Self-organization and confinement in tokamak plasmas with very low edge safety factor

There has long been interest in understanding tokamak behavior for edge safety factor q(a) < 2, in part due to the strong scaling of energy confinement with plasma current. We report internal measurements and nonlinear MHD modeling for very low safety factor, steady, Ohmic tokamak plasmas spanning 0.8 < q(a) < 3. The experiments are conducted in MST (a = 0.5 m, R = 1.5 m, B_T = 0.13 T), where the disruptive kink instability typically encountered at q(a) = 2 is mitigated passively by the close-fitting, thick, conducting shell. A family of self-organized q(r) profiles is discovered in which q(0) ~ 1 as q(a) decreases toward 1, such that the flat core region expands outward toward the wall. Internal electron density and temperature measurements also exhibit increasingly broad, flat profiles. The experimental results are compared to nonlinear MHD simulations with q(a) ≥ 1.5 using the NIMROD code with a similar Lundquist number S = 10⁵, which also show q(0) relaxing toward unity. Experimental data show that periodic sawtooth behavior gives way to irregular fluctuations, and electron energy confinement drops as q(a) decreases below 2. Interesting nonlinear mode coupling behavior near and below q(a) = 1 is also discussed.