The High Current Transport Experiment for heavy-ion inertial fusion

Abstract

THE HIGH CURRENT TRANSPORT EXPERIMENT FOR HEAVY ION INERTIAL FUSION* P.A. Seidl, D. Baca, F. M. Bieniosek, C.M. Celata, A. Faltens, L. R. Prost, G. Sabbi, W. L. Waldron, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA R. Cohen, A. Friedman, S.M. Lund, A.W. Molvik Lawrence Livermore National Laboratory, Livermore, CA 94550 USA I. Haber University of Maryland, College Park, MD 20742 USA Abstract The High Current Experiment (HCX) at Lawrence Berkeley National Laboratory is part of the US program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge-dominated heavy-ion beams at high space-charge intensity (line- charge density ~ 0.2 mC/m) over long pulse durations (>4 ms) in alternating gradient electrostatic and magnetic quadrupoles. This experiment is testing -- at driver- relevant scale -- transport issues resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and beam steering, matching, image charges, halo, electron cloud effects, and longitudinal bunch control. We present the results for a coasting 1 MeV K + ion beam transported through the first ten electrostatic transport quadrupoles, measured with beam-imaging and phase-space diagnostics. The latest additions to the experiment include measurements of the secondary ion, electron and atom coefficients due to halo ions scraping the wall, and four magnetic quadrupoles to explore similar issues in magnetic channels. special limitations associated with magnetic focusing, in particular the onset of transport-limiting effects due to electrons trapped in the potential well of the ion beam. EXPERIMENTAL CONFIGURATION The present configuration begins with the K + ion source and injector, an electrostatic quadrupole matching section (six quadrupoles), and the first 10 electrostatic transport quadrupoles. A multi-purpose diagnostic station (D-end) is at the end of the beam line (Fig. 1). Beam diagnostics are also located at the interface of the matching section and the ten transport quadrupoles (QD1) and after the last (D2) transport quadrupole in the periodic lattice. They include transverse slit scanners, Faraday cups and current transformers. The Gas and Electron Source Diagnostic (GESD) is located at the end of D-end. Most measurements so far have been made at 1.0 MeV, to avoid any high voltage insulation issues in the injector. Though much of the beam physics should scale predictably between 1 and 2 MeV, future measurements will verify this, and also establish operating experience at higher injection energy. The beam energy at present is limited to 1.5 MeV until the water resistor that distributes the voltage along the injector column is modified. The column has operated at 1.8 MV during checks of the injector optics modifications. To date, contact-ionization and alumino-silicate ion sources have been used. The injector beam characterization measurements and the first measurements through the HCX were made using the contact ionization source, before switching to the alumino-silicate source (100 mm diameter) in April 2002. Earlier versions of alumino-silicate sources suffered from poor current- density uniformity. There was a considerable alumino- silicate large-source R&D effort during 2001-2 aimed at improving the uniformity. As a consequence of improvements to the diode optics and the improvements to the ion source [2], the hollowness of the beam has decreased from 20% to 10%. The emittance has decreased from e n =10 x 10 -7 to 6 x 10 -7 m compared to the theoretical minimum, e n = 2R kT / m = 1.7 ¥ 10 m where T is the source temperature and R is the radius. The experimental current is within 2% of the expected value based on 3D PIC simulations. INTRODUCTION The High Current Experiment (HCX) [1] located at Lawrence Berkeley National Lab and carried out by the HIF-VNL (Heavy-Ion Fusion Virtual National Laboratory: a collaboration between groups at LBNL, LLNL and Princeton Plasma Physics Laboratory, which has the goal of developing heavy-ion accelerators capable of igniting inertial-fusion targets for electric power production) is designed to explore the physics of intense beams with line-charge density of about 0.2 mC/m and pulse duration 4<t<10 ms, close to the values of interest for a fusion driver. Experiments are performed near driver injection energy (1-1.8 MeV). HCX beam transport is at present mainly based on electrostatic quadrupole focusing, which provides efficient transport at low energy and provides clearing fields which sweep out unwanted electrons. However, magnetic transport experiments have commenced, to gain operational experience and to explore * Supported by the Office of Energy Research, US DOE, at LBNL & LLNL, contract numbers DE-AC03-76SF00098 and W-7405-Eng-48.