Non-disruptive tokamak operation far beyond traditional safety factor and density limits*

Non-disruptive tokamak plasmas have been produced in the Madison Symmetric Torus (MST) at low field (B_T = 0.13 T) with edge safety factor 0.6 < q(a) < 2, below the traditional disruptive stability limit of q(a) = 2 [Phys. Plasmas 29, 080704 (2022)]; and (separately) with density up to 10 times the Greenwald limit, n_G. Achievable values of q(a) and n/n_G appear to be limited only by hardware and not by instabilities. Low-q(a) operation is possible due to MST’s thick, conductive, close-fitting shell with resistive wall time 800 ms that inhibits resistive wall modes during the 50 ms discharges, and high-voltage, high-bandwidth feedback power supplies capable of sustaining the plasma current in the presence of large resistance and/or rapid MHD dynamics. Plasmas with 1 < q(a) < 2 and q(a) < 1 have been studied previously in other devices, but our work is novel in that steady, controlled equilibria are obtained with detailed internal diagnosis. Measurements reveal self-organized q(r) profiles with q(0) near unity, irregular fluctuations, and decreased confinement for 1 < q(a) < 2; and strong, coherent helical structures for q(a) ≤ 1. Nonlinear MHD simulations conducted with q(a) ≥ 1.5 using the NIMROD code also find q(0) near unity. The capability for n > n_G is thought to be enabled by the advanced power supplies, and the thick shell may also play a role. While other devices have obtained n/n_G as high as 2, the values n/n_G ~ 10 reported here are unprecedented. In the range 1 < n/n_G < 2 the Ohmic power and impurity radiation scale strongly with density, but for n/n_G > 2 the current profile collapses and the scalings weaken. These results may help inform future tokamak design efforts in order to mitigate the disruption problem and extend operational stability boundaries.