No abstract

The work of the ITPA SOL/divertor group is reviewed and implications for ITER discussed. Studies of near SOL gradients have revealed a connection to underlying turbulence models. Analysis of a multi-machine database shows that parallel conduction gradients near the separatrix scale as major radius. New SOL measurements have implicated low-field side transport as driving parallel flows to the inboard side. The high-n nature of ELMs has been elucidated and new measurements have determined that they carry ~10-20% of the ELM energy to the far SOL with implications for ITER limiters and the upper divertor. Analysis of ELM measurements imply that the ELM continuously loses energy as it travels across the SOL-larger gaps should reduce surface loads. The predicted divertor power loads for ITER disruptions has been reduced as a result of finding that the divertor footprint broadens during the thermal quench and that the plasma can lose up to 80% of its thermal energy before the thermal quench (not true for VDEs or ITBs). On the other hand predictions of power loading to surfaces outside the divertor have increased. Disruption mitigation through massive gas puffing has been successful at reducing divertor heat loads but estimates of the effect on the main chamber walls indicate 10s of kG of Be could be melted/mitigation. Estimates of ITER tritium retention have reduced the amount retained/discharge although the uncertainties are large and tritium cleanup may be necessary every few days to weeks. Long-pulse studies have shown that the fraction of injected gas that can be recovered after a discharge decreases with discharge length. The retention rate on the sides of tiles appears to ~ 1-3% of the ion flux to the front surface for C tiles and ~100x less for Mo tiles. T removal techniques are being developed based on surface heating and surface ablation although ITER mixed materials will make T removal more difficult. The use of mixed materials gives rise to a number of potential processes-e.g. reduction of surface melting temperatures (formation of alloys) and reduction of chemical sputtering. Advances in modelling of the ITER divertor and flows have enhanced the capability to match experimental data and predict ITER performance.

The paper describes the results of a physics analysis of a modified divertor cassette for ITER. The issues addressed are the impact on the operational window, the effect of gas leaks through the broader gaps between the divertor cassettes, and radiation power loading of different components of the cassettes. The radiation load on the side walls of the cassette structures in the inter-cassette gaps is identified as a design constraint not previously considered.

Recent results of divertor modelling are summarized and some modelling results incorporated in the ITER-FEAT divertor design, such as the beneficial effect of V-shaped targets, the importance of high gas conductivity between the divertors, and the role of deep core fuelling in maintaining the plasma density, are discussed. Differences between carbon and seeded impurities are pointed out, and the beneficial effect of helium elastic scattering is demonstrated. A number of considerations for modelling reactor-like operation and additional requirements imposed on the code under these conditions are discussed.

No abstract

This paper summarises the modelling studies of steady-state divertor operation being performed for the ITER-FEAT design. Optimisation of the divertor geometry reveals the importance of the proper target shape for a reduction of the peak power loads. A high gas conductance between the divertor legs is also essential for maintaining acceptable conditions in the outer divertor which receives higher power loading than the inner. Impurity seeding, which would be necessary if tritium co-deposition concerns preclude the use of carbon as plasma-facing material, can ensure the required high radiation level at acceptable Z eff , and the divertor performance is not very sensitive to the choice of the radiating impurity.

No abstract

Fuelling requirements for ITER are analysed in relation to pellet fuelling and ELM pacing, and a divertor power load control consistent with the ITER pumping and fuel throughput capabilities. The plasma parameters at the separatrix and the particle sources are derived from scalings based on SOLPS simulations. Effective transport coefficients in the H-mode pedestal are derived from EPED1 + SOLPS scalings for the pedestal height and width. 1.5D transport is simulated in the ASTRA framework. The operating window for ITER DT plasmas with the required fusion performance and level of ELM, and divertor power load control compatible with ITER fuelling and pumping capabilities, is determined. It is shown that the flexibility of the ITER fuelling systems, comprising pellet and gas injection systems, enables operation with Q  =  10, which was found to be marginal in previous studies following a similar approach but with different assumptions. The present assessment shows that a reduction of 〈 n e 〉 by a factor ~2 (from 9 to 5  ×  1019 m−3) in 15 MA H-mode plasmas leads to a reduction in the required pellet fuelling rate by a factor of four. Results of the analysis of the fuelling requirements for a range of ITER scenarios are found to be similar to those obtained with the JINTRAC code that included 2D modelling of the edge plasma.