Observations of Stochastic Ion Heating by Low Frequency Drift Waves

Citation

McChesney, Jon Mearns (1989) Observations of Stochastic Ion Heating by Low Frequency Drift Waves. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/vyaq-ye14. https://resolver.caltech.edu/CaltechETD:etd-02092007-143250

Abstract

Several laser induced fluorescence (LIF) experiments were performed on the Encore tokamak device. These experiments represent the first application of this technique to the majority ions of a tokamak. The main laser system selected consisted of a copper vapor laser (CVL), which pumped a narrowband, tunable dye laser. This system allowed the Doppler-broadened, ion distribution function to be scanned with high resolution, giving accurate ion temperature measurements. As a preliminary test, the diagnostic was used to observe ion heating in the presence of lower hybrid RF power. Ion temperatures were found to increase dramatically with increasing RF power.

By using a second dye laser, actual ion trajectories were determined using the technique of "optical tagging." Tagging involves the use of a so-called "pump" laser to alter the fraction of ions in a particular quantum state. As a preliminary test, this technique was used to demonstrate ion gyro-motion in Encore.

Using the ion distribution functions determined by means of LIF, it was possible to make detailed measurements of ion heating during an ohmically heated tokamak discharge. It was found that the observed rate of ion heating was nearly two orders of magnitude faster than expected from collisional energy exchange with the hot electrons. The high ion temperatures inferred from the LIF measurements were later verified by measuring the Landau damping of ion acoustic waves. The observed damping lengths were roughly in accord with those calculated using measured values of T_e and T_i.

This enhanced ion heating was correlated with the presence of large amplitude, low frequency (ω < ω_ci), drift-Alfvén waves. Using numerical calculations, it was shown that in the presence of electrostatic modes (such as drift waves) of sufficient amplitude, ion motion becomes stochastic or chaotic. In physical terms, stochasticity occurs when the ion displacement that is due to the polarization drift becomes comparable to the perpendicular wavelength, i.e., when α = m_ik²_⊥φ₀/qB²₀ ~ 1. A combination of numerical calculations and experiments was used to demonstrate that stochasticity was indeed responsible for the observed rapid heating.

Finally, we concluded by speculating that stochastic heating may also be the cause of the anomalously high ion temperatures observed in reversed field pinches (RFP's) and in field reversed configurations (FRC's). Intrinsic stochasticity is also important in the field of auxiliary plasma heating. As is now well known, a large amplitude RF electric field can heat particles despite a large mismatch between the wave frequency and the gyrofrequency.

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