Effects of Thermal-Barrier Potentials on the Central-Cell Electron Energy Confinement

The effects of the thermal-barrier potentials ɸ_b on the central-cell electron energy confinement are theoretically and experimentally investigated in the GAMMA 10 tandem mirror. In particular, the scaling of the central-cell electron temperatures T_e with “the central-cell electron-confining potentials” ɸ_b is studied on the basis of the electron energy-balance equation and the generalized Pastukhov theory. The obtained theoretical scaling of T_e with ɸ_b is then compared with the experimentally observed relation between these two parameters. In GAMMA 10, the main tandem-mirror operations are characterized in terms of(i) a high-potential mode having kV-order plasma-confining potentials, and (ii) a hot-ion mode yielding fusion neutrons with 10-20 keV bulk-ion temperatures. In this report, the scaling of T_e with ɸ_b covering over these two representative operational modes is investigated, since the scalings of T_e or the dominant parameters which determine T_e have been remained for a long time as an unresolved important issue for tandem-mirror plasmas. It is found that the data in the two representative operational modes of the high-potential and hot-ion modes in the GAMMA 10 tandem mirror are in good agreement with the theoretically derived scaling formula, though the heating-source parameter dependence in the electron energy-balance equation is quite different in the two modes.