Improved Confinement of C-2 Field-Reversed Configuration Plasmas

C-2 is a unique, large compact-toroid (CT) device at Tri Alpha Energy that produces field-reversed configuration (FRC) plasmas by colliding and merging oppositely directed CTs. Significant progress has recently been made on C-2, achieving ~5 ms stable plasmas with a dramatic improvement in confinement, far beyond the prediction from the conventional FRC scaling. This stable, long-lived FRC plasma state is called the high-performance FRC (HPF) regime. The key approaches to achieve the HPF regime are as follows: (i) dynamic FRC formation by collision/merging of super-Alfvénic CTs, (ii) effective control of stability and transport by end-on plasma guns and neutral-beam (NB) injection, and (iii) active wall conditioning using titanium and lithium gettering systems. Moreover, further improvement in FRC confinement has been obtained with improved open-field-line plasma properties such as a lower fluctuation level, reduced transport rates in radial/axial directions, and lower background neutral density as well as recycling. This open-field-line plasma improvement, mainly obtained by higher magnetic fields in the formation and mirror-plug sections, allows for better NB coupling to the core-FRC plasma. In the recent HPF regime there is a sufficiently large fast-ion population that appears to improve FRC confinement properties as well as stability; the FRC particle and global energy confinement times both increased by ~30% and ~80%, respectively, compared to that of the previously obtained HPF regime.