End-wall plasma characteristics in the Tandem Mirror Experiment

An array of emitting and collecting probes was used to measure the characteristics of the plasma at the end wall of TMX. The emitting probes indicated a plasma potential with a characteristic value of approximately 8 V, measured at 2 cm and 10 cm from the grounded wall. The potential in the plug varied from 300 to 1000 V. The collecting probes indicated a cold plasma present near the wall, with an electron temperature of about 5 eV and a cold-electron density of about 2 × 10 9 cm −3 , while the hot-ion density at the wall was about 6 × 10 8 cm −3 . The density of electrons in the plug was about 2 × 10 13 cm −3 . The electron temperature measured at the wall was decoupled from the temperature of the electrons in the plug and the central cell. The plasma wall sheath was established to balance the flux of hot ions and cold electrons. An additional species of electrons, i.e. secondary electrons produced at the end wall, was also identified. These secondary electrons had a directed energy comparable to the local plasma potential, an energy spread of less than 7 eV, and a density of about 4 × 10 8 cm −3 . The experimental results are shown to be consistent with a simple model involving cold-gas recycling from the end wall.