Observation of hot electrons produced by second harmonic electron cyclotron heating in the axisymmetric tandem mirror GAMMA 10

Abstract

Microwave power, P ECH < 140 kW, has been injected at 28 GHz into the axisymmetric plug/barrier cell in the axisymmetrized tandem mirror GAMMA 10. As observed by soft X-ray measurements, the microwaves generate a hot (50-60 keV) electron population, radially peaked on the magnetic axis, which results in the formation of a thermal barrier. The production mechanism of these hot electrons is found to be second harmonic electron cyclotron heating (ECH), corrected for the effects of the relativistic mass variation and the Doppler shift. This mechanism also explains the first experimental observation of a saturation of the single-component hot electron temperature T eh as being caused by the finite width of the incident microwave lobe. The dependence of the plasma parameters on the filling gas pressure, the plasma density and the ECH power is studied. It is found that the heating process can be interpreted as a competition between electron acceleration by the incident wave, electron deceleration by collisions, and the mirror trapping efficiency of the source electrons for hot electrons. The axial profile of the soft X-rays is investigated in relation to the mechanism of the second harmonic ECH. The heating process is discussed in terms of the electron pitch angle and the magnetic field intensity.

Generalized Scaling Laws of the Formation and Effects of Plasma-Confining Potentials for Tandem-Mirror Operations in GAMMA 10

“…Even at this time, however, the physics-mechanism and scaling studies on the basis of the comparison between experimental data and potential theories [6][7][8][9] are reported in a limited number of publications for specific plasma operations alone [10][11][12].…”

“…In this Letter, from a large number of databases obtained from 1979 to 2000 in GAMMA 10, two representative operations characterized in terms of (i) a high-potential mode having kV-order plasma-confining potentials [10][11][12], and (ii) a hot-ion mode yielding fusion neutrons with 10-20 keV bulk-ion temperatures [13], are investigated. These data summaries are physically interpreted, combined, and then generalized under the proposed consolidation and combination of the two major theories of Cohen's strong electron-cyclotron heating (ECH) theory [6] and generalized Pastukhov's potential confinement theory [8,9] by the use of electron-energy and particle balance equations.…”

“…Thereby, we obtain f c and f b , as F P -F C and F C -F B , respectively. X-ray diagnostics give profiles of electron temperatures T e along with detailed electron-energy spectra in each region [10][11][12]17].…”

“…1(a) and 1(b), scaling relations between f c and f b are summarized in the high-potential [10][11][12] and hot-ion modes [13], respectively. Both figures show a similarly behaved favorable increase in f c with increasing f b .…”

“…GAMMA 10 is a minimum-B anchored tandem mirror with outboard axisymmetric plug and barrier cells [5,[10][11][12][13][14]. It has an axial length of 27 m, and the total volume of the vacuum vessel is 150 m 3 .…”