Results on intense beam focusing and neutralization from the neutralized beam experiment

Results on intense beam focusing and neutralization from the neutralized beam experiment P. K. Roy, S. S. Yu, S. Eylon, E. Henestroza, A. Anders, F. M. Bieniosek, W. G. Greenway, B. G. Logan, W. L. Waldron, and D. L. Vanecek Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA-94720, USA D. R. Welch and D. V. Rose Mission Research Corporation, 5001 Indian School Rd NE , Albuquerque, NM 87110-3946, USA R. C. Davidson, P. C. Efthimion, E. P. Gilson, and A. B. Sefkow Princeton Plasma Physics Laboratory, Princeton, NJ 08543-0451, USA W. M. Sharp Lawrence Livermore National Laboratory, 7000 East Ave., L-645, Livermore, CA 94550, USA We have demonstrated experimental techniques to provide active neutralization for space-charge dominated beams as well as to prevent uncontrolled ion beam neutralization by stray electrons. Neutralization is provided by a localized plasma injected from a cathode arc source. Unwanted secondary electrons produced at the wall by halo particle impact are suppressed using a radial mesh liner that is positively biased inside a beam drift tube. We present measurements of current transmission, beam spot size as a function of axial position, beam energy and plasma source conditions. Detailed comparisons with theory are also presented. I. INTRODUCTION Final focusing has been a subject of intense study [1- 3] from the very early days of heavy ion fusion (HIF). Neutralized ballistic transport (NBT) [4-11] is presently being studied for propagating intense heavy ion beams inside a reactor chamber to an inertial confinement fusion (ICF) target. A recent HIF driver study [12] demonstrates that stringent final-focus requirements [13-15] can be met provided that active neutralization is implemented to overcome the formidable space charge of the intense ion beams. Other beam transport schemes under consideration include self-pinched transport [16-20] and discharge channel [21-23] transport. In the NBT scheme, the individual beams focus outside of the target chamber and enter through ports in the chamber walls. These beams are focused and directed such that they intersect before striking the target and then strike the target as they are expanding into an annular configuration [24]. The target chamber is filled at low pressure with a gas such as flibe. A volumetric plasma is produced as the flibe gas is partially ionized by the beam as well as by xrays emitted by the hot target. The volumetric plasma is not adequate to provide the necessary neutralization. Therefore, additional plasma, the “plasma plug,” is externally injected near the chamber entry port, through which the beam passes. Chamber transport using annular and solid plasma regions in the transport chamber has been examined numerically by several investigators [17-18, 25]. The general concept studied in this paper consists of an initially-non neutralized beam passing through a finite thickness of plasma and dragging along plasma electrons for partial charge and current neutralization. An earlier experiment [26] examined the charge neutralization of a heavy ion beam by electrons drawn from a localized source as the beam was focused. The electron source was a glowing tungsten filament placed in the beam path, enabling the supply of thermionically- emitted electrons inside of the beam. The experiment demonstrated the beneficial effect of charge neutralization on a heavy-ion beam, and these results were confirmed in a series of electrostatic particle-in-cell (PIC) simulations. To quantitatively ascertain the various mechanisms for neutralization, the Neutralized Transport Experiment (NTX) was constructed at Lawrence Berkeley National Laboratory. In this experiment a high quality beam is passed through well-characterized plasma sources. The objective is to provide sufficently detailed experiment data to validate simulation code predictions. Here, we are presenting initial results of neutralization from localized plasma plug on the NTX . This article describes the neutralization physics, NTX beamline system, techniques to control stray electrons in vacuum transport, and beam neutralization using a plasma plug. II. PHYSICS OF NEUTRALIZATION The plasma plug provides electrons that neutralize to >90% the charge of a convergent beam. Typically, n p /Zn b > 1, where n p is the plasma density and n b and Z are the ion beam density and charge state. Ideally, the plasma is in electrical contact with a conducting boundary at large radius enabling a continuous supply of electrons. Stationary plasma can only provide an ion beam electron neutralization down to some minimum space-charge potential. The key scaling parameter for beam transport is the dimensionless perveance defined as the ratio of the beam space charge to kinetic energy (K = 2I b /I A β i 2 , where I A =β i γ i m i c 3 /eZ is the Alfven current with a beam of current I b , velocity β i c, and relativistic factor γ i ). Provided Km i /Zm e > 1, electrons from this plasma can accelerate in