Advances in ion beam intensity at Sandia National Laboratories

Summary form only given, as follows. In 1993 lithium beam intensities /spl ges/1 TW/cm/sup 2/ were achieved and lithium-driven target experiments at the /spl sim/1400 TW/g level were performed on the Particle Beam Fusion Accelerator II (PBFA II) at Sandia National Laboratories. Hohlraum radiation temperatures of up to 60 eV were achieved using this lithium beam. Our 1995 Light-Ion ICF program milestone of achieving a 100 eV radiation temperature in an ion-driven hohlraum will require a lithium beam intensity of 5/spl plusmn/1 TW/cm/sup 2/ on a 4 mm diameter cylindrical target: this will require both an increase in coupled lithium power and a decrease in total lithium beam divergence. The lithium beam power has been limited to /spl sim/5-6 TW by a so called "parasitic load". This parasitic current loss in our ion diodes has recently been identified as being carried by ions that are accelerated from plasmas that are formed when high voltage electrons are lost to anodes with many monolayers of hydrocarbon surface contamination. Control of anode and cathode plasmas on the SABRE accelerator using RF-discharge cleaning, anode heating, and cryogenic cooling of the cathode have increased the efficiency of the production of lithium current by a factor of 2-3. The principal cleaning technique uses an RF-assisted (13.5 MHz, DC bias/spl sim/100 V) Ar/O/sub 2/ glow discharge to chemically combust and sputter the hydrocarbons, followed by an Ar-only discharge to remove the remaining oxides via sputtering. Anode recontamination can be limited by heating the anode to /spl sim/450/spl deg/C to reduce the sticking coefficient and stimulate thermal desorption. A new ion diode incorporating glow discharge cleaning and titanium gettering pumps has been installed in PBFAII and will be tested in December 1994. Anode heaters should be available in January 1995. Circuit model calculations indicate that we can more than double our coupled lithium ion power on PBFA II by eliminating the parasitic current.