Physics
Scientific paper
Feb 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991phdt........76l&link_type=abstract
Thesis (PH.D.)--THE UNIVERSITY OF TEXAS AT AUSTIN, 1991.Source: Dissertation Abstracts International, Volume: 52-12, Section: B,
Physics
4
Scientific paper
We studied the turbulence and fluctuation induced tranport in the edge plasma of the TEXT tokamak using a Langmuir probe array. In this dissertation we present three separate experiments, each of which examines a particular aspect of the edge turbulence and transport. A velocity shear due to a peaking plasma potential close to the outermost flux surface has been characterized on TEXT tokamak. It was found that the turbulence was suppressed, thus confinement was improved due to the velocity shear.^dagger The radial electric field and the corresponding velocity shear can be modified by an externally applied helical magnetic field and a biased limiter. In the first experiment we present an observation of the effects of the stochastic magnetic fields and biased limiter on edge velocity shear and its effect on edge turbulence. In the second experiment we observed that edge conditions are significantly altered during high MHD (m/n = 2/1) discharges as compared to low MHD discharges. The time averaged turbulence level is dramatically enhanced which results in increased transport and reduced confinement times. Local edge density, electron temperature, floating potential, their scale lengths, and high frequency fluctuations are modulated by the Mirnow oscillations. In the scrape -off layer there were indications that the density, floating potential, and temperature fluctuations may be driven by the temperature gradient. However, in the range of 25cm <= r <= 25.6cm, there was no indication of a connection of fluctuations and local density or temperature gradient. In the third experiment we use the technique first described by Robinson and Rusbridge* to investigate the electron temperature fluctuations in the scrape-off layer of the TEXT tokamak. The temperature fluctuations ~ T_ e/T_ e was found to be ~eq 0.4 ~ ne/n_{e }. Particle and energy transport were estimated from <~ ne ~ E_theta> and <~ Te ~ E_theta> correlations. A high correlation of temperature and electric field fluctuations and a low but positive correlation between density and temperature fluctuations were observed. Ch. P. Ritz, H. Lin, T. L. Rhodes, and A. J. Wootton, Phys. Rev. Lett. 65, 2543(1990). *D. C. Robinson and M. G. Rusbridge, Plasma Phys. 11, 73(1969).
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