Ohmic plasma heating in the L-2 stellarator

The paper gives the results of the first experiments on the L-2 stellarator, put into operation at the end of l975. The L-2 stellarator consists of a toroidal magnetic trap with a double (l=2) helical field. The major diameter of the torus D=200 cm, and the cross-sectional diameter of the vacuum chamber d=35 cm. The maximum magnetic field B0=20 kG (in the experiments under consideration, B0<=14 kG). The rotational transform angle on the axis rotational transformsub(delta)(0)=0.2, on the boundary surface rotational transformsub(delta)(a)=0.7, and the shear theta approximately equal to 0.5. The mean radius of the boundary magnetic surface a bar=11.5 cm. The limiting gas-kinetic plasma pressure for B0=20 kG, determined from stability and equilibrium conditions, is 1017 eV cm-3. Ohmic heating of the plasma was used in the first stage of the studies. At a current I approximately equal to 20 kA, a plasma with density n barsub(e) approximately equal to 1013 cm-3 was obtained, with a mean electron temperature T barsub(e) approximately equal to 300 eV and an effective charge Zsub(eff) approximately 3. The energy life-time of the plasma tausub(E) approximately equal to 6-8 ms approximately equal to 3.6x10-8a2B perpendicular, where B perpendicular = (Bsub(S)+Bsub(I)) perpendicular; (Bsub(S)) perpendicular is the average transversal stellarator field, (Bsub(I)) perpendicular is the current poloidal field. The relationship with the Bohm time tausub(Bohm) can be written tausub(E) approximately equal to (40-50) tausub(Bohm). At a current reaching a certain critical value Isub(cr)=2.5 B0 (a/g), corresponding to the total angle of rotational transform on the axis of rotational transformsub(Σ)(0) approximately equal to 1, the appearance of helical perturbations with n/m=1/1, limiting the magnitude of the plasma current, was observed. (author)