Fully Self-Consistent Calculation of Neoclassical Transport in Nonneutral Plasma Columns

Slow radial expansion of a nonneutral plasma column of length L, caused by θ-asymmetric applied static E or B fields, has for decades posed challenges to theory. One issue is that the self-consistent plasma potential in the presence of the field error must be calculated accurately, including finite-length effects. For large B, a simplified bounce-averaged plasma response is sufficient to obtain transport predictions² that match experiments. However at lower magnetic fields (< roughly 1 T for 1 eV electrons at 10⁷ cm^-3) the plasma response is dominated by bounce-rotation resonances, and a fully self-consistent calculation of the plasma potential must be carried out. This involves a numerical iteration technique whereby the plasma distribution function is calculated using the Fokker-Plank equation, in the presence of a given field error potential; then this distribution is used to calculate an improved potential and the process is repeated until convergence in the potential is achieved. For known applied field errors consisting of either a tilt of the magnetic field or an applied electrostatic asymmetry, this numerical method accurately matches experimental transport rates. Results will be presented for both L and B transport scaling from such errors.

²Dubin, Kabantsev and Driscoll, Phys. Plasmas 19, 056102 (2012).