Modelling of L to H-mode Transition in JET

Recently, the linear transport model, allowing for the Low (L) to the high confinement mode (H-mode) transition and tested previously in RITM transport code, has been introduced to the 1.5D transport code JETTO. The model accounts for drift instabilities of different nature, e.g. Ion Temperature Gradient (ITG) and Trapped Electron (TE) modes, which are dominating the transport in the plasma core, as well for modes driven by collisions and current perturbations, like Drift Resistive Ballooning (DRB) and Drift Alfvén (DA) instabilities, which are particularly important at the plasma edge under the L-mode conditions. These edge modes are stabilized by increasing pressure gradient and decreasing collisionality. Local analysis shows that, the reduction of the turbulent transport driven by edge instabilities occurs, if the total heating power exceeds some critical level, Pth, which is interpreted as the H-mode power threshold. Present paper focuses on the validation of the model predictions against experimental results obtained in JET tokamak. Comparison is done for a number of JET discharges, in which the line averaged density and the magnetic field have been varied. Both computed and experimental results were also compared to the inter-machine scaling law. The comparison of the model predictions to results of non-linear computations is outside of the scope of the present paper. Nevertheless, it should be mentioned that, so far, nonlinear computations done for similar plasma conditions do not reproduce this sudden reduction of the turbulent transport with increasing pressure gradient. Reasons for this disagreement are not clear at the moment; the understanding requires further benchmarking activity.
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