Non-Solenoidal Startup via Helicity Injection in the Pegasus ST

Research on the $A\sim 1.2$ Pegasus ST is developing the physics and technology basis for optimal non-solenoidal tokamak startup. Recent work explores startup via Local Helicity Injection (LHI) using compact, multi-MW current sources placed at the plasma edge in the lower divertor region. This minimizes inductive drive from poloidal fields and dynamic shaping. Plasmas with $I_{p} \le 200$ kA, $\Delta t_{pulse} \sim 20$ ms and $B_{T} \le 0.15$ T are produced to date, sustained by two injectors with $A_{inj} =4$ cm$^{\mathrm{2}}$, $V_{inj} \sim 1.5$ kV, and $I_{inj} \sim 8$ kA, facilitated by improvements to the injectors, limiters, and divertor plates that mitigate deleterious PMI. These plasmas feature anomalous, reconnection-driven ion heating with $T_{i} \ge T_{e} \ge 50-100$ eV and large-amplitude MHD activity driven by the injectors. Under some conditions, MHD fluctuations abruptly decrease by over an order of magnitude without loss of LHI drive, improving realized $I_{p} $, and suggesting short-wavelength modes may relate to the current drive mechanism. The high $I_{N} \ge 10$, ion heating, and low $\ell_{i} $ driven by LHI, and the favorable stability of $A\sim 1$ STs allows access to record $\beta_{t} \sim 100\% $ and high $\beta_{N} \sim 6.5$. Such high-$\beta_{t} $ plasmas have a minimum $\left| B \right|$ well spanning $\sim 50\% $ of the plasma volume. Enhancements to the Pegasus facility are considered to increase $B_{T} $ towards NSTX-U levels; establish coaxial helicity injection capabilities; and add auxiliary heating and current drive.