C. Grisolia

Plasma-Wall Interactions in ITER

Abstract This paper reviews the status of the design of the divertor and first-wall/shield, the main in-vessel components for ITER. Under nominal ignited conditions, 300 MW of alpha power will be produced and must be removed from the divertor and first-wall. Additional power from auxiliary sources up to the level of 100 MW must also be removed in the case of driven burns. In the ignited case, about 100 MW will be radiated to the first wall as bremsstrahlung. Allowing the remaining power to be conducted to the divertor target plates would result in excessive heat fluxes. The power handling strategy is to radiate an additional 100–150 MW in the SOL and the divertor channel via a combination of radiation from hydrogen, and intrinsic and seeded impurities. Vertical targets have been adopted for the baseline divertor configuration. This geometry promotes partial detachment, as found in present experiments and in the results of modelling runs for ITER conditions, and power densities on the target plates can be ≤ 5 MW / m 2 . Such regimes promote relatively high pressure (> 1 Pa ) in the divertor and even with a low helium enrichment factor of 0.2, the required pumping speed to pump helium is ≤ 50 m 3 / s . An important physics question is the quality of core confinement in these attractive divertor regimes. In addition to power and particle handling issues, the effects of disruptions play a major role in the design and performance of in-vessel components. Both centered disruptions and VDEs produce stresses in the first-wall/shield modules, backplate and the divertor wings and cassettes that are near or even somewhat in excess of allowables for normal operation. Also plasma-wall contact from disruptions, including at the divertor target, together with material properties are major factors determining component lifetime. Considering the potential for impurity contamination and minimizing tritium inventory as well as thermomechanical performance, the present material selection calls for carbon divertor targets near the strike point, tungsten on the rest of the target and on the baffle where the charge-exchange flux could be high, and beryllium elsewhere. All three materials and relevant joining techniques are being developed in the R&D program and the final selection for the first assembly will be made at the end of the EDA.

Progress in ergodic divertor operation on Tore Supra

Upgrade of the Tore Supra ergodic divertor (ED) has led to significant progress in ED physics. Pulse durations of 30?s with LHCD have been achieved demonstrating the heat exhaust capability of both the actively cooled technology at hand and of this specific divertor concept. The disruptive limit governed by the stochastization of the outer magnetic surfaces is found to occur for a value of the Chirikov parameter reaching two on the magnetic surface q = 2+(3/12). This experimentally observed robustness allows one to operate at very low safety factor on the?separatrix (q~2). Numerical analysis of ballooning turbulence in a stochastic layer indicates that the decay of the density fluctuations is associated with an increase of the fluctuating electric drift velocity. This results in an enhanced cross-field transport in the vicinity of the target plates. This lowering of confinement appears to be compensated by an intrinsic transport barrier on the electron temperature. The three-dimensional response of the temperature field is computed with a fluid code. The code can recover the intrinsic transport barrier at the separatrix, reported experimentally, together with small amplitude temperature modulations in the divertor volume. Experimental evidence for the three density regimes (linear, high recycling and detachment) is reported. The low critical density values for transitions between these regimes indicate that similar parallel physics governs the axisymmetric and ED, despite the open configuration of the latter. Measurement and understanding of these density regimes provide a means for feedback control of plasma density and an improvement in ion cyclotron radiofrequency heating coupling scenarios. Experimental data also indicated that particle control with the vented target plates is effective. Increase of both impurity control and radiation efficiency are reviewed. Global power balance has been analysed in order to account for non-axisymmetric radiation. These results, taken together, confirm the large radiation capability of the ED.

Wall conditioning technique development in Tore Supra with permanent magnetic field by ICRF wave injection

Abstract Wall conditioning techniques tokamaks with a permanent magnetic field have been performed in Tore Supra by using an ion cyclotron range frequency (ICRF) facility. Plasmas have been produced by injection of ICRF power in the range from 40 kW to 350 kW either in helium or deuterium gas. Electron density in the range of 1 · 10 17 to 6 · 10 17 m −3 and electron temperatures from 1.5 to 8 eV have been measured depending on the gas pressure and injected power. Energetic neutral atoms of hydrogen and deuterium with energies up to 50 keV have been produced. High hydrogen removal rates have been obtained in helium discharges, either in a continuous or pulsed operation mode.

Transport in the plasma edge and specific connexion to the wall in the Tore Supra ergodic divertor experiments

The ergodic divertor experiments in Tore Supra can be analysed along two main lines. The first one refers to the change of the heat and particle transport in the ergodized zone. This is especially true for the electron heat transport which is enhanced in the edge layer. But other distinctive features give evidence of the importance of the parallel connexion length between the plasma edge and the wall. The field lines, which are stochastic in the major part of the perturbed layer (10–15 cm) are such that, in the outermost layer (3 cm), the connexion topology is regular. This has obvious effects on the particle and power deposition, but also on the plasma parameters, and consequently influences the particle recycling and impurity shielding processes. The Tore Supra ergodic divertor experiments are reviewed in this framework.

WEST Physics Basis

With WEST (Tungsten Environment in Steady State Tokamak) (Bucalossi et al 2014 Fusion Eng. Des. 89 907-12), the Tore Supra facility and team expertise (Dumont et al 2014 Plasma Phys. Control. Fusion 56 075020) is used to pave the way towards ITER divertor procurement and operation. It consists in implementing a divertor configuration and installing ITER-like actively cooled tungsten monoblocks in the Tore Supra tokamak, taking full benefit of its unique long-pulse capability. WEST is a user facility platform, open to all ITER partners. This paper describes the physics basis of WEST: the estimated heat flux on the divertor target, the planned heating schemes, the expected behaviour of the L-H threshold and of the pedestal and the potential W sources. A series of operating scenarios has been modelled, showing that ITER-relevant heat fluxes on the divertor can be achieved in WEST long pulse H-mode plasmas.

Plasma wall interaction during long pulse operation in Tore Supra

Increase of plasma density observed during long pulse operation is reported. The higher the injected and radiated energies are, the faster the increase of density appears. This behaviour correlated with oxygen and hydrogen plasma density increase is attributed to water desorption induced by wall heating due to the high radiated energy of the plasma. These surfaces are far from plasma and are not baked during the conditioning procedures usually used at Tore Supra. Conditioning evidence is seen during discharge series at high injected energy. But this improvement is prevented if disruptions occur in between.

Experiments on steady state particle control in Tore Supra and DIII-D

Particle control is playing an increasingly important role in tokamak plasma performance. The present paper discusses particle control of hydrogen/deuterium by wall pumping on graphite or carbonized surfaces, as well as by external exhaust with pumped limiters and pumped divertors. Wall pumping is ultimately a transient effect and by itself not suitable for steady state particle exhaust. Therefore, external exhaust techniques with pumped divertors and limiters are being developed. How wall pumping phenomena interact and correlate with these inherently steady state, external exhaust techniques, is not well known to date. In the present paper, the processes involved in wall pumping and in external pumping are investigated in an attempt to evaluate the effect of external exhaust on wall pumping. Some of the key elements of this analysis are: (1) charge-exchange fluxes to the wall play a crucial role in the core-wall particle dynamics, (2) the recycling fluxes of thermal molecules have a high probability of ionization in the scrape-off layer, (3) thermal particles originating from the wall, which are ionized within the scrape-off layer, can be directly exhausted, thus providing a direct path between wall and exhaust which can be used to control the wall inventory. This way, the wall can be kept in a continuous pumping state in the sense that it continuously absorbs energetic particles and releases thermal molecules which are then removed by the external exhaust mechanism. While most of the ingredients of this analysis have been observed individually before, the present evaluation is an attempt to correlate effects of wall recycling and external exhaust.

Plasma edge control in TORE SUPRA

TORE SUPRA is a large superconducting tokamak designed for sustaining long inductive pulses (t approximately 30 s). In particular, all the first wall components have been designed for steady-state heat and particle exhaust, particle injection, and additional heating. In addition to these technological assets, a strict control of the plasma-wall interactions is required. This has been done at low power: experiments with ohmic heating have been mainly devoted to the pump limiter, ergodic divertor and pellet injection experiments. Some specific problems arising in large tokamaks are encountered; the pump limiter and the ergodic divertor yield the expected effects on the plasma edge. The effects on the bulk are discussed.

Temporal evolution and spatial distribution of dust creation events in Tore Supra and in ASDEX Upgrade studied by CCD image analysis

Images of wide-angle visible standard CCD cameras contain information on dust creation events (DCEs) that occur during plasma operations. Analysing the straight line-like dust traces in the shallow volume of scrape-off layer along the vacuum vessel, caused by plasma–dust interaction, the database on the DCEs is built. The database provides short/long term temporal evolution and spatial distribution of origins of DCEs in fusion devices. We have studied the DCEs of CIMES (2006) and DITS (2007) Tore Supra (TS) campaigns, and the DCEs of the 2007 ASDEX Upgrade (AUG) campaign. The results from the TS CIMES campaign show different patterns of DCEs meaning different plasma–wall interaction depending on power coupling. The TS DITS campaign indicates that dusts may be an operational limit if a fixed plasma operation scenario is used repeatedly. Different behaviours of DCEs between the carbon limiter machine and the full tungsten divertor machine are found, which is important for next generation fusion machines like ITER.

Plasma wall particle balance in Tore Supra

A comprehensive study of the particle balance between the carbon wall and the plasma is presented. One finds that the effective particle content of the wall which governs the plasma equilibrium density departs from the deposited number of particles. This effect is dominant for the fully desaturated wall. A scaling law of the plasma density in terms of the wall effective particle content has been obtained. Moreover, the experimental data allows to estimate the plasma particle confinement time. Values ranging from 0.2 to 0.5 s are found depending on the density. An analytical functional dependence of the particle confinement time is obtained.