A Pre-ionization System to Limit Neutral Gas in a Compact Toroid Injector

Fusion plasmas require long lifetimes and high temperatures, both of which are limited by particle loss, among other factors. Therefore, refueling a long-lived advanced beam-driven field-reversed configuration (FRC) plasma in C-2U [1] is necessary, and injecting a supersonic compact toroid (CT) is an effective means of introducing particles into the FRC core [2]. However, neutral gas that trails the CT into the target chamber cools the FRC. Pre-ionization (PI) system assists the break down between electrodes of the CT injector (CTI), so the amount of introduced gas can be lowered by up to a factor of two, effectively increasing the ionization fraction; thus, reducing the amount of neutral gas in the system. Additionally, the PI decreases the delay in CTI breakdown so a highly reproducible operation is achievable. The PI system consists of a fast, high voltage, pulse discharge circuit coupled to a Teflon insulated semi-rigid coaxial cable inserted into the CTI. System details and experimental data will be presented, in addition to issues such as the introduction of impurities and pre-ionizer lifetime. [1] M. Binderbauer \textit{et al}., Phys. Plasmas 22, 056110 (2015). [2] T. Matsumoto \textit{et al}., Rev. Sci. Instrum. 87, 053512 (2016).