Beam Breakup Instability in RF Linacs

Abstract

The Beam Breakup (BBU) instability appears to be the principal limitation to high current operation of RF linacs. This instability results in the loss of part or all of a beam bunch above a certain current or beyond a certain number of bunches. Even when the bunch is not lost it may be deflected sufficiently far off its axis to become useless for its intended application. This paper presents analytical and numerical calculations of the BBU instability in RF linac cavities which should be useful in scaling RF linacs to high current operation. Analytical calculations for single bunch BBU in pillbox cavities show that the beam deflection scales with the initial deflection, the charge per bunch, the accelerating gradient, the accelerator length, and the cavity wavelength in a manner that can be generalized to include the effects of periodic focusing on transverse emittance growth. Numerical simulations of multibunch BBU show the growth of beam deflection amplitude from wake fields generated by preceding bunches. Wake field calculations using a transverse beam cavity interaction code are presented.