High gradient ultra-high brightness RF photoinjector optimization

by Michele Croia (LNF)

Tuesday 3 Apr 2018, 14:30 → 16:00 Europe/Rome

Aula Divisione Acceleratori (LNF INFN)

Most of the accelerator applications demand an high beam quality: ultra low energy spread and ultra high beam brightness i.e. bunches with high peak current and ultra low emittance. These quality parameters are also necessary to perform a good matching between beams from a conventional accelerator and a plasma one in the so called external injection scheme. Beam brightness is a fundamental parameter for applications as the Free Electron Laser (FEL), where the gain length is inversely proportional to the electron beam brightness in the Self Amplified Spontaneous Emission (SASE) X-ray regime. These requests in the quality electron beam means that a perfect control of bunches along the beam line is necessary, starting from the bunch generation up to the accelerator end, especially in the photoinjector region where the beam is not yet relativistic and is in the so called space charge regime. From these requests an optimal transport has to be found i.e. an optimization of some parameters and distances has to be fixed: laser on cathode parameters a proper position for the first accelerating section, an integrated magnetic field of the gun solenoid and an optimal bunch compression scheme. To match these stringent beam quality parameter requests, I optimized the beam dynamics for a new ultra high gradient 1.6 cells C-band (5.712 GHz) gun able to reach 240 MV/m as a peak field. By means of the ultra high gradient a better control of the space charge forces inside the bunch is possible. After optimizations on this electron gun a proper emittance compensation scheme was found through simulations with the software General Particle Tracer (GPT). Simulations showed the possibility to obtain ultra-low emittance and ultra-high beam brightness values. I used this gun to design with GPT the future SPARC_LAB upgrade (EUPRAXIA), where using X-band modules after the photoinjector the final beam energy is about 1.1 GeV, with a very low final emittance and energy spread values.