Study of a final focus system for high intensity beams

STUDY OF A FINAL FOCUS SYSTEM FOR HIGH INTENSITY BEAMS Enrique Henestroza, Shmuel Eylon, Prabir K. Roy, Simon S. Yu, Frank M. Bieniosek, Derek B. Shuman and William L. Waldron Lawrence Berkeley National Laboratory, CA 94720 Abstract The NTX experiment at the Heavy Ion Fusion Virtual National Laboratory is exploring the performance of neutralized final focus systems for high perveance heavy ion beams. The final focus scenario in an HIF driver consists of several large aperture quadrupole magnets followed by a drift section in which the beam space charge is neutralized by a plasma. This beam is required to hit a millimeter-sized target spot at the end of the drift section. The objective of the NTX experiments and associated theory and simulations is to study the various physical mechanisms that determine the final spot size (radius r s ) at a given distance (f) from the end of the last quadrupole. In a fusion driver, f is the standoff distance required to keep the chamber wall and superconducting magnets properly protected. The NTX final quadrupole focusing system produces a converging beam at the entrance to the neutralized drift section where it focuses to a small spot. The final spot is determined by the conditions of the beam entering the quadrupole section, the beam dynamics in the magnetic lattice, and the plasma neutralization dynamics in the drift section. The main issues are the control of emittance growth due to high order fields from magnetic multipoles and image fields. In this paper, we will describe the theoretical and experimental aspects of the beam dynamics in the quadrupole lattice, and how these physical effects influence the final beam size. In particular, we present theoretical and experimental results on the dependence of final spot size on geometric aberrations and perveance. I. INTRODUCTION The topic of final focus systems for high intensity beams has been an important subject of analytical [1-4], and experimental [5] efforts since the beginning of the Heavy Ion Fusion project in 1976. The first designs were derived from systems already in use in particle accelerators for high energy physics where space-charge is negligible, and which incorporated sextupoles and octupoles elements for the correction of chromatic and geometric aberrations. Chromatic aberrations affect the focal spot radius due to deviations from the nominal ion momentum causing a variation in the focal length produced by the magnet system. These aberrations may result from initial transients in the injector, from residual momentum tilt due to beam compression, or from the momentum spread, or longitudinal emittance produced along the beam line. Geometric aberrations are due to nonlinear external fields and also from deviations from paraxial ray optics. In a possible scenario of a final focus system for an HIF driver, the beam is transported in the final focus section through several strong large aperture magnetic quadrupoles, and is then allowed to drift ballistically through neutralizing plasma in a low-density (millitorr) gas onto the target. There are nonlinear processes both in the magnetic section as well as in the neutralized transport section. To investigate these phenomena, the Neutralized Transport Experiment (NTX) has begun at LBNL [6]. The NTX beamline (Fig. 1) consists of 4 large bore quadrupoles followed by a neutralized drift section. A low emittance K+ beam is made to traverse this 4-quad lattice, at the exit of which, the beam enters a 1-meter long drift section with injected plasma. The beam is diagnosed at the exit of the 4-quad section as well as the end of the drift section. The relevance of NTX to the fusion driver is derived from the observation that the key scaling parameter is perveance, defined as Q = 2 ZeI b 4 πe 0 m i v i 3