Designing Neutralized Drift Compression for Focusing of Intense Ion Beam Pulses in Background Plasma*

Neutralized drift compression offers an effective method for particle beam focusing and current amplification. In neutralized drift compression, a linear transverse and a longitudinal velocity tilt are applied to the beam pulse, so that the beam pulse compresses as it drifts in the drift-compression section. The beam intensity can increase more than a factor of 100 in both the radial and longitudinal directions, resulting in more than 10,000 times increase in the beam number density during this process. The self-electric and self-magnetic fields can prevent tight ballistic focusing and have to be neutralized by supplying neutralizing electrons. This paper presents a survey of the present theoretical understanding of the drift compression process and plasma neutralization of intense particle beams. The optimal configuration of focusing and neutralizing elements is discussed in this paper. INTRODUCTION An effective way to achieve high current density of an ion beam pulse on a target is to simultaneously compress the beam in both the radial and longitudinal directions. This is accomplished by applying a velocity tilt to the beam pulse, so that the beam tail is accelerated relative to the beam head. As a result, the beam number density increases during the drift compression, when the beam tail approaches the beam head. Similarly, the beam pulse can be compressed radially by passing the beam pulse through a focusing element. Because the self-electric field of the beam increases rapidly during compression, the beam space charge may prevent the beam from compression, and thus has to be effectively neutralized. A schematic of the beam compression is shown in Fig.1. In this paper we review the conceptual design of the neutralized drift compression system and analyze the factors that may prevent achieving high compression. This paper is organized as follows. The discussion is divided according to the various components making up neutralized drift compression. These are: longitudinal compression; radial and simultaneous compression; and the physics of the neutralization process. LONGITUDINAL COMPRESSION OF INTENSE ION BEAM PULSES Longitudinal compression results from accelerating the tail of the beam relative to the head of the beam. This is accomplished experimentally by passing the beam through a time-dependent acceleration module. For the Neutralized Drift Compression eXperiment-I (NDCX-I) [1,2,3], the accelerating module is the induction bunching module. The module consists of many independentlydriven magnetic cores that generate an inductive electric field, which is applied to two electrodes, as shown in Fig.2. v Beam pulse Conducting wall Figure 1: Schematic of beam compression. Arrows indicate the direction of the local beam velocity. Figure 2: Schematic of the acceleration gap of the induction bunching module. Arrows indicate the direction of the electric field; the dotted line shows the axis of symmetry. The resulting change in the beam velocity ( ) b v τ Δ should be chosen so that all beam ions during the pulse in the interval 0 p t τ ≤ ≤ arrive simultaneously at the same focal plane at a distance f L from the induction bunching module. The beam trajectory is given by ( , ) ( )( ) b z t v t τ τ τ = − , (1) where 0 ( ) ( ) b b b v v v τ τ = + Δ is the beam velocity after the bunching module, and 0 b v is the initial beam velocity before the bunching module. The condition for longitudinal compression is that all parts of the compressed beam pulse arrive simultaneously at the same vb V(t)