Transient beam dynamics in the Lawrence Berkeley Laboratory 2 MV injector

A driver-scale injector for the heavy ion fusion accelerator project has been built at LBL. This machine has exceeded the design goals of high voltage (above 2 MV), high current (more than 0.8 A of K+) and low normalized emittance (less than 1 π mm mrad). The injector consists of a 750 keV gun pre-injector followed by an electrostatic quadrupole accelerator which provides strong (alternating gradient) focusing for the space-charge-dominated beam, and simultaneously accelerates the ions to 2 MeV. A matching section is being built to match the beam to the electrostatic accelerator ELISE. The gun pre-injector, designed to hold up to 1 MV with minimal breakdown risks, consists of a hot alumino-silicate source with a large curved emitting surface surrounded by a thick “extraction electrode”. During beam turn-on the voltage at the source is biased from a negative potential, enough to reverse the electric field on the emitting surface and to avoid emission, to a positive potential to start extracting the beam; it stays constant for about 1 μs, and is reversed to turn off the emission. Since the Marx voltage applied on the accelerating quadrupoles and the main pre-injector gap is a long, constant pulse (several microseconds), the transient behavior is dominated by the extraction pulser voltage time profile. The transient longitudinal dynamics of the beam in the injector was simulated by running the particle-in-cell codes gymnos and warp3d in a time-dependent mode. The generalization and its implementation is WARP3D of a method proposed by Lampel and Tiefenback to eliminate transient oscillations in a one-dimensional planar diode will be presented.