MEASUREMENT AND SIMULATION OF THE UMER BEAM IN THE SOURCE REGION* I. Haber, S. Bernal, R. A. Kishek, P. G. O’Shea, B. Quinn, M. Reiser, Y. Zou University of Maryland, College Park, MD 20742-3511 A. Friedman, D. P. Grote, J. -L. Vay LBNL 1 Cyclotron Road Bldg 47 Berkeley, CA, 94720-8201 Abstract As the beam propagates in the University of Maryland Electron Ring (UMER) complex transverse density structure including halos has been observed. A primary objective of the experiment is to understand the evolution of a space-charge-dominated beam as it propagates over a substantial distance. It is therefore important to understand which details of the beam structure result from propagation of the beam in the ring and which characteristics result from the specific details of the initial distribution. Detailed measurements of the initial beam characteristics have therefore been performed. These include direct measurement of the density using a phosphor screen, as well as pepper pot measurements of the initial transverse distribution function. Detailed measurements of the distribution function have also been obtained by scanning a pinhole aperture across a beam diameter, and recording phosphor screen pictures of the beam downstream of the pinhole. Simulations of the beam characteristics in the gun region have also been performed using the WARP P.I.C. code. From these simulations, the observed behavior has been attributed to a combination of perturbations to the transverse distribution by a cathode grid that is used to modulate the beam current, as well as the complex transverse dynamics that results from the combination of the nonlinear external focusing fields of the gun structure and the nonlinear space charge forces. 1. INTRODUCTION Even though many of the basic characteristics of space-charge-limited diodes have been well known for several decades, this knowledge does not usually extend to the details of the particle distribution function that are necessary for predicting the downstream behavior of modern intense-beam systems. Furthermore, recent simulations and experiments have found that details of the beam distribution emerging from the source can have a strong influence on the downstream beam evolution. Since the phenomena that govern the beam evolution are usually nonlinear and measurements of the beam emerging from the gun region with adequate accuracy are often quite difficult, simulations have become a promising tool for understanding the behavior of space-charge- dominated beams in the source region. However, an important precursor to developing a credible predictive simulation capability that can reliably model the source region is the benchmarking of the numerics against experimental observation. An effort has therefore been undertaken to compare numerical simulation against experiment in those cases where beam diagnostics are sufficient to permit accurate simulation/experiment comparisons for space- charge-dominated sources. It should be noted that even though this work has largely been motivated by developing a predictive capability for the highly space-charge-dominated beams required for heavy ion fusion, the results are applicable to a large class of machines. This is because as the intensity or luminosity of a beam at the high-energy end of an accelerator is increased, it becomes increasingly important to operate the source region in the space-charge- dominated regime. Recent research comparing WARP simulations to experimental observation of gun behavior have centered on two experiments that are particularly suitable for these comparisons because of the diagnostic configurations. * This work is supported by the US DOE under contract Nos. DE-FG02-02ER54672 and DE-FG02-94ER40855 (UMD), and DE-AC03-76SF00098 (LBNL) and W-7405-ENG-48 (LLNL). email: haber@umd.edu