Abstract
The gas injection system in the Tara central cell has been redesigned to overcome problems associated with fueling in the high field region at the ends of the central cell. Previous experimental studies indicated that plasma ions created in the region between the gas injection point and an ICRH antenna were lost radially before passing to the downstream side of the antenna. In the new system gas is injected at two diametrically opposite ports into a cylinder (‘‘gas box’’) located near the midplane, between the two double half turn antennas. A maximum in the magnetic field at the midplane divides the central cell into two R=2 mirrors. Ions trapped in one of these mirrors must scatter into the loss cone before passing into the fueling region, where the loss rate due to charge exchange is highest. This configuration produces separate regions for ionization (at the gas box) and for heating (near the magnetic field minimum). Pumping of neutral gas by the central cell plasma reduces the amount of neutral gas flux into the axicell to acceptable levels. Gas utilization efficiencies are roughly twice as high as in the original configuration.
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