Critical voltage of a rotating plasma

The accessible voltage levels have been determined for a rotating plasma device in which the confinement region surrounds a ring-shaped coil that is suspended by a single rod. (i) There exists a sharply defined critical voltage and a corresponding velocity limit which cannot be exceeded by a moderately large power input. This is clearly shown with the present apparatus where the plasma voltage slowly approaches the critical level from below and where the insulator consists only of a narrow rod. (ii) The critical voltage increases with the mean radial ratio r 0/r w in the plasma. This ratio is defined by a magnetic field line which intersects the midplane of the device at a radial distance r 0 from the axis of symmetry, and cuts the end insulator at the corresponding distance r w. (iii) The results can be explained by an interaction between the plasma and a neutral gas layer at the insulator surface that limits the velocity to the critical value v c earlier introduced by Alfvén. As a consequence, the velocity limit becomes (r 0/r w)v c in the equatorial plane. Deductions of the corresponding voltage limit agree with the experiments, (iv) The plasma is heavily disturbed even by a very small obstacle which is introduced in the equatorial plane. This indicates that the losses of the system are small under normal operation, (v) So far the stability, is not influenced by the fact that most of the field lines pass freely around the confinement region and do not end upon an insulator surface.