Local physics basis of confinement degradation in JET ELMy H mode plasmas and implications for tokamak reactors

First results of gyrokinetic analysis of JET ELMy H mode plasmas are presented. ELMy H mode plasmas form the basis of conservative performance predictions for tokamak reactors of the size of ITER. Relatively high performance for long durations has been achieved and the scaling appears to be favourable. It will be necessary to sustain low Z_eff and high density for high fusion yield. The article studies the degradation in confinement and the increase in the anomalous heat transport observed in two JET plasmas: one with an intense gas puff and the other with a spontaneous transition from type I to III ELMs at the heating power threshold. Linear gyrokinetic analysis gives the growth rate γ_lin of the fastest growing modes. The flow shearing rate γ_E×B and γ_lin are large near the top of the pedestal. Their ratio decrease approximately at the time when the confinement degrades and the transport increases. This suggests that tokamak reactors may require intense toroidal or poloidal torque input to maintain sufficiently high γ_E×B/γ_lin near the top of the pedestal for high confinement.