Space Charge Effects in Energy Recovery Linacs

Abstract

Recent technological advances in energy recovery linear accelerators (ERLs) have enabled us to access high energy and high brightness electron beam, allowing us to gain tremendous insights into fundamental physics while also envisioning diverse and robust applications for electron beams. ERLs are recirculating linacs that generate high quality electron beams, with these beams, energy is gained via multiple passes through a superconducting accelerating cavity. After the fully accelerated beam completes its interaction, for example, with an internal target, the electrons are decelerated in the linac, transferring their energy back to the cavity radio-frequency (RF) fields. However, as the beam brightness is increased, collective effects such as space charge considerably affect the beam quality. Space charge modifies the electron beam dynamics in dispersive regions along the beamline. Further, longitudinal space charge together with dispersion can lead to the amplification of the initial shot noise by density modulation along the beamline. This is known as microbunching instability. In the past two decades, the microbunching instability has been intensively studied for storage rings and linac-based facilities. However, due to the increased intricacy of the machine configuration of ERLs compared to linacs, the existing microbunching instability analysis needs to be extended to maintain the high brightness of the electron beam in an ERL where space-charge-modifies the dispersion. This dissertation focuses on the theoretical investigation of beam matching with space charge and space-charge-induced microbunching instability in the recirculation arcs of an ERL. It is shown that beam envelopes and dispersion along the recirculation arcs of an ERL, including space charge forces, can be matched to adjust the beam to the parameters of the subsequent RF structure. It is also shown that the space-charge-modified dispersion plays a key role in the adjustment of the momentum compaction required for both the isochronous and the non-isochronous recirculation mode of an ERL. A coupled transverse-longitudinal beam matrix approach is used for matching with space charge and computing microbunching instability gain. The beam matrix approach is compared with particle tracking simulations. For a qualitative analysis, it is shown that one can use the smooth focusing approximation but with longitudinal-transverse coupling. Within this simplified model, the scaling of average dispersion, momentum compaction, and momentum deviation with space charge is investigated. As an example case, the above model is applied to the recirculation arcs of the multi-turn Mainz Energy-recovering Superconducting Accelerator (MESA), so as to deliver a continuous wave beam at 105 MeV for physics experiments with a pseudo-internal target. Initially, a low-energy 5 MeV, 180° injection arc, which also works as a bunch compressor, is matched to the subsequent first RF structure of the projected MESA. Then, start to end simulations along the MESA beamline with space charge are conducted. Finally, limitations on beam intensity due to space charge effects are analyzed and models to circumvent these limitations are implemented.