H-mode and Edge Physics on the Pegasus ST: Progress and Future Directions

Ohmic H-modes are routinely attained on the Pegasus ST, in part due to the low L-H power threshold $P_{LH} $ arising from low-$B_{T} $ operation at $A\sim 1$. Characteristics of H-mode include: improved $\tau_{e} $, consistent with $H_{98} \sim 1$; edge current and pressure pedestal formation; and the occurrence of ELMs. Experiments in the past year have examined magnetic topology and density dependencies of $P_{LH} $ in detail. $P_{LH} $ exceeds ITER L-H scaling values by 10--20$\times $, with ${P_{LH} } \mathord{\left/ {\vphantom {{P_{LH} } {P_{ITPA08} }}} \right. \kern-\nulldelimiterspace} {P_{ITPA08} }$ increasing sharply as $A\to 1$. No $P_{LH} $-minimizing density has been found. Unlike at high-$A$, $P_{LH} $ is insensitive to limited or diverted magnetic topologies to date. The low $B_{T} $ and modest pedestal values at $A\sim 1$ afford unique edge diagnostic accessibility to investigate ELMs and their nonlinear dynamics. $J_{edge} (R,t)$ measured through a Type I ELM shows a complex pedestal collapse and filament ejection. These studies are being extended to higher $I_{p} $ and longer pulse length with LHI startup to conserve Ohmic V-s and improve MHD stability. A modest-cost upgrade to the facility will enable detailed validation studies of nonlinear ELM dynamics and ELM control. This initiative will upgrade the centerstack, increasing $B_{T} $ by $\times 3$, Ohmic V-s by $\times 4$, and pulse lengths to 100 ms at $A<1.3$, as well as deploy a comprehensive 3D magnetic perturbation coil system with full poloidal coverage from frame coils and helical centerstack windings.