Temperature and Length Dependence of Finite Length Diocotron Modes

Diocotron modes are surface waves that propagate azimuthally on a nonneutral plasma column, via ExB drifts. Their azimuthal dependence is exp(i l θ). For an infinite length column, the mode frequency ω is independent of temperature in the large B drift limit where ω ~ 1/B. For finite length plasma columns, and for mode number l = 1, there is an approximate theory in the drift limit[1] for the (typically weak) effect of temperature on ω. This temperature dependence is a useful thermometer in experiments [2]. This poster discusses an extension of the Fine-Driscoll theory to mode numbers l > 1, and compares the theory for both l = 1 and l = 2 to numerical solutions of the finite-length bounce-averaged Vlasov equation. Surprisingly, the radial dependence of finite-length mode eigenfunctions is nearly identical to infinite-length theory, even near the plasma ends. The finite-length theory shows that the temperature–dependent frequency shift for l = 2 has the opposite sign to that for l = 1, in agreement with experiments.

[1] K.S. Fine and C. F. Driscoll, Phys. Plasmas 8, 407 (2001).

[2] K. Thompson and A. Kabantsev, adjacent poster