Self-organization and confinement in tokamak plasmas with edge safety factor less than 2

In most tokamaks, operation at low edge safety factor q(a) < 2 is challenging or impossible due to a disruptive kink instability. We report measurements and nonlinear MHD modeling of very low safety factor, Ohmic tokamak plasmas in MST (a = 0.5 m, R = 1.5 m, B_T = 0.13 T) spanning 0.7 < q(a) < 2.9. The external kink instability is mitigated passively by MST’s close-fitting, thick, conducting shell. The experimental results are compared to nonlinear MHD simulations using the NIMROD code with a similar Lundquist number S ~ 10⁵. Equilibrium reconstructions constrained by local magnetic probe measurements at r/a > 2/3 suggest that q(0) ~ 1 for q(a) ≥ 1. The q(r) profile has a flat region in the core that expands outward systematically with decreasing q(a). Sawtooth-like oscillations are observed in the experiment and in NIMROD simulations which cover q(a) ≥ 1.5, implying self-organization of the current profile. When q(a) approaches 1, the dynamics are dominated by a large-amplitude, saturated m/n = 1/1 helical deformation. As q(a) decreases from 2 to 1.5, the measured electron energy confinement time gradually decreases and the sawtooth cycle becomes more irregular, possibly due to the increasing radius of the q = 1 surface. Work supported by US DOE and the WiPPL team.