H. W. Kugel, J. Gorman, D. Johnson, G. Labik, G. Lemunyan, D. Mansfield, and M. Vocaturo
Princeton Plasma Physics Laboratory, Princeton University P. O. Box 451, Princeton, NJ, 08543
Lithium pellet injection has been found to significantly improve TFTR plasma performance. This has involved injecting up to four 3 mg lithium pellets per discharge at velocities of about 500 m/sec to near-core regions. The small pellet size is required to prevent disruptions. The TFTR Lithium Pellet Injector can inject up to four pellets per discharge and has a capacity of 270 pellets. Lithium pellet deposition for plasma surface conditioning requires many discharges and is inefficient. The optimum lithium characteristics have not been found, and little is known about the detailed plasma surface physics and chemistry of lithium deposited on graphite limiter surfaces. The ability to increase the quantity of lithium deposition while minimizing perturbations to the plasma would provide interesting experimental and operational options. This motivated developmental work on two additional lithium deposition tools in 1996: a Solid Target Lithium probe (STL), and a Lithium Effusion Oven.
STL probes were developed for lithium evaporation into the edge plasma from special graphite-felt containing lithium. In principle, a fully loaded probe should be capable of many lithiumizations, and the total evaporation per discharge should be determined by the probe temperature (position), probe surface area, and plasma discharge length. The development and fabrication of STL probes was performed by vaporization of lithium into prototype graphite-felt probe-tips in closed ovens at 850-900 C. These probes were tested off-line but were not installed on TFTR due to the experimental schedule.
In addition to STL probes, a unique miniature Lithium Effusion Oven was developed, fabricated, and operated safely in the TFTR tritium environment from the Bay-D probe. The Bay-D Probe is a generic sample probe with very constrained geometry and existing instrumentation cabling that required oven operation with high voltage, low current rather than the more common high current methods. Lithium was evaporated a distance of about 1 m from the oven to the inner limiter in the absence of plasma. Subsequent operations produced the third highest power TFTR discharge to date.
*Work supported by US DOE Contract No. DE-AC02-76-CHO3073.