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Abstract:
Injection of Lower Hybrid Heating and Current Drive (LHCD) into the current ramp-up phase of JET discharges has produced extremely reversed q-profiles characterized by a core region of near zero current density (within Motional Stark Effect diagnostic measurement errors). Non-inductive, off-axis co-current drive induces a back electromotive force inside the non-inductive current radius that drives a negative current in the plasma core. The core current density does not go negative, although current diffusion calculations indicate that there is sufficient LHCD to cause this. A possible explanation of the core current density clamping near zero is that MHD instabilities redistribute the core current soon after it goes negative. This is seen in reduced MHD simulations in cylindrical geometry and nonlinear resistive MHD simulations in toroidal geometry which predict that these discharges undergo n=0 reconnection events which clamp the core current near zero. Understanding the physics of the current hole in present devices is important for enabling predictions of current profile evolution in next step facilities with strong non-inductive current drive.
