Design of the HIT-SI Machine
There were several design challenges to overcome during the construction of HIT-SI. Injectors are attached to each side of the confinement region, but due to the AC injector drive, the injectors must be electrically isolated from the confinement region. A close fitting flux conserver helps form and confine the spheromak equilibrium. To insure that the current drive is purely inductive the inside of the machine is coated with a thin insulating layer, isolating the plasma from the injectors and the flux conserver.
The injectors are 180° sections of a large aspect ratio reversed field pinch (RFP) that connect to the spheromak confinement volume. Each plasma injector has separate voltage and flux coils to drive current on magnetic field lines, generating magnetic helicity. The solenoid-shaped flux coil (see Fig. 1 below) establishes a flux in the injector while the voltage coil (see Fig. 2) induces a current along that flux. The voltage and flux coils on a given injector are driven in phase, while a 90° phase difference between each injector generates constant helicity injection.
Fig. 1 Flux Coil (black)
Fig. 2 Voltage Coil (black)
Gaps between the blue and yellow portions of the injector in Fig. 1 allow flux generated by the coils to penetrate into the injector.
Spheromak Confinement Volume
The confinement region is bow tie shaped to increase the MHD β limit for the spheromak equilibrium. The bow tie shape provides stability to global modes and the close fitting conducting shell stabilizes local edge modes. Assuming a hollow current profile, the predicted Mercier β-limit for HIT-SI discharges is 10%. The figure below shows the effect of the bow tie flux conserver on the poloidal flux surfaces.