Convective Losses in a Thermionic Plasma with Shear

摘要

Experiments on anomalous transport in a Q machine with a hard core providing magnetic shear show shear-dependent particle losses in the absence of oscillations. It is demonstrated that these Bohm-like losses are caused by steady plasma convection in asymmetric electric fields produced by small temperature gradients in the end plates and that the effect of shear is to symmetrize the convective patterns by twisting the equipotentials into tight spirals. A finite-Larmor-radius theory of transport by convection is presented which correctly predicts the observed increase of confinement with shear. Estimates of the absolute magnitude of losses by this mechanism indicate that at least half of the anomalous transport in Q machines is due to plasma convection, at least in magnetic fields below 4 kG. 41 refs.