CERN Accelerating science

Article
Title	Experimental Investigation of Beam Breakup in the Jefferson Laboratory 10 kW FEL Upgrade Driver
Author(s)	Tennant, Chris ; Bazarov, Ivan V ; Douglas, David ; Hoffstaetter, Georg Heinz ; Jordan, Kevin ; Merminga, Lia ; Pozdeyev, Eduard ; Simrock, Stefan ; Smith, Todd I ; Wang, Haipeng
Affiliation	(Cornell University, Ithaca, New York) ; (DESY, Hamburg) ; (Jefferson Lab, Newport News, Virginia) ; (Stanford University, Stanford, Califormia)
Publication	2005
In:	21st IEEE Particle Accelerator Conference, Knoxville, TN, USA, 16 - 20 May 2005, pp.369
Subject category	Accelerators and Storage Rings
Abstract	In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum current has been 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 recovered pass. This effect is of particular concern in the design of modern high average current energy recovery accelerators utilizing superconducting technology. Experimental 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 presented and compared to the predictions of several BBU simulation codes. To further characterize the instability, beam based measurements were made to determine the orientation of the dangerous HOMs. With BBU posing a threat to high current beam operation in the FEL, several suppression schemes were developed. These include direct damping of the dangerous HOMs and appropriately modifying the electron beam optics. Preliminary results of their effectiveness in raising the threshold current for stability are presented.