Impurities and impurity transport in the spheromak SPHEX

Abstract

Absolute spectroscopic measurements are made of all significant ionization states of the dominant impurities in SPHEX during sustained operation, and these are compared with a diffusive transport model to derive average plasma parameters. Statistical tests are used to establish the reliability of these results. A line ratio measure is also made and compares well with the ionization balance. The electron power balance in these discharges is estimated assuming that only Spitzer dissipation (a small fraction of the total source power) heats the electrons. Titanium gettering is used to reduce recycling, and thus to measure the fraction of impurity due to recycling relative to that directly injected from the Marshall gun. It is established that during sustainment the electrons are radiation dominated everywhere, so that the rise in when Ti is used is quite moderate, from 30 to 50 eV. However, the impurity transport rate is substantially reduced by gettering, implying that high transport rates are not intrinsic to the spheromak configuration. Some more qualitative data pertaining to the decaying spheromak is also presented.