Development and testing of a lithium ion source and injector

Development and testing of a lithium ion source and injector P. A. Seidl,1 w.w. Greenway,1 D. P. Grote,2 J—Y. Jung,‘ J. w. Kwan,' S.M. Lidia,‘ P. K. Roy,1 J. Takakuwa,' J-L. Vay,l and W. L. Waldronl ‘Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA zlawrence Livermore National laboratory, Livermore, California 94550, USA (Received 30 January 2012; published 11 April 2012) We report on the development and testing of an intense lithium ion source and injector for an ion induction accelerator designed for warm, dense matter target heating experiments. The source is a 10.9-cm diameter aluminosilicate emitter on a porous tungsten substrate. For an injector voltage pulse of 120 kV, pulse duration of 1.0- ,u,s FWHM, and an operating temperature of 1250°C, the source emits 35 mA of Li‘“ ions. The results follow experimental studies with much smaller sources. The key challenges included beam quality, source lifetime, and heat management. I. INTRODUCTION Recently, low-energy ion beams are being applied to the field of warm, dense matter physics, that is, the exploration of matter at or near solid density and temperature from 1 eV to several eV. In order to achieve this transient state of matter, the target material, or sample, must be heated faster than the characteristic thermodynamic expansion time of the target, ~ 1 ns. Ion beams complement laser heating experiments, because ion beams deposit their energy volu- metrically in the target material. In particular, low to medium mass ions (ion mass in the approximate range 4 l A l 23) with ion kinetic energy of 1-20 MeV deposit most of their energy near the peak of dE/dx vs E, which is particularly effective for uniform heating [1]. The new neutralized drift compression experiment (NDCX—ll) is based on a Li+ beam accelerated to 1-3 MeV. The ~ 0.1 A beam current at injection (E = 0.1 MeV) is accelerated and compressed to a much higher current and focused to an intense millimeter-sized beam radius at the target [2]. The injector and source are designed for = 1 mA/cm2 space—charge limited extraction from a l0.9—cm diameter emitter. An acceleration electrode follows the main extraction electrode. The potential of the accelerating electrode is used to tune the transverse envelope of the beam within the injector. The injector is designed to create 60 nC, or 4 X 10q ions in a l-/L sec bunch. II. ION SOURCE Various methods for the production of alkali ions, and specifically lithium have been demonstrated, for example, aluminosilicate, vapor spark sources, and contact ionizers from refractory metals. The choice of aluminosilicate was driven by the need for high intensity and repetition rate, coupled with low emittance and low impurities in a single charge state. Very high lithium current has been demon- strated, for example, with LiCl dielectric surface flashover sources [3], operating at injector voltage of 500 kV and creating a Li+ current density of 120 A/cmz. However, the charge state impurities were 25%—35%, greater than that required for our application. Very high current has been demonstrated with vapor Li+ sources ionized by high-power lasers [4], generally at low repetition rate. Contact ionizer sources have shown ion current densities in an acceptable range [5] with very low emittance. The expected lower ionization efficiency for lithium and asso- ciated higher neutral emission from these sources are a concern. The choice of aluminosilicate is based on successes with other alkalis such as K+ [6] which have shown good reproducibility, low emittance, and high current from large surface area sources capable of operating at g1 Hz. Also, there have been demonstrated reliable Li+ sources devel- oped as tokamak plasma diagnostics [7,8]. The results here follow on that work to much larger area lithium sources, at ~ 1 mA/cmz. Ion source preparation Earlier experiments with 6.35-mm emitters [9] estab- lished the sintering temperature, emitter heating and cool- ing rate, extracted current density, and source lifetime. In preparing the large emitters, we have followed most of the steps as described in Ref. [9], with a few changes. (i) High-purity chemicals (AIZCO3, 99.7%; SiO2, 99%; 7Li2CO3, 99.7% isotopically pure, 99.9% chemically pure) are mixed in a stoichiometric ratio (7Li2CO3: AIZCO3: 2 - SiO2) for ,8 eucryptite (LiAlSiO4) and heated to 600°C for one hour, which liberates C02. The chemicals are then thoroughly mixed with a mortar and pestle.