Effects of plasmas on electron beam transport in a fast magnetic kicker

Abstract

The use of a fast kicker is planned for the 2"d axis of DARHT to achieve multiple pulse radiography capability. The kicker is an electromagnetic device that gives a slight magnetic deflection to the electron beam causing the beam to be pushed radially off-center, allowing the beam to be firther deflected in a magnetic septum region. Subsequently, the electron bunch with the pulse length determined by the time duration of the kicker field is transported through the system. In the DARHT design, the magnetic septum consists of a magnetic quadrupole and a magnetic dipole that deflect the electron beam into a pre-determined position. Thus, the 2 ps electron beam is chopped into several pulses of tens of nanoseconds in duration downstream of the septum.In this study, we investigate the processes of plasma formation in the vicinity of the kicker and the effects of plasma contamination on the transport of the DARHT relativistic electron beam (20 MeV, 2.0 -4.0 kA) in the kicker-septum assembly. The effect of varying plasma density on the beam is quantified, and possible mechanisms and time scales for plasma formation are examined. Methods of mitigation will also be discussed.The major target issues concerning the multiple-exposure intense electron-beam radiography are: ( I ) To limit the expansion of the plasma plume from the x-ray converter target. (n) To limit the distance traveled upstream by the ions from the target along the beam. This is because substantial plasma plume expansion can reduce the x-ray output dose and uncontrolled backstreaming ions can cause neuaalization of the e-beam and thus the beam may not be tightly focused on the target.We present two target configurations based on LASNEX simulations that address these two target issues.Multi-foil targets [1,2]: Dividing a solid target into many thin foils over a longer length can lengthen the disintegration time of the target and reduces the expansion of the plasma plume. This improves the production of the x-ray dose. We will present the LASNEX simulations of the target expansion.Pellicle dynamics: Placing a pellicle in front of the target can confine the backstreaming ions to a shorter channel so that the cumulative effect on e-beam focusing is reduced. We will, present the simulation for hydrodynamic expansion, caused by e-beam heating, of the pellicles. We have studied pellicles made of various materials. Both single-layered and multilayered pellicles have been investigated. Simulations show that the range (pr) of the pellicles along the axis remains essentially constant. Therefore, all the pellicles studied here should be able to stop backstreaming ions.Reference:1. Y.-J. Chen et al., Proc. PAC Conf., New York, 1999, p. 1827 2. P. A. Pincosy, UCRL report forthcoming. 283