V. Philipps

Chapter 4: Power and particle control

Progress, since the ITER Physics Basis publication (ITER Physics Basis Editors et al 1999 Nucl. Fusion 39 2137–2664), in understanding the processes that will determine the properties of the plasma edge and its interaction with material elements in ITER is described. Experimental areas where significant progress has taken place are energy transport in the scrape-off layer (SOL) in particular of the anomalous transport scaling, particle transport in the SOL that plays a major role in the interaction of diverted plasmas with the main-chamber material elements, edge localized mode (ELM) energy deposition on material elements and the transport mechanism for the ELM energy from the main plasma to the plasma facing components, the physics of plasma detachment and neutral dynamics including the edge density profile structure and the control of plasma particle content and He removal, the erosion of low- and high-Z materials in fusion devices, their transport to the core plasma and their migration at the plasma edge including the formation of mixed materials, the processes determining the size and location of the retention of tritium in fusion devices and methods to remove it and the processes determining the efficiency of the various fuelling methods as well as their development towards the ITER requirements. This experimental progress has been accompanied by the development of modelling tools for the physical processes at the edge plasma and plasma–materials interaction and the further validation of these models by comparing their predictions with the new experimental results. Progress in the modelling development and validation has been mostly concentrated in the following areas: refinement in the predictions for ITER with plasma edge modelling codes by inclusion of detailed geometrical features of the divertor and the introduction of physical effects, which can play a major role in determining the divertor parameters at the divertor for ITER conditions such as hydrogen radiation transport and neutral–neutral collisions, modelling of the ion orbits at the plasma edge, which can play a role in determining power deposition at the divertor target, models for plasma–materials and plasma dynamics interaction during ELMs and disruptions, models for the transport of impurities at the plasma edge to describe the core contamination by impurities and the migration of eroded materials at the edge plasma and its associated tritium retention and models for the turbulent processes that determine the anomalous transport of energy and particles across the SOL. The implications for the expected performance of the reference regimes in ITER, the operation of the ITER device and the lifetime of the plasma facing materials are discussed.

Boronization in textor

Abstract The liner and limiters of TEXTOR have been coated in situ with a boron containing carbon film using a RG discharge in a throughflow of 0.8 He + 0.1 B2H6 +0.1 CH4. The average film thickness was 30–50 nm, the ratio of boron and carbon in the layer was about 1:1 according to Auger Electron Spectroscopy. Subsequent tokamak discharges are characterized by a small fraction of radiated power ( OVI/ n e of the OVI intensity normalized to the averaged plasma density n e decreases by more than a factor of four. The decrease in the oxygen content manifests itself also as a reduction of the CO and CO2 partial pressures measured during and after the discharge with a sniffer probe. The carbon levels are reduced by a factor of about two as measured by the normalized intensity CII/ n e of the CII line and via the ratio of the C fluxes and deuterium fluxes measured at the limiter (CI/Dα). The wall shows a pronounced sorption of hydrogen from the plasma, easing the density control and the establishment of low recycling conditions. The beneficial conditions did not show a significant deterioration during more than 200 discharges, including numerous shots at ICRH power levels > 2 MW.

Simulation of the plasma-wall interaction in a tokamak with the Monte Carlo code ERO-TEXTOR

The interaction of plasma with the walls has been one of the critical issues in the development of fusion energy research. On the one hand, plasma induced erosion can seriously limit the lifetime of the wall components, while, on the other hand, eroded particles can be transported into the core plasma where they lead to dilution of the fusion plasma and to energy losses due to radiation. Low-Z wall materials induce only small radiation losses in the plasma core but suffer from large physical sputtering rates. Carbon based materials in addition suffer from chemically induced erosion. High-Z wall materials show significantly smaller erosion but lead to large radiation losses. One of the main goals of present plasma-wall studies is to find a special choice of wall materials for steady state plasma scenarios that will provide an optimum with respect to fuel dilution, radiation losses, wall lifetime and fuel inventory in the walls. To obtain a better understanding of the processes and to estimate the plasma-wall interaction behaviour in future fusion devices the 3-D Monte Carlo code ERO-TEXTOR, based originally on the ERO code, has been developed. It models the plasma-wall interaction and transport processes in the vicinity of a surface positioned in the boundary layer of TEXTOR. The main aim is to simulate the erosion and redeposition behaviour of different wall materials under various plasma conditions and to compare this with experimental results. This contribution describes the main features of the ERO-TEXTOR code and gives some examples of simulation calculations to illustrate the application of the code.

Chemical erosion of amorphous hydrogenated carbon films by atomic and energetic hydrogen

The chemical erosion of amorphous hydrogenated carbon films by thermal and energetic hydrogen (deuterium) impact has been studied using atomic and ion beam techniques. The samples were produced during the routine carbonization procedure in TEXTOR as well as in a simulation vessel. In the reaction of thermal hydrogen atoms with these films the radical CH3 is formed accompanied by a wide spectrum of higher hydrocarbons. The overall erosion rate at 200°C is 4 × 10−2 eroded C/H and the temperature dependence of the erosion is similar to that of the reaction of hydrogen ions with graphite. Only few mixed molecules are formed when deuterium instead of protium atoms are used in the reaction on hydrogenated carbon films. Energetic hydrogen ions react with these films forming predominantly CH4 with smaller contributions of higher hydrocarbons. The temperature dependence of the erosion is similar to that of the reaction with thermal atoms. The overall reaction rate is only about twice that of thermal hydrogen exposure. These results agree reasonably well with measurements on the erosion of carbonized TEXTOR surfaces by particles from a RG-glow discharge in hydrogen.

Overview of the ITER-like wall project

Work is in progress to completely replace, in 2008/9, the existing JET CFC tiles with a configuration of plasma facing materials consistent with the ITER design. The ITER-like wall (ILW) will be cr ...

Hydrocarbon formation in the reaction of atomic hydrogen with pyrolytic graphite and the synergistic effect of argon ion bombardment

The reactions of atomic hydrogen with pyrolytic graphite have been investigated by an atomic beam technique. Up to temperatures of 800 K atomic hydrogen reacts with graphite forming CH4 and C2-compounds with a reaction probability of about 2 × 10−4. Above 1000 K no hydrocarbons have been found. A hydrogen atom exposure of the graphite with simultaneous bombardment by Ar+ ions drastically enhanced the hydrocarbon formation up to a factor of 100 exhibiting a characteristic temperature behaviour with a maximum at about 800 K. As main reaction product CH3 is formed together with some CH4 and C2-compounds. When the Ar+ beam is turned off the enhancement of the reaction probability is only slowly decreasing with further hydrogen bombardment. In contrast to ion bombardment no significant influence of simultaneous electron irradiation on hydrocarbon formation has been observed.

First tests of molybdenum mirrors for ITER diagnostics in DIII-D divertor

Metallic mirrors will be used in ITER for optical diagnostics working in different spectral ranges. Their optical properties will change with time due to erosion, deposition, and particle implantation. First tests of molybdenum mirrors were performed in the DIII-D divertor under deposition-dominated conditions. Two sets of mirrors recessed 2cm below the divertor floor in the private flux region were exposed to a series of identical, lower-single-null, ELMing (featuring edge localized modes) H-mode discharges with detached plasma conditions in both divertor legs. The first set of mirrors was exposed at ambient temperature, while the second set was preheated to temperatures between 140 and 80°C. During the exposures mirrors in both sets were additionally heated by radiation from the plasma. The nonheated mirrors exhibited net carbon deposition at a rate of up to 3.7nm∕s and suffered a significant drop in reflectivity. Net carbon deposition rate on the preheated mirrors was a factor of 30–100 lower and their...

Plasma?surface interaction, scrape-off layer and divertor physics: implications for ITER

Recent research in scrape-off layer (SOL) and divertor physics is reviewed; new and existing data from a variety of experiments have been used to make cross-experiment comparisons with implications for further research and ITER. Studies of the region near the separatrix have addressed the relationship of profiles to turbulence as well as the scaling of the parallel power flow. Enhanced low-field side radial transport is implicated as driving parallel flows to the inboard side. The medium-n nature of edge localized modes (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. The predicted divertor power loads for ITER disruptions are reduced while those to main chamber plasma facing components (PFCs) increase. Disruption mitigation through massive gas puffing is successful at reducing PFC heat loads. New estimates of ITER tritium retention have shown tile sides to play a significant role; tritium cleanup may be necessary every few days to weeks. ITERs use of mixed materials gives rise to a reduction of surface melting temperatures and chemical sputtering. Advances in modelling of the ITER divertor and flows have enhanced the capability to match experimental data and predict ITER performance.

Hydrogen inventories in nuclear fusion devices

Hydrogen retention in tokamaks is due to implantation into plasma-facing materials and trapping in deposited layers. In the limiter tokamak TEXTOR-94 hydrogen-rich deposited layers with thicknesses up to 1 mm are observed on recessed parts of the limiters, areas perpendicular to the magnetic field in the scrape-off layer (SOL), neutralizer plates of the pumped limiter and inside the pumping ducts. In the divertor tokamak JET the main deposition is observed in the divertor, additional deposits are observed in the main chamber on the sides of the guard limiters. Codeposition of carbon ions with hydrogen is the major mechanism of layer growth at areas with direct plasma contact. At remote areas without direct plasma contact, sticking of neutral hydrocarbon radicals seems to play an important role for hydrogen trapping.

Differences in the CH3 and CH4 formation from graphite under bombardment with hydrogen ions and hydrogen atoms/argon ions

Abstract In the surface reactions on graphite induced by atomic hydrogen/ energetic ion irradiation the predominant reaction product is the radical CH3 whereas with H2+ bombardment mainly CH4 is formed. In both cases the temperature dependence is identical, indicating the same rate-determining steps in the reaction path. From the mass spectra of the reaction product on graphite by H0/D2+ irradiation it is concluded that the reaction by energetic hydrogen ions occurs at the inner surfaces whereas with irradiation with H0/energetic ions CH3 is formed at the surface. Both types of reaction are additive and do not mix. On the basis of these results a reaction mechanism is proposed.