Lifetime scaling of field-reversed configurations

Field-reversed configurations are elongated compact toroidal plasmas confined solely by poloidal fields. Their particle lifetimes have been described by analytical models assuming a lower hybrid drift (LHD)-governed anomalous resistivity. While these models have correctly predicted the observed particle lifetimes, they do not correctly reproduce the measured flux lifetimes. Moreover, the predicted particle lifetimes, for present small experiments, depend more on open than closed field line confinement and therefore comparisons with theory are not very accurate in illuminating the true nature of the closed field line confinement. Measurements on the TRX field reversed theta pinch display flux decay rates which imply a more spatially uniform resistivity than predicted by the LHD model. The detailed measurements over a wide range of conditions also suggest a much weaker dependence on open field line confinement. A parameter s=Δrsub(FL)/rhosub(i0), where Δrsub(FL) is an equivalent poloidal flux layer thickness and rhosub(i0) is the ion gyroradius in the external field, has been shown to be the relevant parameter for examining the contribution of open field line effects. In the presence of drift parameter-related microinstabilities, s must be greater than 3 for closed field line confinement to dominate. For present experiments, s rarely exceeds 2. For large s, where the closed field line confinement will truly dominate, the particle confinement is predicted to approach an s4 dependence for an LHD-governed resistivity. (author)