Neutral beam system for the C-2-Upgrade Field Reversed Configuration Experiment

In the C-2 field-reversed configuration (FRC) experiment, tangential neutral beam injection (NBI), coupled with electrically-biased plasma guns at the plasma ends and advanced surface conditioning, led to dramatic reductions in turbulence-driven losses [1]. Under such conditions, highly reproducible, macroscopically stable, hot FRCs with a significant fast-ion population, total plasma temperature of $\sim$ 1 keV and record lifetimes were achieved [2]. To further improve the FRC sustainment and provide a better coupling with beams, the C-2 device has been upgraded with a new NBI system, which can deliver up to a total of 10 MW of hydrogen beam power (15 keV, 8 ms pulse), by far the largest ever used in compact toroid plasma experiments. The NBI system consists of six positive-ion based injectors featuring flexible, modular design. This presentation will provide an overview of the C-2U NBI system, including: 1) NBI test facility, beam characterization, and acceptance tests, 2) integration with the machine and operating experience, 3) improvements in plasma performance with increased beam power.\\[4pt] [1] M. Tuszewski et. al, Phys. Rev. Lett 108, 255008 (2012).\\[0pt] [2] M.W. Binderbauer et al., Phys. Plasmas 22, 056110 (2015).