A heterodyne interferometer for electron density measurements in low-density plasmas is described. It operates at 309 GHz. The available power amounts to 5 mW. A reference microwave field, shifted by 10 kHz, is generated by the Doppler shift connected with the back reflection from the grooves of a grating on the circumference of a rotating wheel. Collimation and focusing of the microwave radiation are performed with elliptic aluminum mirrors. Schottky barrier diodes operated in mixer configuration are used as detectors. The interferometer is applied to the line-of-sight integrated electron density measurement of low-density plasmas. Line densities down to several 10 14 m −2 can be measured; the spatial resolution is slightly better than 5 mm.
Experiments are performed in which a toroidal d.c. current is induced in a compact plasma torus by applying a linearily polarised high frequency field which is perpendicularly orientated to a static field along the torus axis. It is shown that a toroidal current can be induced. having the same magnitude as the high frequency current in the coils. Depending on the plasma density, the toroidal current may have the direction of the diamagnetic current or the opposite one.
A plasma source is described which produces a plasma beam with a density ne=1013 cm-3 and a temperature of 5-25 eV, 50 cm outside the actual discharge. In this plasma unipolar arcs are ignited without applying an external voltage. It is shown that by plasma cleaning of the surface this type of arcing can be completely suppressed for the above stated parameters. A microcontamination deliberately attached to a wall probe of stainless steel enables one to predetermine the ignition point of a unipolar arc, and gives the possibility of measuring the arc current. The erosion phenomena of single arc events are compared with those from bipolar arcs and wall probes in tokamaks.
In experiments, concerned with the RF heating of plasma, the very large high frequency fields (1 kV cm-1, 1 MHz in these experiments) cause characteristic distortions of the signals of electrostatic double probes and particle spectrometers: The probe current appears as a series of narrow spikes, while the electric fields inside the plasma change sinusoidally with time. A model is presented explaining the experimental observations and allowing the evaluation of the signals with respect to Te and ne.
In connection with the problem of radio frequency heating of plasma, the interaction of a high power wave (0.1 GWatts) with a cylindrical plasma is investigated experimentally for frequencies near the ion cyclotron frequency and in the region between the ion cyclotron frequency and the lower hybrid frequency. It is found that a geometric resonance can be excited in which more than 90% of the wave energy is absorbed by the plasma. At frequencies around the ion cyclotron frequency, the macroscopic properties of the plasma, including distortion of the wave by large amplitude effects, can be fairly well explained by a MHD-model assuming an anomalous resistivity. The enlarged collision frequency is attributed to the presence of turbulence which is excited by drift currents connected with the wave.
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