Bean-Shaped Spherical Tokamak Equilibrium With Ergodic Limiter

Equilibrium simulations of a spherical tokamak (ST) with a bean-shaped plasma in rail limiter geometry are presented for the first time. Preliminary results in a not fully self-consistent scenario show this equilibrium is possible and, compared with the more usual ST rail limiter D-shaped geometry, retain consistently the same kinetic storage energy. The related parameters such as volume-averaged beta increase by 17% due to pure geometric effects, i.e., the increase of plasma major radius (10%), which then increases the aspect ratio (1.4-1.6) due to the simultaneous reduction of plasma minor radius (9%), while the elongation increases (1.2-1.3). The plasma volume is virtually the same but the plasma triangularity increases substantially (0.32-0.45) thus favoring beta limits because of shaping. These results have been obtained after a careful pressure tailoring by fixing the plasma outboard within a 3% error. This unique bean-shaped equilibrium can be attained simultaneously (but independently) setting an edge localized ergodization as observed via a Poincaré mapping, using a low level of nonresonant perturbation to the plasma current ( ~ 2%). This combination can theoretically lead to a higher beta limit ST plasma with different thermal power loads distribution, which is an interesting area of study for larger devices.