The major increase in discharge duration and plasma energy in a next step DT fusion reactor will give rise to important plasma-material effects that will critically influence its operation, safety and performance. Erosion will increase to a scale of several centimetres from being barely measurable at a micron scale in today's tokamaks. Tritium co-deposited with carbon will strongly affect the operation of machines with carbon plasma facing components. Controlling plasma-wall interactions is critical to achieving high performance in present day tokamaks, and this is likely to continue to be the case in the approach to practical fusion reactors. Recognition of the important consequences of these phenomena stimulated an internationally co-ordinated effort in the field of plasma-surface interactions supporting the Engineering Design Activities of the International Thermonuclear Experimental Reactor project (ITER), and significant progress has been made in better understanding these issues. The paper reviews the underlying physical processes and the existing experimental database of plasma-material interactions both in tokamaks and laboratory simulation facilities for conditions of direct relevance to next step fusion reactors. Two main topical groups of interaction are considered: (i) erosion/redeposition from plasma sputtering and disruptions, including dust and flake generation and (ii) tritium retention and removal. The use of modelling tools to interpret the experimental results and make projections for conditions expected in future devices is explained. Outstanding technical issues and specific recommendations on potential R&D avenues for their resolution are presented.

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Plasma{material interactions in current tokamaks and their implications for next step fusion reactors

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.

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Chapter 4: Power and particle control

The physics of divertors in tokamaks is reviewed, primarily from an experimental point of view, although where possible simple analytic modelling is included. The paper covers the four main subject areas at issue in divertor research: (1) the wide dispersal of plasma power exhausted from the main plasma, (2) the production of sufficiently high gas pressures in the vicinity of pump ducts to enable the removal of fuel and helium (`ash') gas from the system, (3) the elimination or reduction of impurity production and (4) the screening of impurities produced, or intentionally added, at the plasma boundary from the plasma core. A simple analytic model, the `two-point' model, is introduced early in the paper and provides a framework for comparison of the experimental results, drawn from many machines, with simply derived expectations. Conclusions regarding the direction of future research priorities are made.

https://iopscience.iop.org/article/10.1088/0741-3335/39/6/001/pdf

Experimental divertor physics

Positive muons implanted into diamond and zincblende-structured semiconductors often form hydrogen-like paramagnetic muonium (${\ensuremath{\mu}}^{+}\ensuremath{-}{e}^{\ensuremath{-}}$) states whose characteristics can be investigated with the "muon spin rotation" ($\ensuremath{\mu}\mathrm{SR}$) technique. In contrast to the case of hydrogen, which is not known to form a paramagnetic state in semiconductors, two coexisting types of muonium states are seen. "Mu" with a large isotropic hyperfine interaction, and "${\mathrm{Mu}}^{*}$" with a small [111]-axially symmetric hyperfine interaction. Both "spectroscopic" properties of these states, such as the electronic $g$ factors and the nuclear hyperfine interactions, and "dynamic" properties, such as their diffusion rates and their rates of interconversion, are accessible with $\ensuremath{\mu}\mathrm{SR}$. Direct information about the site of the muonium states is available using the channeling effect of the positron from muon decay in a crystalline host. The techniques for probing semiconductors with positive muons are described in this review, and the results they have provided to date are critically discussed. The considerable amount of theoretical work that has been invested in microscopic models of Mu and ${\mathrm{Mu}}^{*}$ is also summarized.

Muonium states in semiconductors

Theories of the electric field effect on toroidal plasma confinement are reviewed with the emphasis placed on recent progress in the areas of anomalous transport, structural formation and bifurcation, research which has been motivated by the discovery of improved confinement. Topics include single-particle physics, such as particle orbits or collisional transport, turbulent transport, transport matrix, structural formation and dynamics, bifurcation, and improved confinement.

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The role of the electric field in confinement

A method for the determination of the depth distribution of light elements in heavy materials is described. It involves the detection of light elements recoiling under the bombardment by a 35Cl beam. A resolution of 300 A was achieved for the lithium present in a thin sample. The measures were done with layers of 1016 atoms/cm2 and it is estimated that quantities as small as 1014 atoms/cm2 can be located without much difficulty.

B. Terreault

Papers

An accurate and sensitive method for the determination of the depth distribution of light elements in heavy materials

The use of 6Li and 35Cl ion beams in surface analysis

XPS study of the process of oxygen gettering by thin films of PACVD boron

Hydrophobic fluorinated carbon coatings on silicate glaze and aluminum

Preliminary observations of blistering of niobium by 1–15 keV helium ions