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Description
Electron beam-driven plasma wakefield accelerators (PWFAs) are currently being intensively developed for future applications in high-energy physics and industry due to their ability to provide a high acceleration gradient. Among the key unresolved challenges of PWFAs is maintaining high quality of accelerated (witness) and drive bunches (emittance, energy spread, efficiency) during their transport over long distances along the accelerating structure. This problem is especially relevant if PWFAs are considered for possible use in future electron-positron colliders, which require TeV-level accelerated particle energies.
A Layered Plasma Wakefield Accelerator (LPWA) [1], that is a modification of the PWFA, has been proposed to improve the transport of accelerated electron/positron bunches. Unlike the vacuum channel design for bunch transport [2], in the LPWA, the transport channel is filled with plasma, but with a density lower than the surrounding tubular plasma. The outer plasma provides high accelerating fields, and the inner plasma provides the focusing fields.
Here we investigate the excitation of wake fields by a ramped train of relativistic electron bunches in LPWA. The ramped bunch train allows increasing the maximum energy of accelerated particles, characterized by the transformer ratio R (the maximum energy gain of the witness bunch to the maximum energy loss of the drive bunch). We consider a ramped train of four bunches following with a period of 1.5 accelerating field wavelength. The bunch charges from the head to the tail of the train increase as odd integers. The transverse sizes of the inner and outer plasmas are 150 µm and 600 µm, respectively. The outer tubular plasma provides a resonant frequency of the accelerating wave of 600 GHz, and the density of the inner plasma corresponded to the Langmuir frequency of 200 GHz. Analytical study brings R ~ 7 (the limiting predictable value R for the train of 4 drive bunches is 8). PIC simulation yields an R value of ~3.8 (estimated based on the last drive bunch energy loss). This is due to increasing plasma inhomogeneity from the head to the tail of the train. That leads to poor bunch focusing, an increase in their transverse size, and energy losses that increase with bunch number. The condition of equality of losses in the drive beams is broken, and the transformer ratio calculated using the energy loss of the last bunch becomes less than the calculated one. To reduce the disruption of transverse plasma homogeneity, PIC simulation was performed with a thin quartz capillary (20 µm thick) inserted between two plasma layers. The resulting R value was 5.34. LPWA optimization may further increase the transformer ratio.
The study is supported by the National Research Foundation of Ukraine under the program “Excellent Science in Ukraine” (project # 2023.03/0182).
References
1. G.V. Sotnikov, K.V. Galaydych, P.I. Markov. arXiv:2601.04903 [physics.acc-ph].
2. S. Gessner, E. Adli, J. M. Allen et al, Nature Communications. 2016, 7, 1-6