CANCELLED Microwave Ion Source and Beam Injection for an Accelerator-Driven Neut ron Source

M I C R O W A V E ION SOURCE AND B E A M INJECTION FOR AN ACCELERATOR-DRIVEN NEUTRON SOURCE * J. H. Vainionpaa, R. Gough, M. Hoff, J. W. Kwan, B. A. Ludewigt, M. J. Regis, J. G Wallig, R. Wells , LBNL, Berkeley, California Abstract An over-dense microwave driven ion source capable of producing deuterium (or hydrogen) beams at 100- 200 mA/cm and with atomic fraction > 90% was designed and tested with an electrostatic low energy beam transport section (LEBT). This ion source was incorporated into the design of an Accelerator Driven Neutron Source (ADNS). The other key components in the ADNS include a 6 MeV RFQ accelerator, a beam bending and scanning system, and a deuterium gas target. In this design a 40 mA D^ beam is produced from a 6 mm diameter aperture using a 60 kV extraction voltage. The L E B T section consists of 5 electrodes arranged to form 2 Einzel lenses that focus the beam into the RFQ entrance. To create the ECR condition, 2 induction coils are used to create « 875 Gauss on axis inside the source chamber. To prevent HV breakdown in the L E B T a magnetic field clamp is necessary to minimize the field in this region. Matching of the microwave power from the waveguide to the plasma is done by an autotuner. We observed significant improvement of the beam quality after installing a boron nitride liner inside the ion source. The measured emittance data are compared with PBGUNS simulations. Figure 1: ADNS system schematics. In producing high ion current density, and low min order to minimize neutron generation at tin- test stand, the ion source and L E B T tests were made using Hydrogen gas instead of Deuterium. Thus the required peak current scales to « 57 mA of H* for a matched perveance. ION SOURCE Microwave source was selected due to it's high power efficiency, and reliability. The design of the ion source is presented in Fig. 2. The 2.45 GHz microwave is generated using magnetron and transmitted to the ion source using a wave-guide. RF is impedance matched to ion source using an auto-tuner. A pair of solenoids is used to generate an axial magnetic field required to form an ECR condition. It is necessary to shield the L E B T from the stray magnetic field because it affects the beam optics and also causes high voltage break down problems. This was done by installing a magnetic steel plate at the source exit. Unfortunately the magnetic field clamp at the present location has an adverse effect to the beam current and species. In the source we used boron nitride (BN) lining to increase the atomic fraction and extracted current density. [1,7,8] The source has optimum operation when the gas flow was -1.5 seem equivalent of-0.15 Pa. INTRODUCTION An accelerator-driven neutron source (ADNS) for scanning cargo containers to detect shielded nuclear material [1] was designed at Lawrence Berkeley National Laboratory (LBNL). The key components of the ADNS include a high current D* ion source, a low energy beam transport (LEBT) section, a RFQ accelerator, beam bending and scanning magnets, and a deuterium gas target. The system can produce neutrons with energy up to 8.5 MeV in a forward directed flux of up to 2.0E7 n/cm /s at 2.5 m distance from the target. [2] A schematic diagram of the ADNS is shown in Fig.l. Topic of this paper is the ion source and LEBT section shown in Fiq.2. [3-4]. Our design goal is to have a time-averaged beam current of 1.5 mA. at « 5 % duty factor. Taking beam loss into consideration, the required peak current from the ion source is » 40 mA D* ions with a pulse length of * 0.3 ms and 180 Hz repetition rate. For this application, we have chosen to use the 2.45 GHz microwave ion source because of its capability •Work supported by US Dept. of Homeland Security and the Dept. of Energy under contract number DE-AC03-76SF00098.