The role of axisymmetric reconnection events in JET discharges with extreme shear reversal

Injection of lower hybrid heating and current drive into the current ramp-up phase of JET discharges can produce extremely reversed q-profiles characterized by a core region of very small or zero current density (within motional Stark effect diagnostic measurement errors) and q_min > 1. T_e-profiles show sawtooth-like collapses and the presence of an internal transport barrier. Accurate equilibrium reconstructions of these discharges are obtained using the ESC code, which was recently extended to allow equilibrium reconstructions in which a free boundary solver determines the plasma boundary and a fixed boundary solver provides the magnetic geometry and current density profile. The core current density does not appear to go negative, although current diffusion calculations indicate that sufficient non-inductive current drive to cause this is present. This is explained by nonlinear resistive MHD simulations in toroidal geometry which predict that these discharges undergo n = 0 reconnection events (axisymmetric sawteeth) that redistribute the current to hold the core current density near zero.