Radial space potential measurements in the central cell of the tandem mirror experiment with a heavy-ion-beam probe

Spatial and temporal profiles of the space potential in the central-cell midplane of TMX have been obtained with a heavy-ion-beam probe. The absolute accuracy of measurements is +- 25 volts (with respect to the machine vacuum walls) with a resolution of approx. 2 volts. During moderate fueling with the gas boxes (i/sub gas/ approx. = 1200 Atom-Amperes D2), the plasma potential is parabolic to at least 25 cm radius, with phi/sub e/ approx. = phi/sub max/[1-(r/32)2] and 300 < phi/sub max/ <450 volts. With puffer-valve fueling, the space potential is relatively flat to at least 27 cm radius, with 250 < phi/sub e/ < 350 volts