Local Physics Basis of Confinement Degradation in JET ELMy H-Mode Plasmas and Implications for Tokamak Reactors
Authors: The JET Team (presented by R.V. Budny)
Date of PPPL Report: November 2000
Presented at: the 18th International Atomic Energy Agency's (IAEA) Fusion Energy Conference 2000 (FEC-2000) held in Sorrento, Italy, October 4-10, 2000 and to be published in an unedited proceedings in electronic format (CD-ROM) only. An expanded version of this paper was submitted to Nuclear Fusion for publication in a Special Issue of the conference papers which will be refereed and edited -- see PPPL-3542.
ELMy H-mode plasmas form the basis of conservative performance predictions for tokalmak reactors of the size of ITER. Relatively high performace for long durations has been achieved and the scaling is favorable. It will be necessary to sustain low Zeff and high density for high fusion yield. This paper studies the degradation in confinement and increase in the anomalous heat transport observed in two JET plasmas: one in which the degradation occurs with an intense gas puff, and the other with a spontaneous transition at the heating power threshold from Type I to III ELMs. Linear gryokinetic analysis gives the growth rate, gammalin of the fastest growing mode. Our results indicate that the flow-shearing rate omegaExB and gammalin are large near the top of the pedestal. Their ratio decreases approximately 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 |omegaExB|/gammalin near the top of the pedestal for high confinement.