Numerical simulation of current-free double layers created in a helicon plasma device

Two-dimensional simulations reveal that when radially confined source plasma with magnetized electrons and unmagnetized ions expands into diverging magnetic field B, a current-free double layer (CFDL) embedded in a conical density structure forms, as experimentally measured in the Australian helicon plasma device (HPD). The magnetized electrons follow the diverging B while the unmagnetized ions tend to flow directly downstream of the source, resulting in a radial electric field (E-perpendicular to) structure, which couples the ion and electron flows. Ions are transversely (radially) accelerated by E-perpendicular to on the high potential side of the double layer in the CFDL. The accelerated ions are trapped near the conical surface, where E-perpendicular to reverses direction. The potential structure of the CFDL is U-shaped and the plasma density is enhanced on the conical surface. The plasma density is severely depleted downstream of the parallel potential drop (phi(parallel to o)) in the CFDL; the density depletion and the potential drop are related by quasi-neutrality condition, including the divergence in the magnetic field and in the plasma flow in the conical structure. The potential and density structures, the CFDL spatial size, its electric field strengths and the electron and ion velocities and energy distributions in the CFDL are found to be in good agreements with those measured in the Australian experiment. The applicability of our results to measured axial potential profiles in magnetic nozzle experiments in HPDs is discussed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754598]