Abstract
In recirculating accelerators, and, in particular, energy-recovery linacs, the maximum current can be limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovering pass. This effect is of particular concern in the design of modern high average current energy-recovery accelerators utilizing superconducting rf technology. Experimental characterization and observations of the instability at the Jefferson Laboratory 10 kW free electron laser (FEL) are presented. Measurements of the threshold current for the instability are made under a variety of beam conditions and compared to the predictions of several BBU simulation codes. This represents the first time in which the codes have been experimentally benchmarked. With BBU posing a threat to high current beam operation in the FEL driver, several suppression schemes were developed. These include direct damping of the dangerous HOM using cavity feedback and modifying the electron beam optics so as to reduce the coupling between the beam and mode. Both methods were shown to increase the threshold current for stability. Beam optical suppression techniques, in particular, have proved to be so effective that they are routinely used in the normal operations of the FEL Upgrade Driver.
Highlights
IntroductionEnergy recovering linacs (ERLs) that utilize superconducting radio-frequency (SRF) technology offer an attractive alternative as drivers for, among other things, synchrotron light sources and free electron lasers (FEL), as they combine the desirable characteristics of both storage rings (high efficiency) and linear accelerators (superior beam quality) [1]
Energy recovering linacs (ERLs) that utilize superconducting radio-frequency (SRF) technology offer an attractive alternative as drivers for, among other things, synchrotron light sources and free electron lasers (FEL), as they combine the desirable characteristics of both storage rings and linear accelerators [1]
Dipole higher-order modes (HOMs) can drive the multipass beam breakup (BBU) instability. This effect is of particular concern in the design of modern high average current energy-recovery accelerators utilizing SRF technology due to the relatively high quality factors of HOMs
Summary
Energy recovering linacs (ERLs) that utilize superconducting radio-frequency (SRF) technology offer an attractive alternative as drivers for, among other things, synchrotron light sources and free electron lasers (FEL), as they combine the desirable characteristics of both storage rings (high efficiency) and linear accelerators (superior beam quality) [1]. Dipole HOMs can drive the multipass beam breakup (BBU) instability. This effect is of particular concern in the design of modern high average current energy-recovery accelerators utilizing SRF technology due to the relatively high quality factors of HOMs. Because BBU imposes a real threat to the operation of high current ERLs, a better understanding of the instability and development of suppression techniques are vitally important. The 7 MeV energy recovered beam is extracted to a dump
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