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(EXP5/19) Local Physics Basis of Confinement Degradation in JET ELMy H-mode Plasmas and Implications for Tokamak Reactors

R. V. Budny¹⁾, the JET Team^2)
 
1) PPPL, POBox 541, Princeton University, Princeton, NJ 08543, USA
²⁾ JET JOINT UNDERTAKING, Abingdon, UK

Abstract.  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 is favorable. It will be necessary to sustain low Z_eff 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 spontanous transition at the heating power threshold from Type I to III ELMs. Linear gyrokinetic analysis gives the growth rate, $\gamma_{\mathrm{lin}}^{}$ of the fastest growing mode. Our results indicate that the flow-shearing rate $\omega_{\mathrm{ExB}}^{}$ and $\gamma_{\mathrm{lin}}^{}$ 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 |$\omega_{\mathrm{E\times B}}^{}$|/$\gamma_{\mathrm{lin}}^{}$ near the top of the pedestal for high confinement.

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