Surface cleaning techniques and efficient B-field profiles for lithium ion sources on extraction ion diodes

Summary form only given. Application of ion beams to inertial confinement fusion requires efficient production, transport and focusing of an intense, low microdivergence beam of an appropriate range ion. At Sandia, we are studying the production of lithium ion beams in extraction applied-B ion diodes on the SABRE accelerator (5 MV, 250 kA). Evidence on both SABRE (1 TW) and PBFA-II (20 TW) indicates that the lithium beam turns off and is replaced by a beam of mostly protons and carbon, possibly due to election thermal and stimulated desorption of hydrocarbon surface contamination with subsequent avalanche ionization. Turn-off of the lithium beam is accompanied by rapid impedance collapse. Surface cleaning techniques are being developed to reduce beam contamination, increase the total lithium energy and reduce the rate of diode impedance collapse. Application of surface cleaning techniques has increased the production of lithium from passive LiF sources by a factor of 2. Improved diode electric and magnetic field profiles have increased the diode efficiency and production of lithium by a factor of 5, without surface cleaning. Work is ongoing to combine these two advances. Our principal surface cleaning technique consists of 13.5 MHz RF and RF-assisted discharges for efficient physical and chemical sputtering of surface impurities from both the anode and cathode.