Control of Plasma Drift in the Thetatron

The linear theta pinch drifts laterally with an acceleration directly proportional to the magnetic field gradient and temperature; for example, a magnetic field gradient of only 1 G in 100 kG causes a 1-keV plasma to drift to the wall of a 10-cm-diam tube in 100 μsecs. Four main sources of magnetic field gradient are proposed and examined experimentally; the coil end, the collector shape, the collector-coil discontinuity and the plasma influence. The magnetic field gradient may be controlled by sectionalizing the coil at regular intervals, or by a tab between the coil and collector or, more conveniently, by the use of an ``inductive lens.''