Plasma convection and line-tying

The directed motion of a low-β plasma across a toroidal magnetic field is examined as the degree of electrical contact between the plasma and a conducting end wall (line-tying) is varied in a controlled manner. Experimentally, little difference is found between the motion of a completely isolated plasma and that of a plasma in contact with a wall that does not emit electrons. As the degree of line-tying is increased by making the end wall electron-emitting, the cross-field convection of the plasma is increasingly suppressed. Beyond a critical emission current density, no further suppression is observed. When the system is in the surface-line-tying configuration, the suppression of the directed cross-field motion is accompanied by a large increase in the radial transport of plasma.