Initiatives in Non-Solenoidal Startup and H-mode Physics at Near-Unity $A$

Research on the $A\sim 1$ Pegasus ST is advancing the physics of non-solenoidal tokamak startup and the H-mode confinement regime. Local helicity injection (LHI) uses current sources in the plasma edge to initiate and drive $I_{p} $ via DC helicity injection, subject to constraints from helicity conservation and Taylor relaxation. To date, $I_{p} \sim 0.18$ MA has been initiated with $I_{inj} \sim 6$ kA. A predictive 0-D power balance model of LHI $I_{p} (t)$ evolution matches present discharges with strong PF induction. It projects $I_{p} \sim 0.3$ MA operation in Pegasus will achieve the LHI-dominated physics regime expected for 1 MA NSTX-U startup. Ohmic H-mode plasmas are routinely attained, due to the low $P_{th} $ at the low $B_{T} $ of $A\to 1$ plasmas. However, both limited and favorable $\nabla B$ SN plasmas have $P_{th} \sim 11$ times higher than expected from high-$A$ scalings. They have improved $\tau_{e} $ ($H_{98} \sim 1)$ and a quiescent $J_{edge} $ pedestal between edge localized modes (ELMs). Unique $J_{edge} (t)$ measurements through a single Type I ELM show a complex, multimodal pedestal collapse and filament ejection. A proposed Pegasus-U initiative will upgrade the centerstack assembly and LHI injector systems, increasing $B_{T} $ to 1 T, Ohmic V-s by $\times 6$, and pulse length to 100 ms at $A=1.2$. This allows the physics and technology of LHI to be validated at NSTX-U relevant parameters, supports studies of nonlinear ELM dynamics, and will test high-$\beta_{T} $ tokamak stability.