Behaviour of a High-Temperature Plasma under Current Heating in the High-Shear Stellarator ''Uragan''

In this paper the results are presented of an investigation into the behaviour of plasma produced by ohmic discharges in the high-shear stellarator ''Uragan'' (''Hurricane''). During the past year the magnetic system of the stellarator has undergone changes: new rectilinear cylinder elements have been installed. Measurements conducted with an electron beam have shown that with the new cylinder elements operation is possible with a magnetic system of the following parameters: maximum shear and rotational transform angle of 0.12 and 1.8 respectively on magnetic surfaces having a radius of 6 cm. Ohmic discharges were produced by means of a discharge from a capacitor bank via a corresponding transformer directly on a metal discharge chamber having an insulating ceramic section. Preliminary ionization was effected with the help of a high frequency generator having a frequency of 100 kc/s and an output of up to 200 kW. The following plasma parameters were measured: electron density, by means of a phase meter, and electron temperature, on the basis of the plasma conductivity and the ratio of the intensities of the spectral lines of helium. The radial distribution of the electron density and temperature was measured with the help of electric probes. Under some conditions control measurements were made of the radial distributions of the electron temperature and concentration on the basis of the Thomson scattering of light from a ruby laser. The results are given of measurements of the energy lifetime and the charged particle lifetime as a function of the configuration of the magnetic field in the stellarator. When measuring the charged particle lifetime account was taken of the effect of ionization by the spectroscopic method. In typical operating régimes the electron density attained 1013 cm-3 and the electron temperature up to 100 eV. The results are discussed of attempts to separate the effect of shear and of the rotational transform angle using the specific behaviour of the shear and rotational transform values when the increase ϵ = h3 /Hz (h3 is the fundamental harmonic of the helical field and Hz the strength of the longitudinal magnetic field). (author)