Simulation of vapor formation for the LEVIS lithium ion source

The Laser Evaporation Ion Source (LEVIS) is an active ion source developed for light ion inertial confinement fusion. A 1.06 μm Neodymium YAG laser creates a neutral vapor from a thin (0.2--0.5 μm) lithium-rich surface. An incident energy density of 0.1--0.2 J/cm2 is needed to form a thin vapor layer near the anode with a short-pulse (8 nns FWHM) laser. A 1 μs dye laser, tuned to the Li I 2s-2p resonance transition at 6708 A and with an energy density of about 0.05 J/cm 2, ionizes the vapor. LEVIS experiments on the Sandia Accelerator and Beam Research Experiment (SABRE) and the Particle Beam Fusion Accelerator II (PBFA II) use LiMo or LiAg films deposited on an insulating (LIF) substrate, or in-situ lithium evaporated onto a metallic substrate just prior to the accelerator shot to reduce contamination of the lithium surface by impurities. A spectrometer/streak camera diagnostic characterizes the species, temperature, and center of mass velocity of the neutral vapor. Two-color laser deflection measurements of refractive index gradients (RING) are used to separate electron and neutral components of the medium. The authors are modeling lithium vapor formation at these energy densities. Different models for the reflectivity and opacity of lithium-bearing metals affect the temperature and velocity of the neutral vapor obtained with the one-dimensional radiation/hydrodynamics code, TITAN. For 0.05--0.15 J/cm2 on a pure lithium surface, the vapor is un-ionized and transparent to 1.06 μm light for reasonable values of reflectivity, opacity, and thermal conductivity. The calculated time-dependent density and temperature profiles from the Nd:YAG laser vaporization will be used to interpret the spectroscopic and RING data and as input to a code that calculates the resonant ionization produced by the dye laser. The present status of the TITAN simulations will be discussed