Potential and current structures of auroral-like plasma terminating on neutral gas

The 3-D potential and current structures of a quiescent ($\delta $n/n $<$5{\%}), magnetized plasma terminating on a neutral gas have been measured and compared to theory. A thermally emissive LaB6 cathode biased $<$400 V ionizes a background helium gas. The 30 m plasma is not terminated by an electrode so there are no net currents in it. There are, however, complicated auroral-like closed-current structures. Hot electrons carry a field aligned currents and ions carry a cross field current to closure currents in the low density plasma halo around the plasma. Probes are used to measure the plasma's electric fields, currents, and basic plasma properties. A strong field-aligned neutral collision dominated double layer ($\Delta \phi $/kT$_{e} \quad \sim $ 1) terminates the auroral structure where the plasma pressure matches the neutral pressure. In this region, electric fields and neutral-collision dominated conductivities create comparable cross field and field aligned currents, closing the current loop. The experiment was carried out at the ETPD at UCLA, a large toroidal device (major radius = 5 m, 2 m wide, 3 m tall) with a pulsed (1 Hz) DC plasma discharge (t$_{d}\sim $20 ms, B$_{t}\sim $250 G, and B$_{v}<$6G, R$_{plasma}$=10cm, n$_{e}<$10$^{13}$cm$^{3}$, T$_{e}<$10eV, and T$_{i}\sim $T$_{e})$.