The HIT-SI Approach
The Helicity Injected Torus with Steady Inductive drive (HIT-SI) uses a novel inductive approach for injecting magnetic helicity to form and sustain a spheromak without the use of electrodes. Helicity is generated and injected into the confinement region by two semi-toroidal injectors located on opposite ends of the device. In each injector the ‘toroidal flux’ and induced ‘loop voltage’ are time varying sinusoids in phase, producing a sine-squared helicity injection rate. The two HIT-SI injectors are driven 90° out of phase with each other, producing a near constant rate of total helicity injection. The lack of electrodes or other strong plasma-wall interactions make HIT-SI an ideal platform for clean studies of helicity injection current drive and current-profile relaxation.
A sketch of the evolution of the field lines of one injector can be seen in the figure below. Injector filed lines are denoted in red with the spheromak flux surfaces in blue.
Figure a shows the start of an injector cycle driving maximum current in one direction.
As the cycle continues, figure b, the injector flux is reduced by introducing fields of the opposite direction. The edge fields reconnect and move to the spheromak equilibrium.
Once the injector flux changes sign, figure c, the injector operates as a stabilized pinch. The injector flux begins to link with the spheromak equilibrium.
Figure d shows the completion of the half-cycle with maximum current dive in the opposite direction.