Experimental study of MHD-activity and the effect of a limiter in the L-2 stellarator with Ohmic heating. Theory of stellarator equilibrium and stability

Plasma stability is studied in the L-2 stellarator in the Ohmic-heating regime both with X-ray diagnostic and magnetic probes. The radial electron temperature distributions are measured and the current density distributions are calculated. At the centre of the plasma column, m=0 sawtooth oscillations are observed. In the peripheral plasma region, large-n and -m oscillations are found. - A limiter is used to decrease the wall-plasma interaction. The limiter which is introduced 3 cm into the plasma decreases the wall-plasma interaction and reduces the fraction of radiative loss to half of its value, which does not, however, bring about a substantial decrease in the Zsub(eff) value. - The possibility of additional plasma refuelling with solid hydrogen pellets is demonstrated. The efficiency of pellet plasma trapping is studied. - A system of MHD equations for magnetic systems with a periodic magnetic field is obtained by an averaging procedure. On the basis of these equations, the plasma stability is studied as a function of plasma pressure and Ohmic-heating current in stellarator-type devices. It was shown that in some cases plasma stability improves with increasing pressure. (author)