Electron-cyclotron heating and associated parallel cooling

Abstract

We have observed experimentally that during electron-cyclotron heating the electron longitudinal temperature drops as the perpendicular temperature increases. The experiment was carried out in a linear mirror machine with a low-density (10' cm ') weakly ionized (~1.0%) plasma.It has been shown numerically by Busnardo-Neto, Dawson, Kamimura, and Lin' and later analytically using quasilinear theory by Arunasalam that during ioncyclotron heating the increase in the perpendicular ion temperature is associated with a decrease in the parallel ion temperature. Since there is no reason to believe that this does not also work for electrons, we may think that the same effect would occur during electron-cyclotron heating and, indeed, there are recent reports in the literature to this effect. ' It should also be mentioned that if in fact this effect occurs, then the electron-cyclotron current drive picture in tokamaks should be changed, since the cooling would change both the collision frequency and the density of the trapped particles in the banana orbits of tokamak. To the best of our knowledge these effects have not been investigated yet and this is the subject of future work.The main purpose of this article is to show that parallel cooling occurs for electrons during electron-cyclotron heating with the anisotropy in the temperature reaching a maximum of r, /T, = 1.55. The experiment was carried 'll out on LISA, a linear mirror machine now operating at Universidade Federal Fluminense (UFF) ( Table I and Fig. 1). This machine is 2.55 m long with an internal diameter of 0.17 m and it has a 1-m-long central section where the magnetic field is uniform (within 2%%uo). More details on the machine can be found elsewhere. For the experiments described here the dc current in the field coils was finely controlled so that the magnetic field Parameter Value Total length L Inner radius a Uniform magnetic field B Mirror region B Extension of the uniform magnetic field Electron density n, Electron temperature T, Ion temperature T; 255 cm 8.5 cm 10.5 kG 13.0 kG 100 cm 10' cm 80 eV 10 eV TABLE I. Summary of the basic LISA and target plasma parameters.varied from 150 to 1150 G in steps of 10 G, except for two points where a change of range in the control switch occurred. The diamagnetic effects are not very important because they are only of the order of 1.5 G. A Hall probe was used for careful measurements of the field which were later confirmed by means of a small magnetic probe used with a small ac current superimposed on the main dc current of the field coils. The helium plasma was produced by a microwave generator, which produces a power of 800 W at a frequency of 2.45 GHz, at 10 torr filling pressure with a background pressure of 10 torr, and the wave is injected through a rectangular waveguide at the side where there is a dip in the dc magnetic field (Fig. 1). The electron density n, and the parallel temperature T, were measured with a small planar Langmuir probe. The average electron temperature ( T, ) was measured spectroscopica11...