Design Considerations for a Transient CHI Gas Injection System for Pegasus-III

Due to restricted space for a full-sized solenoid, compact tokamaks and spherical tokamaks such as the ST40 and the next-step ST80 device planned by Tokamak Energy, Oxfordshire, U.K., could considerably benefit from solenoid-less plasma startup capability. Transient coaxial helicity injection (CHI), a method first developed on the small helicity injected torus-II (HIT-II) experiment and then validated on national spherical torus experiment (NSTX), is a method to initiate an inductive-like tokamak plasma discharge without reliance on the central solenoid. In both these devices, toroidal ceramic insulators were used to electrically separate the inner and outer vessel components so that the magnetic flux that initially connects the inner and outer vessel components could be grown into the vessel using J $\times$ B forces to generate a closed magnetic field line configuration. In reactors, the installation of large toroidal insulators as part of the vacuum vessel boundary may not be feasible. To address this design requirement, a first of its kind, floating double-biased reactor-relevant CHI configuration is being developed for Pegasus-III. A particularly challenging requirement for a successful transient CHI discharge generation is the need to reduce the amount of fuel gas injected to the levels normally used for discharge initiation using the inductive solenoid. While higher levels of gas injection ease the plasma breakdown requirements, it also favors discharge initiation outside the CHI injector region. This stringent requirement generally requires that a small gas plenum be located close to an appropriately designed gas injection manifold in the injector region in a high gas conductance configuration. Plasma startup using transient CHI and the design aspects for a transient CHI gas injection system for the Pegasus-III geometry are described.