Pozdeyev, E.
(Eduard Pozdeyev)

WEBOS03 Suppression of Multipass, Multibunch Beam Breakup in Two Pass Recirculating Accelerators
Chris Tennant, David Douglas, Kevin Jordan, Lia Merminga, Eduard Pozdeyev (Jefferson Lab, Newport News, Virginia), Todd I. Smith (Stanford University, Stanford, Califormia)

Beam Breakup (BBU) occurs in all accelerators at sufficiently high currents. In recirculating accelerators, such as the energy recovery linacs used for high power FELs, the maximum current has historically been limited by multipass, multibunch BBU, a form that occurs when the electron beam interacts with the high order modes (HOMs) of an accelerating cavity on one pass and then again on the second pass. This effect is of particular concern in the designs of modern high average current energy recovery accelerators utilizing superconducting technology. In such two pass machines rotation of the betatron planes by 90°, first proposed by Smith and Rand in 1980 [1], should significantly increase the threshold current of the multibunch BBU. Using a newly developed 4-dimensional tracking code, we study the effect of rotation on the threshold current of the JLAB FEL Upgrade. We examine several optical rotator schemes based on quadrupoles and solenoids and evaluate their performance in terms of the instability threshold current increase and their effect on the FEL optics.