Toroidal confinement experiment featuring direct ohmic heating of the ions

Recently a toroidal plasma device with the confinement being provided by surface multipole magnetic cusps generated by permanent magnets has been operated at UCLA. The cusps are oriented in the poloidal direction and have a maximum field strength of 1.3 kG. A toroidal current is driven by a transformer; the loop voltage is about 100 V. It is found that for a hydrogen plasma a current of 3 to 6 kA is driven and that the current is carried by the ions. This gives direct ion heating to temperature of 40 to 100 eV in a 1013 cm-3 density hydrogen plasma. The electron temperature is lower, being in the 20-40 eV range. The #betta# of the plasma is high, in the 20-30% range. We have simulated the operation of this device using a 2 1/2 dimensional electromagnetic particle code. The model duplicates many of the features of the experiment, including the restraint of electron current by the cusps while the ions flow relatively freely, the focussing of the streaming ions to the center of the chamber by the magnetic lens effect of the cusps, and the pinch effect. A simple theoretical treatment explains both the experiments and the simulations, although the detailed simulation results indicate the need for a more complete theory. (orig.)