Measurements of negative potentials in a tandem mirror experiment

Negative plasma potentials as large as -400 V were measured in the magnetic fusion experiment Tandem Mirror Experiment-Upgrade (TMX-U). Normally, a plasma confined by an open ended magnetic field produces a positive potential. To measure the negative potentials, neutral beams were injected into the plasma region at angles within the loss cone. The greatest negative potentials were observed after the electron cyclotron heating power was turned off but while energetic ions were still being introduced into the plasma with large-angle neutral beams. These negative potentials are explained in terms of thermal ions electrostatically trapped by long lived, hot, mirror confined electrons. (author). Letter-to-the-editor. 9 refs, 3 figs