Beam-breakup calculations for the DARHT accelerator

An induction line that will accelerate a 4-MeV, 3-kA beam of electrons to 16- to 20-MeV in 64 gaps is modeled. To suppress beam-breakup (BBU) instabilities induced by excitation of RF deflecting modes, the growth factor Gamma must be kept sufficiently small (e.g. <3). On prototype DARHT cavities, RF measurements have shown that the normally degenerate TM modes are split in frequency by the asymmetry that the two pulsed-power drive rods present to the cavity. If half the cavities had vertical and half had horizontal drive-rod orientations, the effective number of gaps would be reduced by half if there were no coupling between the modes by the solenoid focusing and if the split modes had no overlap. The LLNL (Lawrence Livermore National Laboratory) code BREAKUP was used to study BBU growth for drive rod alternation patterns of blocks of 1, 2, 4, 8, 16, 24, 32, or 64 (no alternation) for both constant and alternating polarity solenoids.<>