H. D. Pacher

The ITER divertor concept

Abstract The ITER divertor must exhaust most of the alpha particle power and the He ash at acceptable erosion rates. The high recycling regime of the ITER-CDA for present parameters would yield high power loads and erosion rates on conventional targets. Improvement by radiation in the SOL at constant pressure is limited in principle. To permit a higher radiation fraction, the plasma pressure along the field must be reduced by more than a factor 10, reducing also the target ion flux. This pressure reduction can be obtained by strong plasma-neutral interaction below the X-point. Under these conditions Te in the divertor can be reduced to

Scaling laws for edge plasma parameters in ITER from two-dimensional edge modelling

Results of a detailed study of the parameter space of the ITER divertor with the B2-Eirene code are presented. Relations between plasma parameters at the separatrix, the interface between the core and edge plasma, are parametrized to provide a set of boundary conditions for the core models. The reference ITER divertor geometry is compared with the straight target option, and the possibility of controlling the edge density by shifting the plasma equilibrium in ITER is explored.

Effect of neutral transport on ITER divertor performance

This paper describes the present state of the development of the computational model of the ITER edge plasma. Neutral–neutral collisions and molecular dynamics are introduced into the self-consistent scheme. First results of ITER modelling including these effects indicate that the operational window for the ITER divertor shifts towards higher neutral pressure in the private-flux region, retaining the operational flexibility determined in the previous analyses.

Basic divertor operation in ITER-FEAT

The modelling studies being performed for steady state divertor operation of the ITER-FEAT design are summarized. Optimization 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 a higher power loading than the inner divertor. Impurity seeding, which would be necessary if tritium co-deposition concerns precluded the use of carbon as the plasma facing material, can ensure the required high radiation level at acceptable Zeff, and the divertor performance is not very sensitive to the choice of radiating impurity.

Erosion lifetime of ITER divertor plates

Abstract For ITER divertor operation in the detached regime, erosion of divertor plates made of beryllium, carbon fibre composites and tungsten-rhenium alloys is evaluated. Erosion due to (a) physical and, for CFC, chemical sputtering (including surface temperature variation over the lifetime because of neutron irradiation), (b) disruption thermal quench using published data and (c) evaporation and melt layer loss (metals) for infrequent high-power transients (20 MW/m 2 , up to 10 s) is evaluated both with and without the reduction of net incident power by radiation from the evaporated impurities (‘low density vapour shield’). The composite lifetime is calculated for normal operation (detached, 10% transients, 10% disruptions); the effect of an increase in transient frequency and in incident power (semi-attached operation) is determined. The resulting erosion lifetime excludes beryllium, whereas both CFC and W-3%Re yield acceptable lifetimes.

Effect of conditions for gas recirculation on divertor operation in ITER

The latest results of B2-Eirene modelling of ITER divertor operation are presented. The operational window is further explored with an improved model of the neutral transport, the effect of gas leaks between the divertor cassettes is assessed and the sensitivity of the results to the features of the divertor geometry and to the gas puffing arrangement is analysed. The presence of neutral–neutral collisions in the model makes the results rather insensitive to the detail of the divertor shape. The analysis shows that the effect of the gas leak on the divertor performance in ITER is weak and therefore inter-cassette sealing may be not critical.

2D modelling of radiating divertor regime for ITER

Abstract A modelling study of the ITER divertor is done using the B2-Eirene code package with full multi-species impurity treatment and volumetric recombination of deuterium ions. A ‘virtual target regime’ is identified, for which strong recombination and ionization form a double layer which acts like a target surface inside the divertor plasma. The power loads, erosion, and helium removal seem to be within the acceptable range for the ITER design, making this regime, together with operational points close to it, a reasonable starting point for the optimization of the ITER divertor.

Critical issues in divertor optimisation for ITER–FEAT

Abstract A strong effect of the shape of the divertor floor near the separatrix strike point on power loading of the divertor targets in ITER–FEAT is found in the modelling studies. A similar effect has been demonstrated in JET experiments with Mark I and Mark II divertors (A. Loarte, Nucl. Fusion 38 (1998) 587; R. Monk, et al., in: Proceedings of the 24th EPS Conference on Controlled Fusion and Plasma Physics, Berchtesgaden, 1997, vol. 21A, p. 117) thus providing experimental verification of the modelling results. An operational window in six-dimensional phase space is found and different controls, like pumping speed or impurity seeding, are considered for ITER–FEAT.

Comparison of B2-EIRENE calculations with multi-machine experimental measurements

The B2-EIRENE code has been compared with plasma edge measurements from Alcator C-MOD, DIII-D, JET, JT-60U in L-mode and H-mode regimes. Similarities in L-mode discharges between JET and JT-60U are well reproduced by the code as well as the sudden approach to detachment seen in JT-60U when a 1% chemical sputtering yield for carbon is assumed. Comparisons between similar L-mode regimes in JET, JT-60U and Alcator C-MOD reveal the larger importance of anomalous energy convection in Alcator C-MOD with regard to larger tokamaks. H-mode discharges of DIII-D and JET can be satisfactorily modelled with similar values of the transport coefficients (approximately an order magnitude smaller than in L-mode) when the power flow into the scrape-off layer (SOL) is dominated by the ion channel, as previous EDGE2D simulations for JET suggested.

Operating window for high divertor radiation in ITER

The operating parameters of the ITER divertor are varied around the partially attached operating point of Refs. [A.S. Kukushkin et al., Berchtesgaden, 1997; A.S. Kukushkin et al., Contributions to Plasma Physics 38 (1998) 20], to obtain an operating window with low heat load, acceptable He pumping and high divertor radiation. Lower heat diffusivities lead to higher heat loads which can be compensated by somewhat lower input power. The divertor geometry is varied: longer domes lead to higher heat loads, whereas the case without dome exhibits poorer helium pumping. Shorter divertor length cases show higher heat load and little change in helium pumping.