Compensation of wakefield-driven energy spread in energy recovery linacs

Abstract

Energy Recovery Linacs provide high-energy beams, but decelerate those beams before dumping them, so that their energy is available for the acceleration of new particles. During this deceleration, any relative energy spread that is created at high energy is amplified by the ratio between high energy and dump energy. Therefore, Energy Recovery Linacs are sensitive to energy spread acquired at high energy, e.g. from wake fields. One can compensate the time-correlated energy spread due to wakes via energy-dependent time-of-flight terms in appropriate sections of an Energy Recovery Linac, and via high-frequency cavities. We show that nonlinear time-of-flight terms can only eliminate odd orders in the correlation between time and energy, if these terms are created by a beam transport within the linac that is common for accelerating and decelerating beams. If these two beams are separated, so that different beam transport sections can be used to produce time-offlight terms suitable for each, also even-order terms in the energy spread can be eliminated. As an example, we investigate the potential of using this method for the Cornell x-ray Energy Recovery Linac. Via quadratic time-of-flight terms, the energy spread can be reduced by 66%. Alternatively, since the energy spread from the dominantly resistive wake fields of the analysed accelerator is approximately harmonic in time, a high-frequency cavity could diminish the energy spread by 81%. This approach would require bunch-lengthening and recompression in separate sections for accelerating and decelerating beams. Such sections have therefore been included in Cornell's x-ray Energy Recovery Linac design.

Transverse emittance-preserving arc compressor for high-brightness electron beam-based light sources and colliders

“…For this reason, if such a peak current is needed, for instance, at the end of recirculating accelerators like energy-recovery linacs (ERLs) driving free-electron laser (FEL) and/or ERL-based particle colliders, the photoinjected beam is recirculated in isochronous beam lines and magnetically time-compressed only after the very last stage of acceleration [2], e.g. with a magnetic chicane [3,4]. Although this approach tends to preserve beam brightness during recirculation [5], it may put an upper limit to either the compression factor or the beam charge or both, thus to the final peak current, because of CSR-induced emittance growth in a single-stage compression [6].…”

Optimizing the configuration of a superconducting photonic band gap accelerator cavity to increase the maximum achievable gradients

“…The beam breakup threshold due to HOM wakefields in the linac scales inversely proportional to frequency squared [5]. Therefore, much of the effort in the field of SRF for high current electron accelerators was directed to ensure strong damping of parasitic HOM oscillations [2,[5][6][7].…”

First High Power Test Results for 2.1 GHz Superconducting Photonic Band Gap Accelerator Cavities

“…The HOMs can greatly reduce luminosity, increase emittance and strongly affect interaction of the beams at the collision point [5]. Studies of the beam breakup mechanisms and the efficient methods for the higher order mode suppression have become a critical area of research for the future high duty factor and high current SRF accelerators (including energy recovery linacs) for linear colliders [6] and free-electron lasers [7,8]. Photonic Band Gap (PBG) [9] cavities have the unique potential to absorb all HOM power and greatly reduce the wakefields.…”