Neutralized Drift Compression Experiments (NDCX) with a High Intensity Ion Beam

Neutralized Drift Compression Experiments (NDCX) with a High Intensity Ion Beam P. K. Roy 1* , S. S. Yu 1 , W. L. Waldron 1 , A. Anders 1 , D. Baca 1 , J. J. Barnard 2 , F. M. Bieniosek 1 , J. Coleman 1 , R. C. Davidson 3 , P. C. Efthimion 3 , S. Eylon 1 , A. Friedman 2 , E. P. Gilson 3 , W. G. Greenway 1 , E. Henestroza 1 , I. Kaganovich 3 , M. Leitner 1 , B. G. Logan 1 , A. B. Sefkow 3 , P. A. Seidl 1 , W. M. Sharp 2 , C. Thoma 4 and D. R. Welch 4 Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, California 94720, USA. Lawrence Livermore National Laboratory, Livermore, California 94550, USA. Princeton Plasma Physics Laboratory, New Jersey 08543-0451, USA. Voss Scientific, Albuquerque, NM 87108, USA. Elsevier use only: Received date here; revised date here; accepted date here Abstract To create high energy density matter and fusion conditions, high-power drivers, such as lasers, ion beams, and x-ray drivers, are employed to heat targets with pulses short compared to hydro-motion. Both high energy density physics and ion-driven inertial fusion require the simultaneous transverse and longitudinal compression of an ion beam to achieve high intensities. We have previously studied the effects of plasma neutralization for transverse beam compression. The scaled experiment, the Neutralized Transport Experiment (NTX), demonstrated that an initially un-neutralized beam can be compressed transversely to ~1 mm radius when charge neutralization by background plasma electrons is provided. Here we report longitudinal compression of a velocity-tailored, intense, neutralized 25 mA K + beam at 300 keV. The compression takes place in a 1-2 m drift section filled with plasma to provide space-charge neutralization. An induction cell produces a head-to-tail velocity tilt that longitudinally compresses the neutralized beam, enhances the beam peak current by a factor of 50 and produces a pulse duration of about 3 ns. The Physics of longitudinal compression, experimental procedure, and the results of the compression experiments are presented. © 2006 Elsevier Science. All rights reserved. Keywords: Beam; ion; longitudinal compression; plasma; neutralization; diagnostics; induction cell; experimental articles on beam neutralization and transverse compression have been published elsewhere[1-6]. In neutralized drift compression, the beam is longitudinally compressed by imposing a linear head-to-tail velocity tilt that produces a pulse duration of several ns. Longitudinal compression of space-charge-dominated beams has been studied extensively in theory and simulations [7-12]. Longitudinal space-charge forces limit the beam compression ratio, the ratio of the initial to final current, to about ten in most applications. An experiment with five- fold compression has been reported [13]. Recent theoretical models and simulations predicted that much higher compression ratios (of order 100) can be achieved if the beam compression takes place in a plasma-filled drift region in which the space-charge forces of the ion beam are neutralized [14, 15]. Experimentally, we have reported 50- fold compression in a brief letter [16]. The physics and technical issues, fast diagnostics, experimental results on longitudinal beam compression are presented in this article. 1. Introduction Intense ion beams of moderate energy offer an attractive approach to heating dense matter uniformly to extreme conditions, because their energy deposition is nearly classical and shock-free. High energy density physics and ion-driven inertial fusion require the simultaneous transverse and longitudinal compression of an ion beam to achieve high intensities. A beam of ~200 A (1 beam, 23 MeV Na+) with a 1-mm focal spot radius and pulse length of ~1 ns would be ideal as a driver for Warm Dense Matter experiments. These beam spot sizes and pulse lengths are achievable with beam neutralization and longitudinal compression in background plasma. In beam neutralization, electrons from a plasma or external source are entrained by the beam and neutralize the space charge sufficiently that the pulse focuses on the target in a nearly ballistic manner to a small spot. Several numerical and