Spheromak injection into a tokamak

Recent results from the Caltech spheromak injection experiment [to appear in Phys. Rev. Lett.] are reported. First, current drive by spheromak injection into the ENCORE tokamak as a result of the process of magnetic helicity injection is observed. An initial 30% increase in plasma current is observed followed by a drop by a factor of 3 because of sudden plasma cooling. Second, spheromak injection results in an increase of tokamak central density by a factor of 6. The high-current/high-density discharge is terminated by a sharp peaking of the density profile followed by an interchange instability. In a second experiment, the spheromak is injected into the magnetized toroidal vacuum vessel (with no tokamak plasma) fitted with magnetic probe arrays. An m=1 (nonaxisymmetric) magnetic structure forms in the vessel after the spheromak undergoes a double tilt; once in the cylindrical entrance between gun and tokamak, then again in the tokamak vessel. In the absence of net toroidal flux, the structure develops a helical pitch (the sense of pitch depends on the helicity sign). Experiments with a number of refractory metal electrode coatings have shown that tungsten and chrome coatings provide some improvement in spheromak parameters. Design details of a larger, higher-current spheromak gun with a new accelerator section are also discussed.