THE CHARGE STRIPPING INTERACTIONS OF ENERGETIC LITHIUM IONS AND THE IMPLICATIONS FOR MAGNETIC FUSION APPLICATIONS

This paper describes the first experimental measurements of sigma23p, the cross section for removal of the final electron from lithium by ch arge exchange and impact ionization with protons. The implications tha t this cross section has with regard to lithium ion beam heating and/o r current drive in tokamaks are discussed. Since the expected cross se ction is quite small, non-standard techniques were employed to make th is measurement. Singly ionized lithium ions, formed in a magnetically insulated racetrack diode, were passed through an energy selector and then directed along the axis of a theta pinch produced hydrogen plasma . The selector restricted the energy range of the lithium beam to the neighbourhood of 400 keV. The theta pinch coil was 71 cm long and had an inner diameter of 10.5 cm. It was energized by a 25 kJ main capacit or bank operating at 150 kHz. After exiting the theta pinch plasma the ion charge states were analysed with a Thomson spectrometer. The cros s section was determined by comparing the measured charge state distri bution of exiting lithium ions to the results of a Monte Carlo code wh ich used a theoretical expression for sigma23p. It was determined that the theoretical value for the cross section needed to be scaled by a factor of 1.51+/-0.75 to best fit the data. These experimental results were therefore in general agreement with theory. At a lithium impact energy of 406+/-23 keV the experimentally measured cross section, sigm a23p, had a value of 2.64+/-1.31 x 10(-18) cm2. Scaling to beam energi es that would be required for injection into the centre of a tokamak r eactor results in values for the stripping cross section that predict lithium beams would be appropriate for heating and current drive.