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| Book/Report | FZJ-2018-03088 |
1989
Kernforschungsanlage Jülich, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/18611
Report No.: Juel-2258
Abstract: The magnetic confinement of plasma in a tokamak is of growing interest as a concept for controlled thermonuclear fusion. In a tokamak, a toroidally symmetric plasma is confined by an external toroidal field B$_{t}$ and the poloidal field B$_{p}$ of a toroidal plasma current I$_{t}$. In addition, the current provides ohmic heating of the plasma. Since tokamak discharges are limited in time, the ratio between the current inward diffusion time and the total discharge time, and hence generally the kind and duration of current diffusion processes, is important. The present work investigates the influence of external discharge parameters like the pulse shape of the total current, the particle density, the toroidal magnetic field, or the plasma contamination on current diffusion processes in the TEXTOR tokamak (plasmaradius a $\simeq$ 0.5 m, major radius R$_{o}$ $\simeq$ 1.7 m). Special emphasis is given to the question of in which cases (if any) the current diffusion can be described by a simple Ohm's law (i.e. current density $\varpropto$ electric field) with a proportionality factor equal to the resistivity of a fully ionized plasma corrected for toroidal effects ("neoclassical resistivity"). For these investigations, the temporal and spatial distributions of the poloidal magnetic field have to be known. This information is obtained from a far-infrared ($\lambda$ = 337 $\mu$m) polarimeter/interferometer measuring simultaneously Faraday rotation angles and lineaveragedelectron densities along nine vertical chords. The electron temperature which is required to prove the validity of Ohm's law in this simple form is derived from themeasurement of the electron cyclotron radiation. In all TEXTOR discharges under investigation, the simple Ohm's law with neoclassical resistivity is approximately valid, at least during most of the time that current is diffusing. In contrast to other experiments where strong deviations from this relation have been observed and attributed to the occurrence of resistive plasma instabilities, the law also holds on TEXTOR during such instabilities. In order to describe the influence of the external discharge parameters on the current diffusion, an "effective" diffusion time $\tau_{eff}$ is introduced. In the start-up phase, the dependences are quite well reflected by the scaling law $\tau_{eff}$ $\varpropto$ a$^{5/2}$ R$_{0}^{-1}$ B$_{t}$ I$^{1/4}_{t,stat}$ which does not contain the current ramp rate, the particle density, and the impurity concentration because of their weak influence on $\tau_{eff}$$^{1}$. For a physical interpretation of this scaling law, a simple theoretical model is formed on the basis of the current diffusion equation and the electron energy conservation equation. The model makes use of the above-mentioned simple relation between current density and electric field and equates the ohmic heating term with the electron heat conduction term. An empirical formula obtained for stationary conditions is adopted for the latter term. Furthermore, an upper limit for the central current density is assumed for cases with periodic instabilities in the plasma center ("sawtooth activity"). The dependences which are derived from the model agree with the experimental scaling law.
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