Experiments are performed at the Enormous Toroidal Plasma Device at UCLA to study the neutral boundary layer (NBL) between a magnetized plasma and a neutral gas along the direction of a confining magnetic field. This is the first experiment to measure plasma termination within a neutral gas without the presence of a wall or obstacle. A magnetized, current-free helium plasma created by a lanthanum hexaboride (${\mathrm{LaB}}_6$) cathode terminates entirely within a neutral helium gas. The plasma is weakly ionized ($n_e/n_n\sim 1\%$) and collisional ${\lambda }_n\ll L_{\mathrm{plasma}}$. The NBL occurs where the plasma pressure equilibrates with the neutral gas pressure, consistent with a pressure balance model. It is characterized by a field-aligned ambipolar electric field, developing self-consistently to maintain a current-free termination of the plasma on the neutral gas. Probes are inserted into the plasma to measure the plasma density, flow, temperature, current, and potential. These measurements confirm the presence of the ambipolar field and the pressure equilibration model of the NBL.

• Received 1 February 2013

DOI:https://doi.org/10.1103/PhysRevLett.110.265001