Spheromak global instabilities and stabilization by nearby conductors

Publisher Website
Google Scholar
Abstract
By means of a three-dimensional (3-D) magnetohydrodynamic (MHD) simulation, the stabilization effects of conducting walls on the tilting instability of spheromaks created by the Princeton induction method are extensively studied. The nonlinear development and deformation of the spheromak resulting from global instabilities with toroidal modes n=1 (tilting), 2, and 3 are also studied in detail. Simulation results have shown that while the induction effect of the wall current is insufficient, the line-tying effect is much more powerful for the suppression of the strong tilting instability. For a perfect line-tying stabilization, a certain amount of surface magnetic flux around the spheromak should be tied to the conducting wall. Next, the influence of the shape of the cylindrical vacuum vessel on the stabilization is studied. It is found that the created spheromak assumes a similar figure irrespective of the vessel shape; thus we conclude that controlling the spheromak shape by changing the vacuum vessel is ineffective. Also investigated is the tilting behavior when the magnetic index is changed. The result is not conclusive in the sense that a well-developed oblate spheromak could not be created when the magnetic index was close to 1. Simulation runs in which n=2 and n=3 perturbations are applied, in addition to the tilt mode (n=1), have shown that the spheromak with a large aspect ratio is unstable to all of these modes. Interestingly, it is found that as the n=2 mode develops sufficiently, the spheromak torus is severely folded and twisted; consequently, the folded arms undergo reconnection internally.