Ion production and rf generation in the DARHT-II beam dump

The DARHT-II accelerator produces a 2-kA, 17-MeV beam in a 1600-ns pulse. After exiting the accelerator, the pulse is sliced into four short pulses by a kicker and quadrupole septum and then transported for several meters to a high Z material target for conversion to x-rays for radiography. The un-kicked beam is diverted to a graphite dump. The interaction of this beam with the dump produces ions that propagate back to the target beamline and affect the beam properties. Coupling of the electron beam and ions to the beam dump vacuum chamber can produce significant amounts of ions and radiofrequency fields (rf) that disrupt the beam transport to the target. This requires a change in the nominal tune to the target. An alternative is to redesign the beam optics in the dump line. The results of a 3D PIC simulation and experimental data are presented along with mitigation techniques to suppress and/or eliminate these effects.