Theory of asymmetry-induced transport in a non-neutral plasma

Abstract

Radial transport produced by static nonaxisymmetric fields is thought to limit the confinement of non-neutral plasmas and experiments with applied asymmetries have verified that such fields do produce transport. A theoretical model of such transport is presented which is appropriate for long, thin plasmas. The theory allows for asymmetries with nonzero frequency and includes the linear collective response to applied wall voltages. For the regime where the effective collision frequency is large, the asymmetry-induced radial particle flux is derived from the drift kinetic/Poisson equations including collisions. For low collision frequencies a heuristic derivation is given. In both regimes the resulting transport is dominated by particles that move in resonance with the asymmetry. Possible applications of the theory to several experiments are discussed.

Confinement Asymmetry and Trapped Particle Effects on Radial Transport in Charged Plasmas

“…Indeed, experiments over a wide range of parameters have confirmed that the tilt-asymmetry-induced transport is directly proportional to the trapped particle mode damping rate γ T , when there is a significant fraction of electrically trapped particles to measure the dynamics of this mode. This transport differs markedly from the traditional perspective of single particle resonant transport theory [11]. We emphasize that the effect is dominant in low-collisionality plasmas even though very few particles are trapped, because this separatrix dissipation scales with collisionality like (ν ee /ω E ) 1/2 , rather than (ν ee /ω E ) 1 .…”

Amplitude scaling of asymmetry-induced transport

“…By judiciously selecting the voltage applied to each sector we can produce an asymmetry consisting of essentially a single Fourier mode, thus eliminating the sum over modes in the transport theory [9] and making for a simpler comparison between theory and experiment. We use AC voltages at a variable frequency f since this gives us an additional experimental parameter that can be varied independently of other quantities.…”

“…Current theory [9] predicts two transport regimes depending on the amplitude of the asymmetry in the plasma ¢. For smaller amplitudes (the plateau regime), the radial flux is proportional to the square of the asymmetry potential ¢2.…”

Trapped particles and asymmetry-induced transport

“…The broad scaling characteristics of this ''anomalous'' background transport have been measured experimentally over the past 20 years, [3][4][5][6][7][8][9] but theoretical understanding of the transport kinetics has developed only in a few limited parameter regimes. 10 Here we present measurements which unambiguously demonstrate that the dominant asymmetry-induced transport mechanism on the ''CamV'' apparatus involves diffusion across the velocity-space separatrix between axially trapped and passing particles; and that these dissipative separatrix crossings also cause damping of the observed trappedparticle modes. Similar trapped-particle EϫB drift modes and instabilities have been extensively analyzed theoretically [11][12][13] for neutral plasmas; and instability has been identified experimentally in some regimes.…”