 coupling cavities. To guarantee the symmetry of the field, the structure is doubly-fed. Operating with the pi/2 mode standing wave, it is much less sensitive than the standing-wave structure of pi-mode. Additionally, the microwave mode is TM02 in coupling cavities that are larger and even less sensitive than the traditional bi-periodic structure. The shape of the coupling cavity can be redesigned to make it tunable. In the subsequent study, we will work on the experiment of the cryogenic structure.
S-band high-gradient accelerating structures were proposed to accelerate protons from 30MeV to 230MeV for a compact therapy Linac in IMP. A backward traveling wave structure and a high gradient standing wave structure were developed, and the differences between the two types of structures were analyzed. In this paper, A new-shaped accelerating structure with reduced coupling holes and thermal stress was developed, along with a novel cooling channel design, which allow the cooling water flow in the middle of the disk, thus make it possible for higher duty cycle and longer RF pulse operation. It provides a wider application scenario for the high gradient accelerating structure. The research of high gradient accelerating structure is consistent with the trend of linear accelerator technology (compact, flexible and economical). Design and optimization of the linac and the status of the prototype cavity will be discussed in this paper.
The occurrence of breakdown events is a primary limiting factor for future accelerator applications aiming to operate under high field-gradient environments. Experimental evidence often leads to a hypothesis that breakdown events are accompanied by elevated temperature and dark current spike due to high asperity nano-structure formation which significantly enhances the local electric field. However, the mechanistic origin of such field enhancement under typical operational fields and joule heating conditions remains poorly understood.
In this work, we built a model describing the evolution of a typical copper surface driven by a electric field and temperature spikes. Implementing a mesoscale curvature-driven diffusion model, we identify the critical regimes where electric fields and thermo-elastic driving forces combine to lead to the spontaneous formation of sharp surface features. These regimes strongly resonate with previous experimental findings on breakdown of copper electrodes, suggesting surface diffusion to be a strong candidate for breakdown precursor formation mechanism.
Currently SSRF/SXFEL is ongoing to develop the advanced transverse deflecting structure TTDS(Two-mode operation transverse deflecting structure) to carry out variable polarization based on the design of dual-mode RF structure. Driven by two different RF power sources, this novel TDS can work on both HEM11 and HEM12 modes in vertical and horizontal deflecting directions simultaneously, and consequently it is ultra-fast to fix and change linear polarization, in particular circular and elliptic polarizations as well by flexible vector combination of these two modes, actually in engineering which is very practically realized by amplitude and phase modulation from LLRF. The work presented in this paper is focused on analysis and design of the variable polarization TDS, consisting of dual-mode cells and two dual-mode couplers. Considering the complexity of dual-mode design and optimization, the advanced optimization procedure based on both neural network and multi-objective algorithms, is successfully developed to improve the accuracy and efficiency of the RF structure design. After many iterative optimizations, the dual-mode cells in the final design can work on high impedance and similar RF performance for both HEM11 and HEM12, in particular the two couplers for RF power input and output are also approached to the ideal design as well. Based on the optimized design and RF sensitivity analysis, the engineering design is finally completed and ready for manufacture.
Increasing energy of proton beam at the Los Alamos Neutron Science Center (LANSCE) from 800 MeV to 3 GeV will improve radiography resolution ten-fold. This energy boost can be achieved with a compact cost-effective linac based on normal conducting high-gradient (HG) RF accelerating structures. Such an unusual booster is feasible for proton radiography (pRad), which operates with short beam pulses at very low duty. The pRad booster starts with a short L-band section to capture and compress the 800-MeV proton beam from the existing linac. The main HG linac is based on S- and C-band cavities. An L-band de-buncher at the booster end reduces the beam energy spread at 3 GeV three times below that at the exit of the existing 800-MeV linac. We are developing proton HG standing-wave structures with distributed RF coupling for the booster. Results of this development, as well as breakdown rate measurements for a two-cell C-band test cavity at the LANL RF Test Stand, will be presented.
Advances in high-gradient accelerating technologies have facilitated commercial interest in producing compact, pulsed, electron-driven neutron facilities. Here we present studies undertaken for the design of an accelerator, optimised for cost and compactness, that delivers 30 nC trains in < 1 µs timescales.
The Very-compact Inverse Compton gamma source (VIGAS) is underconstructing in Tsinghua University in Beijing. It is based on a 350-MeV electron linac and to generate up to 4.8MeV photon by head-on interaction between the electron bunch and the laser pulse. The accelerator consists of S-band photo-injector and X-band main linac. The six 630-mm-long x-band accelerating structures will provide 80MV/m accelerating gradient to boost the electron beam energy from 50MeV to 350MeV. This talk will present the recent progress of the project.
The CXFEL Project encompasses the Compact X-ray Light Source (CXLS) that is now commissioning in the hard x-ray energy range 4-20 keV, and the Compact X-ray Free-Electron Laser (CXFEL) designed to lase in the soft x-ray range 300-2500 eV. CXFEL has recently completed a 3-year design phase and just received NSF funding for construction over the next 5 years. These instruments are housed in separate purpose-built laboratories and rely on inverse Compton scattering of bright electron beams on powerful lasers to produce femtosecond pulses of x-rays from very compact linacs approximately 1 m in length. Both instruments use recently developed X-band distributed-coupling, room-temperature, standing-wave linacs and photoinjectors operating at 1 kHz repetition rates and 9300 MHz RF frequency. They rely on recently developed Yb-based lasers operating at high peak and average power to produce fs pulses of 1030 nm light at 1 kHz repetition rate with pulse energy up to 400 mJ. We present the current commissioning performance and status of CXLS. We also review the design and initial construction activities of the large collaborative effort to develop the fully coherent CXFEL.
Inverse Compton Scattering (ICS) sources are becoming more popular as future lab-based x-ray sources. Smart*Light is one such facility, under commissioning at Eindhoven University of Technology (TU/e). This compact X-ray source aims at bridging the gap between conventional lab X-ray sources and synchrotrons.
A 100 kV DC photo electron gun is used in combination with a bunching cavity to produce electron bunches that are injected in a X-band accelerator. The high gradient X-band accelerator is adapted from an original design for the Compact Linear Collider (CLIC). The accelerated electron bunches are focused and collide with a focused 12 mJ/pulse 800 nm laser beam thereby producing X-ray photons with energies between 10 and 40 keV. The physical basis behind the production of the X-rays is the ICS process in which photons from the laser pulse are bounced off a relativistic electron bunch, turning them into X-ray photons through the relativistic Doppler effect.
This work introduces the design of the low- and high power RF system, gives an overview of measurements of the electron bunch quality, and shows the first results of conditioning the high gradient accelerator.
This work is financed by the "Interreg programme Flanders-Netherlands" with financial support of the European Fund for Regional Development.
In this talk we will present an update on the design and R&D plans for C3 (Cool Copper Collider). C3 is a Higgs factory concept based on cryogenic copper distributed-coupling accelerator technology. An update on the accelerator complex design, cryogenics and cryomodule design will be presented. Ongoing R&D (high gradient testing, vibrations, beam dynamics, etc.) and future R&D needs (alignment, emittance preservation, wakefields, etc.) will be presented.
To reduce energy consumption and cost has become a prime objective of the development and operation of high energy particle accelerators. RF sources are major energy consumption components of any RF system for the particle colliders. The 30 GeV injector linear accelerator (LINAC) for the CEPC (Circular Electron and Positron Collider) requires S-band klystrons with higher efficiency to reduce energy consumption and cost. In this paper, two novel bunching method, including COM (Core Oscillation Method) and BAC (Bunching Alignment and Collecting), are applied to the BEPCII (Beijing Electron Positron Collider) S-band klystron. These methods increase the efficiency of klystron from 45% to 55%, therefore increasing the output power to 80 MW with the same operation voltage. The preliminary optimization design is completed with 1-D disk model based AJDISK code and further checked by 2-D code EMSYS. The density modulation of electron injection is improved by selecting a suitable cavity string structure and thereby to optimize electron bunching. Further improvement in the RF conversion efficiency of the klystron is also planned and is in line with a requirement to go green.
Several types of X-band accelerating tubes for varies applications have been developed in Tsinghua University, with beam energy from 6MeV to 12MeV. Beam-break-up was also observed in a 6MeV tube with 10microsecond 200mA pulse. A backward-travelling-wave strcuture was developed as an injector for VHEE beamline. This presentaton will talk about the research and develop of these X-band linac tubes.
After the discovery of the Higgs particle at Large Hadron Collider (LHC) in 2012, a 240 GeV circular electron positron collider (CEPC) was proposed in China. The beam power of CEPC is about 60 MW. To reduce the energy demand and operation cost for CEPC, a 800 kW CW klystron with efficiency of more than 80% are being developed as a priority key technology at Institute of High Energy Physics, Chinese Academy of Sciences. So far, several klystron prototypes have been designed, manufactured and high power tested. The first klystron prototype with a second harmonic cavity successfully obtained 804 kW pulse power with output efficiency of 65.3% and 700 kW CW power on March, 2020. To improve the efficiency to more than 75% in the second stage, a lower perveance and CSM bunching method have been applied. Due to the crack of ceramic window and spurious oscillation in RF cavity, the klystron just obtained 630 kW CW power with output efficiency of 70.5% on July, 2022. After problem analysis, this klystron is being repaired and the high power test will be carried out on October, 2023. The third prototype is a Multi-beam klystron with efficiency of more than 80%, which is being manufactured and will be tested in the middle of 2024. In this presentation, the progress on high efficiency klystron for CEPC will be introduced. Also the detailed design and test results will be presented.
Operation of high-gradient structures require the delivery of extremely high RF power---from hundreds of kW to MW---by klystrons, usually at the expense of high overall energy consumptions. A collaboration between CERN and Canon Electron Tubes and Devices has been established to optimise the efficiency of commercially available klystrons, resulting in the proposal of novel technological solutions. An existing 6 MW, X-band klystron (Canon E37113) has been redesigned and successfully rebuilt to ensure 8 MW output power and >10% efficiency increase. We will describe the setup, calibration, fine tuning, and results for the acceptance tests performed on this prototype at the CERN X-band facilities. Revealing a remarkable ~60% efficiency, as opposed to the ~40% efficiency of the E37113 model, these results validate the proposed HE klystron technology and represent a significant leap forward in klystron technology. Future work will seek to apply similar advances to the LHC klystron, in preparation for its possible deployment in the high luminosity HL-LHC project.
A pulse compression system based on double-height waveguides was designed for the Klystron-based CLIC main linac. The system has been optimized to achieve a power gain of 3.81 with the specific pulse shape required for the CLIC-K accelerating structure. This pulse compression system is composed of a main pulse compressor based on the Barrel Open Cavity (BOC) design and 4 correction cavities based on the bowl cavity design. The BOC pulse compressor operates in the TM1,1,32 mode and has a Q0 of 2.35×105 and a 𝛽 of 6.6. To simplify the machining process, a novel coupling waveguide network was designed for the BOC pulse compressor. The correction cavities are based on the bowl cavity operating in the TE2,2,3 mode, with a Q0 of 7.5×104 and a 𝛽 of 1.95.
Recently,a soft X-ray FEL facility, Shenzhen Superconducting Soft-X-ray Free Electron Laser(S3FEL), has been approved by the national government of China. S³FEL is a high repetition rate soft-X-ray super-conducting free-electron laser facility that consists of a 2.5 GeV CW superconducting linear accelerator and four initial undulator lines, which aims at generating X-Rays between 40 eV and 1 keV at repetition rates up to 1 MHz. In S3FEL, Transverse Deflecting Structures (TDS) that work at S-band (2997.222 MHz) and X-band (11988.889 MHz) will be utilized for the diagnosis and analysis of longitudinal phase-space of electron bunches. According to the preliminary design, about 60~80 MW RF power is needed for 1 X-band TDS system. Meanwhile, the output power of X-band klystron is 20 MW, which means pulse compressors are necessary. Three pulse compressor schemes are under considering: SLED, spherical cavity and bowl-shaped cavity. The comparison between these three schemes will be carried out later and one of them will be chosen for S3FEL.
High gradient electron linacs are a key ingredient for advanced accelerator applications ranging from particle-driven radiation sources to lepton colliders. The overall performance of such machines strongly relies on the quality of the electron beams they transport and accelerate. In particular, due to the high phase space density, the dynamics of high brightness beams are affected by strong self-induced electromagnetic fields causing mutual interaction of the charged particles through space-charge and wakefield mechanisms. Such effects become extremely important in high gradient RF linacs where the small irises enhance the wakefield coupling whereas, at the same time, the radial focusing acts against significant space charge forces. As a consequence, beam dynamics studies capable of investigating the performance, as well as the operational limits, of a given machine are crucial. However, the modeling of collective effects in simulation codes tracking large particle ensembles often requires significant, time-intensive numerical resources. Here we thus present alternative, simplified approaches for the description of space-charge and wakefield effects that permit streamlined computations. The features of such models are discussed and validating comparisons with existing tracking codes are shown. In addition, we present examples of applications to state-of-the-art facilities currently under design.
There are various hypotheses for vacuum breakdown trigger mechanisms in normal-conducting accelerating structures. It has been experimentally turned out that the dominant trigger of RF breakdowns in normal-conducting UHF CW cavities is a hot micro-particle with a high sublimation point [Phys. Rev. Accel. Beams 21, 122002 (2018)], later named a "fireball" by this presenter. In principle, this fireball breakdown can occur in any normal-conducting HG accelerating structures. On the other hand, SuperKEKB accelerator is suffering from a serious obstacle of sudden beam losses in the collider rings, that limits the luminosity improvement.
Recently, this presenter has proposed a hypothesis that the same mechanism as in the fireball breakdown can be a trigger
of the sudden beam losses, where fireballs could interplay through the accelerator. In this presentation, the presenter will explain the fireball hypothesis and show some plans to perform further high-power tests of RF cavities to demonstrate the hypothesis and to better understand the breakdown mechanism.
After fabrication, high-gradient structures need to be conditioned to provide high gradients of 100 MV/m, and sustain even higher gradients at the copper surfaces, while achieving a breakdown rate below $3*10^{-7}$ BD/pulse. Estimating the energy unaccounted for during a breakdown may be important in order to avoid damage to the structure as well as to understand the breakdown process itself. We are establishing an energy balance analysis of individual breakdowns and compare it to the overall conditioning process.
After refining the calibrations for the direct RF power measurements, we are performing simulations and calorimetry measurements of the power lost by dissipation in the copper.
Future work will seek to apply CST PIC-Solver analysis to investigate the “Dark Current” and “Breakdown Current”.
All these factors affect how well we can measure the missing energy during a breakdown. The first results of this analysis will be shown.
RF-Track is a CERN-developed particle tracking code that can simulate any particle species' acceleration and tracking. RF-Track tracks particles in time or in space through field maps and conventional elements and considers a large set of single-particle and collective effects: space-charge, beam-beam, short- and long-range wakefield effects, synchrotron radiation emission, multiple Coulomb scattering in materials, and particle lifetime. A self-consistent model that simulates beam loading effects in standing- and travelling-wave structures was recently added. All these effects make RF-Track an ideal tool for simulating high-intensity machines. RF-Track has been used to simulate electron linacs for medical applications, inverse-Compton-scattering sources, positron sources, protons linacs, electron cooling, and the ionization cooling channel of a future muon collider. An overview of the code is presented, along with some significant results.
High gradient RF cavities, especially those at frequencies above S-band, are critical in several concepts for future electron linacs such as the ultra-compact X-ray free electron laser (UCXFEL) and the Cool Copper Collider ($C^3$). These two designs in particular rely on the cryogenic operation of RF cavities, taking advantage of several complementary physical effects. It is then advantageous to develop in greater detail an understanding of the complex surface and material physics associated with high gradient RF fields in normal conducting cavities. We present here measurements made at the CYBORG (CrYogenic Brightness-Optimized Radiofrequency Gun) beamline at UCLA. In this first phase of study we will relate our temperature dependence measurements of dark current and other RF cavity figures of merit to the general material science research goals via a modified theory of anomalous skin effect regime. The theory will also be presented with additional results from low level RF cavity testing. Future directions will be presented including the status of the second phase of the beamline in which photoemission of novel cathodes in extreme conditions will be studied.
Dielectric Assist Accelerating (DAA) structures based on ultralow-loss ceramic are being studied as an alternative to conventional disk-loaded copper cavities. This accelerating structure consists of dielectric disks with irises arranged periodically in metallic structures working under the TM02$-\pi$ mode.
Here, the numerical design of an S-band DAA structure for low beta particles, such as protons or carbon ions used for hadrontherapy treatments, is shown. Four dielectrics with different permittivity and loss tangent are studied as well as different particle velocities depending on the energy range.
Through optimization, most of the RF power is stored in the vacuum space near the beam axis, leading to a significant reduction of power loss on the metallic walls. This allows to fabricate cavities with extremely high quality factor over 100 000 and shunt impedance over 300 M$\Omega$/m at room temperature.
During the numerical study, the design optimization has been improved by adjusting some of the cell parameters in order to both increase the shunt impedance and reduce the peak electric field in certain locations of the cavity, which can lead to instabilities in its normal operation. In addition, first multipactor simulations are being carried out, using several coatings to reduce SEY, which has also been taken into account in the electromagnetic result. Finally, thermal and mechanical analysis has been performed in order to estimate cavity performance and cooling.
In 2020, following a successful commissioning period, CERN’s linear accelerator 4 (Linac4) became the proton source for the CERN accelerator complex. The first RF accelerating structure in Linac4 is a 3 m long Radio-Frequency Quadrupole (RFQ) which operates with an inter-vane voltage of 78 kV and a peak surface electric field of 34 MV/m.
To monitor the cavity field profile, the RFQ is equipped with 16 probes and the measured signals are logged automatically during operation. An analysis of the signals recorded during breakdowns has been performed, and in this talk a novel method of breakdown localisation is described. The results and their implications are then briefly discussed.
The new C-Band RF gun, developed in the context of the European I.FAST and INFN Commission V TUAREG projects, has been realized. It is a 2.6 cell standing wave cavity with a four-port mode launcher, designed to operate with short rf pulses (300 ns) and cathode peak field larger than 160 MV/m. The gun has been realized with the new technology without brazing, developed at INFN, that allows to assemble the gun cells with special gaskets and to proceed, after the vacuum test, directly to the rf characterization. In the paper we illustrate the realization procedure and the results of the vacuum and low power RF tests. The gun has been installed at PSI (Switzerland) and is now ready for the high power test
The current trend in accelerator facilities is to increase the beam brightness, which places higher demands on both the electron gun and the acceleration unit. With high shunt impedance and expected gradient, cryogenic structure has the ability to provide beam with high peak brightness and has the potential to operate at a higher repetition rate. In SSRF/SXFEL, the scheme and preliminary experimental studies about the cryogenic RF structure have been developed. Foremost, theoretical and experimental studies of the RF characteristics in cryogenic environments are investigated. Then the study of beam dynamics and the RF design of the cryogenic gun, as well as the fabrication of the gun are performed. An upgraded prototype of the electron gun with the cryostat, which can make the gun compatible with the beam has been designed. In the subsequent study, we will work on the high power experiment of the cryogenic gun, and continue developing cryogenic acceleration units.
HB2TF is a project funded by INFN CSN5 related to the development of a High Brightness Beams Test Facility (HB2TF) at the INFN-LASA laboratory. The Test Facility will allow to perform developments in different accelerator technologies and to carry out experiments with a high current CW electron beam in frontier areas of accelerator physics.
The Test Facility setup will comprise a high-performance DC Gun followed by a normal conducting RF buncher-acceleration section to provide 1 MeV 5 mA CW electron beam.
A HV DC gun provides a robust and already well-developed solution with proven and well documented successful operations at high repetition rate. Our reference solution is the “inverted insulator” design developed at JLAB.
The DC Gun UHV chamber is made of 316L Stainless Steel and hosts an array of six NEG pump modules. The spherical cathode holder is attached to the narrow end of a tapered conical insulator designed by INFN. The cathode electrode is made of two hydroformed hemispherical shells (316L stainless steel) welded together. The photogun is connected to a −350 kV dc Cockcroft-Walton high voltage power supply (HVPS).
In this paper we will present the rationales of the full design.
Nowadays, production of ultrashort and high-current electron bunches of few fs to hundred fs with several pc charge is a very attractive subject in the context of many recent applications, specially, in light sources, free electron lasers and plasma wake field accelerators. In the light sources and free electron lasers, the radiation gain is directly proportional to the beam pick current and so obtaining very short bunches on the order of fs plays an essential role for high gain machines.
Consequently, recent approaches for production of such electron bunches are based on usage of photo injectors in combination with a chain of bunch compressors. In the Iranian light source project attempts are directed toward the designing a novel compact soft X-ray synchrotron beyond the state of the art, using the latest advanced studies worldwide. The project will be started form a novel photoinjector which can provide very high quality, and in the meantime, ultra-short electron bunches in a compact structure. Moreover, finding the best structures for the rf components, specifically, the rf gun and its associated buncher cavity has a crucial effect on the function of the whole light source in near future. In this talk I will show some of the photocathode RF gun designing and compare the results
In this talk we will present results from high gradient structure testing of single and multi-cell accelerating structures and components at C-band. Structures were tested with a variety of fabrication techniques including brazing, diffusion bonding and clamped plated structures. Target gradients of 120 MeV/m were achieved and exceeded. Designs and performance of rf components (loads, windows, etc.) for these tests will also be discussed. Plans for future experiments will be presented.
This talk will report on the status high gradient C-band accelerator structure research at Los Alamos National Laboratory (LANL). Current activities at LANL include construction of the Cathodes And Radio-frequency Interactions in Extremes (CARIE) high gradient C-band RF photoinjector test stand and research on the higher order mode (HOM) absorbers for high gradient cavities. We are putting together the high gradient photoinjector test stand capable of producing electric fields at the cathodes up to 250 MV/m. The photoinjector will be powered by a 50 MW, 5.712 GHz Canon klystron. The klystron was delivered to LANL in July of 2023, installed, and is being tested. A concrete vault was renovated, capable to provide radiation protection for electron beams with beam power up to 20 kW. All required waveguide and vacuum components have been received and the waveguide line to the photoinjector is being constructed. The all-copper photoinjector was designed and is currently in fabrication with the expected delivery in September of 2023. The second version of the photoinjector will be built with replaceable high quantum-efficiency cathodes and produce an ultra-bright 250 pC electron beam accelerated to the energy of 8 MeV. Adding capability to operate the photoinjector at cryogenic temperatures is considered. The status of the facility, the designs of the photoinjector and the beamline, and plans for photocathode testing will be presented. As a separate synergistic activity, we designed and are currently fabricating a test two-cell accelerating structure with nickel-chrome HOM absorber waveguides. The structure will be tested at LANL’s C-band Engineering Research Facility in New Mexico (CERF-NM) with the goal to determine high gradient conditioning and operation limitations of these novel absorbers. The talk will also cover the design of the structure and fabrication and testing plan.
FLASH Therapy represents a revolutionary advancement in cancer treatment, offering the potential to spare healthy tissue from ionizing radiation damage while maintaining its efficacy in tumor control. The practical clinical integration of FLASH therapy, particularly for treating deep-seated tumors, requires the attainment of Very High Electron Energy (VHEE) levels within the 50-150 MeV range. To this end, Sapienza University has proposed a linac-based facility, the SAFEST project, for radiobiological and pre-clinical studies. In this framework, Sapienza in collaboration with INFN is developing a compact C-band high-gradient electron linac. This paper outlines the design strategy, the electromagnetic characteristics and the prototypes testing.
A dielectric assist accelerating (DAA) structure, a type of dielectric loaded accelerating structures, is greatly superior in power efficiency compared with the conventional disk-loaded copper structures. The advantage of DAA structure is that it has an extremely high quality (Q0 > 10^5@C-band) factor and a high shunt impedance at room temperature since the electromagnetic field distribution of the accelerating mode can be controlled by the geometry of its structure to reduce the wall loss on metallic surface. However, an accelerating gradient of C-band DAA structure is currently limited about 12 MV/m due to multipactor and discharge.
We are developing X-band DAA structures to achieve higher accelerating gradient in DAA structure. These structures are tested using the X-band high power test facility named Nextef2 at KEK. In the high-power test, we plan to perform short pulse RF excitation using step pulse input in DAA structures to verify the potential of them to generate a high accelerating field. In this conference, we will report the achievements and progress in the development of DAA structure.
With high shunt impedance and expected gradient, cryogenic structure has the ability to reduce accelerators' length and has the potential to operate at a higher repetition rate. In SSRF/SXFEL, the scheme and preliminary experimental studies about the cryogenic RF structure have been developed. Foremost, theoretical and experimental studies of the RF characteristics in cryogenic environments are investigated. Then we designed a cryogenic C-band standing wave bi-periodic accelerating structure. It is a 17-cell structure consisting of 9 accelerating cavities and 8 coupling cavities. To guarantee the symmetry of the field, the structure is doubly-fed. Operating with the pi/2 mode standing wave, it is much less sensitive than the standing-wave structure of pi-mode. Additionally, the microwave mode is TM02 in coupling cavities that are larger and even less sensitive than the traditional bi-periodic structure. The shape of the coupling cavity can be redesigned to make it tunable. In the subsequent study, we will work on the experiment of the cryogenic structure.
Abstract: In electron linear accelerators, the improvement of the acceleration gradient of the acceleration structure has been a continuous research topic for scientists, which can reduce the construction cost of the entire accelerator by increasing the acceleration gradient. Normally, large acceleration structures work on traveling wave made of ordinary conductive material oxygen free copper at room temperature. Recently, SLAC proposed C3 project. The distributed coupling acceleration structure of C-band cool copper material can achieve an accelerating gradient of over 120 MV/m in 77k liquid nitrogen, and has broad prospects for future applications. Institute of high energy physics has research on distributed coupling low-temperature structures at C-band. A parallel coupled standing wave structure was designed, manufacture, and cold tested. This structure consists of 20 cavities, with a length of approximately 0.5 meters. The cryogenic system has designed fabricated. High-power test will be done on existing test C-band test bench in Dongguan. This paper will provide a detailed introduction to the design process, project progress, and next step work plan
A cryogenic DC HV system integrated in a stand-alone cryocooler has been constructed at Uppsala University in order to investigate the fundamental mechanisms of field emission and breakdown nucleation. A series of high-field measurements has been carried out with pairs of copper and niobium electrodes at temperatures ranging from ambient down to 4K. We observed a significant increase in the field holding capability of the electrodes when cooled and conditioned at cryogenic temperatures.
A significant reduction of fluctuations and greater stability of the field emitted current from fully conditioned electrodes operated at cryogenic temperatures was also observed, together with an increase of the maximum current enabled by the larger attainable field.
The results show a general agreement of BD characteristics with the proposed theoretical models, where the temperature enters exponentially via $e^{{1}/{k_b T}}$ term, and confirm the increase in field holding capability at cryogenic temperatures observed in other studies.
The present work provides experimental data that can be used to refine such models and potentially discriminate between various underlying physical mechanisms, thus eventually improving our understanding of BD phenomena.
Finally, our study provides valuable data for the design of future normal-conducting accelerators at cryogenic temperatures with very high gradient and reduced breakdown rates, with the potential of a reduced cost and optimized performance.
Recently, much work has been directed at R&D a ultra-compact X-ray FEL (UC-XFEL) based on rapidly emerging techniques in high field cryogenic acceleration, attendant dramatic improvements in beam brightness, and state-of-the-art concepts in beam dynamics, magnetic undulators, and X-ray optics. A full conceptual design of a 1 nm XFEL with a length and cost over an order of magnitude below current XFELs has been developed. This instrument has been conceived with an emphasis on permitting exploratory scientific research in a university setting. Concurrently, compact FELs are undergoing rapid development for use in next-generation chip manufacturing as a high flux, few-nm lithography source. This new role suggests consideration of XFELs to address urgent demands in this sector, as identified by recent national need studies, for new radiation sources aimed at chip manufacturing: a coherent hard X-ray source which enables frontier metrology methods. Indeed, it has been shown that one may use coherent X-rays to perform few nm-resolution surveys of macroscopic structures using ptychographic tomography. As the XFEL is an extremely promising candidate for realizing such methods, we present here an analysis of the issues and likely solutions associated with extending the UC-XFEL to X-rays above 7 keV, much higher fluxes, and methods of applying such a source to ptychographic tomography for micro-electronic device measurements. We discuss the development path to move the concept to rapid realization.
Hollow-core dielectric Electromagnetic Band Gap (EBG) microstructures powered by lasers represent a new and promising area of accelerator research thanks to the dielectric higher damage threshold and greater accelerating gradients with respect to the metallic counterparts. In this paper, we present MeV-scale 3D beam-dynamics simulations and fabrication results relative to a silicon, woodpile-based travelling-wave structure operating at the frequency of 60 THz (wavelength λ = 5 μm). The simulated CST and HFSS electric field has been employed as input for the Astra simulation package, in order to perform beam-dynamics calculations considering beam injection and extraction into the structure and effects on the main beam parameters, aiming at improving the beam brightness. In order to mitigate space charge effect without reducing the beam current, CW operation is desirable: therefore, the temperature distribution and heat flow under a CW laser power for accelerating gradients ≥ 250 MV/m are analysed.
Finally, we show the first Si prototypes of the woodpile structure obtained by Two Photon Polymerization fabrication process. This technique allows to reach resolutions down to hundreds of nanometers, offering the possibility to print Si-rich structures, or woodpile skeletons to be infiltrated with Si by CVD technique.
The exploration of procedures, materials, technological methods, and welding techniques applied in the production of high-acceleration components to achieve an exceeding accelerating gradient (>100 MV/m) and minimal occurrence of RF breakdown, has guided us towards proposing the utilization of hard-copper structures composed of multiple segments in the Ka-Band spectrum. Among research applications, we mention the study of RF Breakdown Rate and RF Breakdown Physics; electron beam longitudinal phase-space manipulation in FELs; multi-frequency accelerators; development of single and multi-frequency high-power RF klystrons.
In this talk, we detail our investigation into TIG welding evaluations, encompassing both visual scrutiny and temperature surveillance, concerning metallic RF cavities operating in the Ka-band frequency. These examinations, from which several facets of the welding process were taken as benchmarks, encompass both two-half and four-quadrant models.
The operational principles and the approach to mitigate electromagnetic field breakdowns in a corrugated structure of a sub-THz wakefield accelerator will be discussed. A design of the structure and its test using an electron beam will be presented. The analysis of an impulse heating of the structure with a high charge electron bunch will be shown. The results of calculations using COMSOL’s multi-physics model defining a threshold for beam induced heating of the structure will be discussed.
High intensity THz pulsed radiation was used to investigate the damage induced on low roughness copper substrates and thin films deposed on copper. Using the THz Free Electron Laser available at the ISIR facility of the Osaka University [1], we irradiated samples in air at different angles, with an energy density of ~100 GW/cm$^2$. We induced a reproducible electric field gradient up to ~4 GV/m and, at this intensity, breakdown phenomena may occur.
In the case of copper, since at THz wavelengths the reflectivity is ~99%, irradiation at normal incidence does not induce any damage and no signature of breakdown phenomena is detectable on the surface. At variance, decreasing the angle of incidence the damage of the surface occurs with a central spot of ~200 μm diameter, surrounded by a visible corona associated to the intense heating induced by the pulsed beam. In addition, in the central region tips made by copper oxides identified by Raman microscopy are induced by the multiple breakdowns [2,3].
To better understand the damage, we performed simulations of irradiations on copper surfaces using state of the art two-temperature modelization of high-intensity ultrafast radiation interacting with matter.
We also tested thin MoO$_3$ films deposed on copper to probe the resistance to breakdown and to probe the damage induced by multiple breakdowns. This van der Walls material, is characterized by a high work function (6.7 eV) and, with its higher mechanical resistance compared to copper, it is an optimal candidate to reduce the damage of the copper surface [2,3]. In spite of the limited thickness, i.e., ~100-200 nm, much lower than the wavelength, experiments again performed at different incidence angles showed that the oxide coatings minimize the damage of the copper surface. In spite the extremely high electric field, there is no evidence of tips in the central irradiated region and no copper oxidation is detected by Raman microscopy on the irradiated surfaces. [4]
References
1. A. Irizawa, S. Suga, T. Nagashima, A. Higashiya, M. Hashida and, S. Sakabe, Laser-induced fine structures on silicon exposed to THz-FEL. Applied Physics Letters, 111, 251602 (2017)
2. S. Macis, C. Aramo, C. Bonavolontà, G. Cibin, A. D’Elia, I. Davoli, M. De Lucia, M. Lucci, S. Lupi, M. Miliucci, A. Notargiacomo, C. Ottaviani, C. Quaresima, M. Scarselli, J. Scifo, M. Valentino, P. De Padova, and A. Marcelli, MoO3 films grown on polycrystalline Cu: Morphological, structural, and electronic properties, Journal of Vacuum Science & Technology A 37(2), 021513 (2019).
3. S. Macis, L. Tomarchio, S. Tofani, S.J. Rezvani, L. Faillace, S. Lupi, A. Irizawa, A. Marcelli, Angular Dependence of Copper Surface Damage Induced by an Intense Coherent THz Radiation Beam. Condens. Matter 5, 16 (2020).
4. S. Macis A. D’ Elia, A. Irizawa, M. Carillo, B. Spataro, Z. Ebrahimpour, L. Mosesso, J.S. Rezvani, S. Lupi and A. Marcelli, ‘High electric field induced damage using a pulsed THz source: a new technique to test metallic surfaces’, INFN-22-05/LNF, 14 Novembre 2022
Hefei Advanced Light Facility (HALF) is a fourth-generation low-energy synchrotron radiation light source which will be started from September 2023 to August 2028. It consists of a 2.2 GeV full energy linac and a storage ring. This contribution presents optimizations on the C-band accelerating structures and pulse compressors for HALF linac. A new design method for matching couplers to an accelerating structure in a more efficient way is also presented. It combines the Kroll’s method with Kyhl’s method, thereby simplifying the tuning and simulation process of a coupler cavity.
Linear accelerators used in high-energy physics, industry, and medicine need compact, durable, and cost-effective accelerating structures. To achieve a smaller footprint, linacs require high accelerating gradients. Currently, stable operating gradients exceeding 100 MV/m have been successfully demonstrated at SLAC National Accelerator Laboratory, CERN, and KEK, particularly at X-band frequencies. Recent experiments have indicated that accelerating cavities constructed from hard copper alloys outperform their soft copper counterparts in achieving higher gradients.
We present the status of a continuative 15-year-long collaboration between SLAC, INFN-LNF, and KEK that focuses on advanced high-gradient RF structures. The study of this collaboration specifically explores technological advancements aimed at demonstrating the feasibility and high-power testing of innovative RF structures made out of different geometries, materials, fabrication technologies and joining techniques.
Various working groups have been set up in the context of the European Strategy for Particle Physics. We report on surveys carried out by two of the working groups; Normal conducting high-gradient RF and high-efficiency power sources. The surveys give an overview of the status of development initiatives, infrastructure at the European and international level.