Electron thermal diffusivity in ISX-B from observations of sawtooth oscillations

Author

John D. Bell

Date of Award

12-1983

Degree Type

Thesis

Degree Name

Master of Science

Major

Physics

Major Professor

Edward G. Harris

Committee Members

R. W. Childers, S. B. Elston

Abstract

Significant interest exists in studying energy confinement in tokamaks. Previous analyses of the power balance in the ISX-B tokamak have suggested that electron thermal conduction is the largest energy loss channel and have deduced values for X_e. the electron thermal diffusivity, which increase as the auxiliary (neutral beam) heating power is increased. The goal of the research presented here is to obtain an independent value for X_e analysis of "sawtooth" oscillations, which generate electron "heat pulses" that propagate from the core to the outside of the tokamak plasma.

The heat pulse is observed via soft x-rays (SXR, 0.5-5 keV), whose emission depends on local electron temperature (which undergoes ~20% fluctuations during a sawtooth cycle), and on local electron density (which shows much smaller fluctuations in the cases studied). The x-rays are collected by arrays of collimated surface barrier diode detectors, and their time histories are recorded.

In previous work on this problem, X_e values were obtained by comparing SXR data (from oscilloscope photos) with T_e values from simulations based on analytic models of the heat pulse. The present research employs digital SXR data from many detectors. To get X_e two simulations are done. The first solves a diffusion equation in electron temperature to track the heat pulse propagation for a specific value of X_e; the second uses this electron temperature solution to model line-integrated SXR signals, whose characteristics are compared to the same characteristics of the experimental signals. We iterate on X_e until a match is obtained. The value of X_e from heat pulse propagation can then be compared with the value from the power balance analysis. Also, the simulations allow us to study their sensitivities to various assumptions included in the model.

The heat pulse X_e values for six ISX-B cases studied are in good agreement with the power balance analysis, confirming the importance of electron thermal conduction in ISX-B's energy transport mechanisms.

Recommended Citation

Bell, John D., "Electron thermal diffusivity in ISX-B from observations of sawtooth oscillations. " Master's Thesis, University of Tennessee, 1983.
https://trace.tennessee.edu/utk_gradthes/14755