Design considerations for a transient CHI gas injection system for Pegasus-III

Transient Coaxial Helicity Injection, a method first developed on the HIT-II experiment and then validated on the NSTX device 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. Magnetic flux that initially connects the inner and outer vessel components could be grown into the vessel using J x 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 possible. To address this design requirement, a first of its kind, floating double biased reactor-relevant CHI configuration is being developed for Pegasus-III. An equally important requirement for successful transient CHI discharge initiation is the need for injecting a relatively small amount of gas while simultaneously satisfying the requirements for gas breakdown in the injector region and avoiding breakdown in other parts of the vessel. This stringent requirement generally requires that a small gas plenum be located close to a toroidal gas manifold in the injector region in a high gas conductance configuration. Design aspects for a transient CHI gas injection system for the Pegasus-III geometry will be described.