Plasma ion dynamics in large-amplitude drift waves : stochasticity, collisions, orbit loss, and recycling

Citation

Sanders, Steven Jay (1998) Plasma ion dynamics in large-amplitude drift waves : stochasticity, collisions, orbit loss, and recycling. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/11sj-hm29. https://resolver.caltech.edu/CaltechETD:etd-01252008-115408

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.

The dynamics of plasma ions in the presence of large-amplitude electrostatic waves are investigated experimentally. The work was conducted in Caltech's Encore research tokamak. The toroidal plasma current excites coherent, poloidally propagating drift waves which stochastically heat ions in the poloidal plane. Ion distribution functions f (x, v ,t) are probed via Laser-Induced Fluorescence along three orthogonal velocity directions. Wave phase resolution is provided by the narrow laser pulse width and by a novel data-acquisition system which ensures synchronization between laser trigger and drift wave.

Time-resolved measurements show a multi-step heating process during each wave period: (i) The wave electric field excites stochastic ion orbits in the poloidal [...] plane, resulting in [...] heating. (ii) ion-ion collisions impart energy to the toroidal [...] direction, raising [...] to relax the [...] temperature anisotropy. (iii) Hot ions with large gyroradius escape confinement, reaching the chamber wall and cooling the distribution. (iv) Cold ions from the plasma edge are convected back into the plasma (recycled), significantly replenishing the density depleted by orbit losses.

The ion-ion collision period [...] is highly time-dependent due to intense ([...]50%) fluctuations in both n and T. The anisotropic temperature relaxation rate is found to be consistent with Fokker-Planck theory when the time-dependence of the collision period is properly taken into account. Thus, classical Fokker-Planck correctly describes the evolution of f (vil), despite the intrinsic single-particle stochasticity in the [...] direction.

Evidence for ion recycling is given by observations of significantly non-Maxwellian (NM) ion velocity distributions near the plasma edge. These appear periodically, synchronous with the drift wave phase at which, simultaneously, ion fluid flow from the wall toward the plasma center peaks, ion density is a local minimum, and ion temperature is high. The appearance of NM features at this phase is consistent with the intantaneously low ion collision rate which allows non-equilibrium features to be long-lived. The observed NM distributions are bimodal and indicate the presence of a group of cold ions (0.4 eV) superimposed on a hot background plasma (8 eV) of roughly equal density.

Thesis Files