Trapped-Particle-Mediated Collisional Wave Damping and Transport Effects Can Dominate over Electron Landau Damping
Author: Andrey Kabantsev
Submitted: 2005-12-21 13:20:28
University of California, San Diego
9500 Gilman Drive
La Jolla, CA 92093-0
Trapped-Particle-Mediated (TPM) wave damping effects may be important in critical burning plasma modes such as Toroidal Alfven Eigenmodes (TAEs); but few experiments have incisively probed this kinetic process. Here, we present pure electron plasma experiments on plasma wave damping effects that occur due to collisions between passing and trapped electrons, and which often dominate over the electron Landau damping. Our main concern is the damping (and transport) effects induced by a small population of electrons trapped by weak magnetic ripples or by potential variations.
We find experimentally that even small (~1%) fractions of such locally-trapped particles give a dominant contribution to collisional transport and mode damping. In essence, plasma rotation or wave induced currents cause phase-space discontinuities at the trapping separatrix, so small velocity scatterings cause large changes in particle orbits and energy. This near-discontinuous phase-space distribution enhancement was first considered by Rosenbluth, Ross and Kostomarov (1972) with regard to damping of the dissipative trapped-ion instability; but the broader implications of this approach to various dissipative processes have not yet been reconciled experimentally and theoretically. Theory shows that separatrix-enhanced perturbative effects scale as the square root of the electron-electron collision frequency, so they dominate in low collisionality regimes.
To date, our experiments have characterized the TPM collisional damping of electron plasma modes, and damping of novel "trapped particle diocotron modes"; in both, TPM damping dominates over electron Landau damping. When rotational asymmetries are present in the main confinement fields, TPM effects produce strong particle transport and strong damping of conventional diocotron drift modes. The observed rates for all these processes are related by simple scalings for magnetic field, electron density, and asymmetry strength, since they are all caused by the same separatrix crossings.
Analagous trapped electrons collisional damping have been postulated as main contributor to kinetic damping of TAEs in burning plasmas. Our experiments emphasize the effectiveness of even small trapped-electron populations from weak magnetic ripples. This suggests that similar TPM effects may play a crucial role for TAEs stability in fusion plasmas.
*Supported by NSF PHY-03544979.