Control of Plasma/Wall Interactions in Tandem Mirrors

Data from the Tandem Mirror Experiment (TMX) and other recent research show how to control plasma/wall interactions in tandem mirrors (TMs). Based on current knowledge, plasma/wall interactions will not limit the performance of TM reactors—either at the end walls or the radial walls. Magnetic field expansion and gas pumping can be used to regulate the plasma conditions at the end wall. Specifically, in TMX the plasma density at the end wall was found to be ≈2 × 10⁹ em ⁻³, whereas the end-plug density was ≈2 × 10¹³ cm⁻³; also, the sheath potential at the wall (8 V) was <10% of the end-plug electron temperature. The "natural divertor" effect-by which positively charged plasmas in magnetic mirror machines exhaust particles and energy to the end wall—can be used to both control the plasma conditions at the radial walls and divert impurities to the end wall. These techniques, the data that support them, and needed areas of further research are discussed.