Experimental investigation of multibunch, multipass beam breakup in the Jefferson Laboratory Free Electron Laser Upgrade Driver

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.

A proposed VUV oscillator-based FEL upgrade at Jefferson Lab

“…The modular recirculator design described above provides redundant control of transverse phase space and allows control of both turn-toturn phase advances and beam envelope functions. This can be used to limit the magnitude of system response to HOM effects and avoid troublesome instability thresholds [31,32].…”

Beam breakup current limit in multiturn energy recovery linear accelerators

“…Method (1) Vary t b Method (2) Vary phase advance Method (3) Introduce x-y coupling Both the second and third method involve modifying the optics of the recirculation beamline between the pairs of multipass cavities. The idea of modifying beam optics to improve the I th was first suggested in 1980 [11], and has been tested out at the Jefferson Lab's free electron laser [12][13][14]. The effect of all three methods are simulated using BMAD following only the design particle, with results presented in the three following subsections.…”

“…The dependence however vanishes in the approximate formula for the trough region [Eq. (13)]. It is interesting to investigate how I th of CBETA varies with t b using simulation.…”

Technical Applications of the Physics of High Energy Densities

“…The free-electron laser at the Jefferson Laboratory, USA, was used to advantage for the simultaneous generation of high-power femtosecond X-ray radiation (by the interaction of Thomson scattering and the infrared radiation of the electron beam in the wiggler), infrared free-electron laser radiation, and terahertz radiation in the linear accelerator [37,38,63,71].…”