ITER

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Boundary and Divertor Physics
The ITER plasma boundary will comprise the thin (~0.2-m thick), relatively low-temperature periphery of the ITER plasma, plus the magnetically-connected plasma scrape-off layer and lower poloidal divertor , where the fully-ionized boundary plasma  —  including thermalized alpha particles  —  will be converted to neutral gas, to be exhausted by pumps external to the torus vacuum vessel. In addition to this 'fusion ash' helium exhaust function — essential to the maintenance of a sustained fusion burn in the plasma core — the ITER divertor plasma will also radiate a substantial fraction — up to 80% — of the highly localized plasma thermal power flux that would otherwise impinge on the divertor target surfaces. The combination of particle (D, T and He) and thermal power exhaust that the divertor provides gives rise to description of its functions as being ones of power and particle exhaust.

Divertor Region Physics and Modelling
The physics basis for the power and particle exhaust aspects of the ITER plasma boundary deal primarily with the characteristics of relatively low-temperature (1-100 eV) plasmas, the atomic physics of plasma recombination and ionization in this low-temperature regime and the major role that radiation from hydrogenic and trace impurity (carbon, oxygen, etc.) species and localized plasma convection plays in attainment and control of the thermal and particle exhaust functions of the divertor. Divertor studies in present tokamakin present tokamaks have elucidated both the complex plasma and atomic physics involved and the utility of 2-D numerical boundary simulation models (UEDGE, B2-EIRENE) which can now accurately reproduce present experimental data and which provide the design basis for extrapolating such data to the ITER/reactor-tokamak regime. Operation of the ITER divertor will constitute a final 'reactor-regime' validation of such modelling.

Divertor and Edge Region Physics and Modelling Bases
Divertor and Edge Physics Opportunities in ITER

Pedestal Region Physics
The inside 'plasma-facing' portion of the ITER plasma boundary comprises the transition region between the low-temperature divertor plasma and fusion-producing plasma core. For the planned 'ELMy H-mode' operation scenario, the transition from 'outer' ~100 eV plasma temperature to the inner ~5 keV 'fusion core' is anticipated to occur over a radial dimension of ~0.1 m. The resulting plasma temperature and pressure gradients in this 'H-mode edge' or 'H-mode pedestal' region are high and a unique set of physics basis considerations applies to the 'self-organizing' development of the strong temperature gradient and energy transport barrier that is characteristic of the H-mode and also to the regulation of the H-mode edge pressure gradient by the periodic MHD instabilities called edge localized modes (ELMs). The inner temperature of the H-mode edge — the so-called pedestal temperature — and the magnitude and frequency of ELMs have separate but equally great import for ITER. Present data show a clear correlation between higher pedestal temperature (pressure) and better plasma energy confinement, and models of ITER performance based on numerical micro-turbulence simulations in the plasma core predict that an edge temperature of about 5 keV will be required to obtain the desired fusion power and Q, The same data also shows that the large, low-frequency Type I ELMs, predicted for this edge temperature regime, will lead to rapid erosion of the ITER divertor targets. But at the present time, physics understanding of the pedestal region and the ability to reliably predict ITER pedestal and ELM properties is arguably less well developed than the corresponding understanding of the plasma core and divertor regions. Accordingly, the urgent need for development of better understanding of pedestal and ELM physics leads to science opportunities for present fusion experiments and theory and to unique opportunities for validation of such understanding in ITER.

Pedestal and ELM Physics and Understanding
Divertor and Edge Physics Opportunities in ITER

DIII-D plasmas with an ITER-like shape and divertor show radiation from the plasma boundary and divertor. In-divertor radiation is strongly localized near the plasma x-point and outboard target strikepoint

B2-Eirene modelling reproduces the 2-D radiation intensity profiles observed in the ASDEX Upgrade tokamak divertor

Confinement in DIII-D plasmas increases with increasing edge pedestal pressure. But higher pressures yield large 'Type I' ELMs, which may be problematical for ITER
ELITE MHD code modelling shows edge 'peeling-mode' structure and radial depth differences for plasmas with large and small ELMs