Energy balance in a low- high-density helium plasma in the ST Tokamak

Helium discharges have been produced in the ST Tokamak with electron densities exceeding 1014 cm−3, with less than 1% oxygen (and other light impurities) and less than 0.05% metallic impurities. In such discharges, the effect of the impurities on the plasma resistivity is relatively unimportant, and, consequently, the usual ambiguities about the radial current distribution are unusually small. We find, consistently with previous experiments and in spite of various quantitative differences, that the plasma resistance is determined by Spitzer resistivity, that the rise of electron temperature is limited by the development of the q = 1 surface and that the electron kinetic energy confinement time is equal to the average particle confinement time. Energy loss by electrons constitutes about three quarters of the total and is roughly divided between line radiation and kinetic energy transport.