Poloidal Ohmic Heating in a Multipole.

The feasibility of using poloidal currents to heat plasmas confined by a multipole field has been examined experimentally in Tokapole II. The machine is operated as a toroidal octupole, with a time-varying toroidal magnetic field driving poloidal plasma currents I(,plasma) ('(TURN)) 20 kA to give densities n(,e) ('(TURN)) 10('13) cm('-3) and temperatures T(,e) ('(TURN)) 30 eV. The measured plasma resistivity ranges from ('(TURN))(eta)(,Spitzer) to ('(TURN)) 1500 x (eta)(,Spitzer) and scales like(' )SQRT.(T(,e)/n(,e) as expected from mirror- and fluctuation-enhanced resistivity theory. The enhanced resistivity allows large powers (('(TURN))2 MW) to be coupled to the plasma at modest current levels. However, the poloidal ohmic heating reduces the confinement time from ('(TURN)) 1 msec to ('(TURN))30 (mu)sec. This is apparently due to a combination of the unfavorable location of the input power near the wall of the vacuum tank and fluctuation-enhanced transport. A one-dimensional transport code calculation shows that the location of the input power near the walls yields a confinement time four to ten times shorter than if the power were deposited on the octupole separatrix. The heating produces large ((delta)n/n ('(TURN)) 100%) low-frequency (f(' )<(' )10 kHz) fluctuations which are apparently current-driven. Local electric field measurements corellated with local density measurements show that these fluctuations further degrade the confinement time by a factor of two to five. Current-driven drift instabilities and resistive MHD instabilities appear to be the most likely causes for the fluctuations.