11 mars 2014

First quantitative prediction of the transient energy losses in the ITER divertor

**Within the frame of an F4E contract, the IRFM team has succeeded in predicting for the first time the transient energy losses in the ITER divertor using the magneto-hydrodynamic (MHD) code JOREK computing the full edge relaxation dynamics that takes place at the edge of the ITER plasmas. **

ITER high fusion performance relies on high confinement plasma characterized by strong edge plasma pressure and gradients which are driving quasi-periodic fast (~250ms) edge plasma relaxations: the Edge Localized Modes (ELMs). ELMs not only limit the maximum pressure and hence plasma confinement, but they are predicted to be extremely harmful for the ITER divertor due to the large heat and particle fluxes expelled by each ELMs. In particular considerations on the divertor plasma facing material (tungsten) safety point of view show that the energy released by each ELM should remain smaller than ~1MJ. However the present estimates based on experimental scaling laws predict that the ELM energy released in ITER may reach ~20MJ. A major concern for ITER operation in the long run is thus to be able to control and/or mitigate the onset of such large ELMs.

The present work conducted in the framework of the F4E-2011-GRT-265 Grant had the two following goals: first, estimate the stability limits for pedestal pressure in most of the relevant scenarios foreseen for ITER; second, model the dynamics of the ELM crash including its onset and development in the relevant ITER geometry and estimate the impact of ELMs on the ITER divertor. .

The stability code JOREK is internationally recognized as a leading tool in this field. It has been applied to present day tokamaks reaching the level of possible direct comparison with experimental data. In the present work the resulting dynamics of the ELM crash has been modelled, starting from the growth of the main unstable instability (ballooning-peeling modes) predicted in the linear MHD analysis and up to the full highly ELM relaxation and calculation of the resulting energy deposition into ITER divertor. For example, in the nominal 15MA/5.3T scenario relatively small ELMs are predicted: the plasma thermal energy loss being about 1.1% (~5MJ). This ELM leads to a maximum heat flux of about ~17GW/m^{2} (Fig.1-a). In the half current scenario 7.5MA/2.65T, 2.7% energy loss is found, corresponding to a ~2.5 MJ ELM characterized by a maximum heat flux of about ~3GW/m^{2}(Fig.1-b). The thermal energy in the ELM is essentially conducted to the divertor following the perturbed by ELM (ergodic) magnetic field lines (Fig.1c) and particles are expelled from the plasma essentially via convective transport (Fig.1d).

Maj : 11/03/2014 (372)