Plasma Motion and Confinement in a Toroidal Octupole Magnetic Field

Abstract

Plasma of approximately 100 eV in ion energy was injected into and trapped within an octupole magnetic field with closed lines of force contained within a toroidal region of magnetohydrodynamic stability. A zero-field locus was near the middle of the region, and fields of 1000 to 8000 G existed along the wall and around the current-carrying hoops forming the multipole field with 4.7 to 9.6 gyroradii across the stable plasma region. Observed electric fields caused by the self polarization charge at the edge of a moving plasma cloud enabled the injected plasma stream to penetrate the octupole field and to flow rapidly throughout the low-field region. The high-polarization potentials disappeared after 100 μsec as the trapped plasma evolved into a quiescent state having typically a lifetime of 0.4 msec for 100-eV ions. The electron temperature was approximately 10 eV at all times. Density fluctations and field fluctuations during confinement were negligibly small compared to those observed in plasmas afflicted with anomalous diffusion. Electric potential fluctuations were less than 2% of the electron temperature. This quiescent condition continued while the plasma density was smoothly decreasing by loss on probes and hoop supports.