The role of axisymmetric reconnection events inJET discharges with extreme shear reversal

Abstract

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