Helical structures in low-safety-factor toroidal plasmas near the Kruskal-Shafranov limit*

In the region of edge safety factor 0 < q(a) < 2 between the reversed-field pinch and the tokamak, there exists a regime of low-safety-factor toroidal plasmas that is difficult or impossible to access in most devices due to disruptive kink instabilities. In the Madison Symmetric Torus (MST), this regime can be accessed in steady conditions due to a thick, conductive, close-fitting wall that delays growth of the resistive wall mode, and a high-voltage, high-bandwidth, feedback-controlled power supply driving the plasma current [N. C. Hurst, et al., Phys. Plasmas 29, 080704 (2022)]. We present results of experiments with plasmas in MST near the Kruskal-Shafranov limit, q(a) = 1, providing insight into a regime of toroidal plasma operation that is not well characterized. As the limit is approached from above, strong, global m/n = 1/1 helical structures are observed rotating past several diagnostics. When q(a) is very close to 1, these modes lock to the machine wall and grow to large amplitude up to 10% of the poloidal field at the plasma edge. The interaction of these modes with an applied resonant magnetic perturbation is discussed. Discharges with 2/3 < q(a) < 3 /4 are also presented, where two nearly resonant modes compete for dominance.