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

The understanding and predictive capability of transport physics and plasma confinement is reviewed from the perspective of achieving reactor-scale burning plasmas in the ITER tokamak, for both core and edge plasma regions. Very considerable progress has been made in understanding, controlling and predicting tokamak transport across a wide variety of plasma conditions and regimes since the publication of the ITER Physics Basis (IPB) document (1999 Nucl. Fusion 39 2137-2664). Major areas of progress considered here follow. (1) Substantial improvement in the physics content, capability and reliability of transport simulation and modelling codes, leading to much increased theory/experiment interaction as these codes are increasingly used to interpret and predict experiment. (2) Remarkable progress has been made in developing and understanding regimes of improved core confinement. Internal transport barriers and other forms of reduced core transport are now routinely obtained in all the leading tokamak devices worldwide. (3) The importance of controlling the H-mode edge pedestal is now generally recognized. Substantial progress has been made in extending high confinement H-mode operation to the Greenwald density, the demonstration of Type I ELM mitigation and control techniques and systematic explanation of Type I ELM stability. Theory-based predictive capability has also shown progress by integrating the plasma and neutral transport with MHD stability. (4) Transport projections to ITER are now made using three complementary approaches: empirical or global scaling, theory-based transport modelling and dimensionless parameter scaling (previously, empirical scaling was the dominant approach). For the ITER base case or the reference scenario of conventional ELMy H-mode operation, all three techniques predict that ITER will have sufficient confinement to meet its design target of Q = 10 operation, within similar uncertainties.

https://iopscience.iop.org/article/10.1088/0029-5515/47/6/S02/pdf

Chapter 2: Plasma confinement and transport

The use of the compressional hydromagnetic mode (also called the magnetosonic or, simply, the fast wave) is examined in some detail with respect to the heating of a tritium plasma containing a few percent deuterium. Efficient absorption of wave energy by the deuteron component is found when ω = ωC (deuterons), with Qwave 100. Reasonable efficiencies are found also for electron heating, but coherence effects between transit-time and Landau damping for electrons reduce the total absorption for both processes to one-half of the transit-time power, calculated separately.The fusion output of a two-component neutral-injected plasma can be enhanced by selective heating of the injected deuterons. Also, selective deuteron absorption may be used for ion-tail creation by radiofrequency excitation alone, as an alternative to neutral injection. The dominant behaviour of the high-energy deuteron distribution function is found to be f(v) ~ exp[(3/2)∫vdv / ], where is the Chandrasekhar-Spitzer drag coefficient, and is the Kennel-Engelmann quasi-linear diffusion coefficient for wave-particle interaction at the deuteron cyclotron frequency. An analytic solution to the one-dimensional Fokker-Planck equation, with r.f.-induced diffusion, is developed, and using this solution together with Duane's fit to the D-T fusion cross-section, it is found that the nuclear-fusion power output from an r.f.-produced two-component plasma can significantly exceed the incremental (radiofrequency) power input.

Fast-wave heating of a two-component plasma

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

Internal transport barriers in tokamak plasmas are explored in order to improve confinement and stability beyond the reference scenario, used for the ITER extrapolation, and to achieve higher bootstrap current fractions as an essential part of non-inductive current drive. Internal transport barriers are produced by modifications of the current profile using external heating and current drive effects, often combined with partial freezing of the initial skin current profile. Thus, formerly inaccessible ion temperatures and QDTeq values have been (transiently) achieved. The present paper reviews the state of the art of these techniques and their effects on plasma transport in view of optimizing the confinement properties. Implications and limits for possible steady state operations and extrapolation to burning plasmas are discussed.

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Internal transport barriers in tokamak plasmas

A beam of injected fast atomic hydrogen presents a superb probe for hot fusion plasmas. The neutral particles experience excitation and ionization by collisions with electrons and ions as they penetrate into a plasma. The emitted characteristic line radiation is Doppler shifted and the spectral lines are split due to motional Stark fields. Measurements of wavelength, intensity and polarization of the Balmer- alpha emission reveal information about the neutral beam, such as beam attenuation, beam-geometry, beam-divergence and species mix. Local pitch angles and toroidal fields can be derived from the simultaneous measurement of the polarization pattern and the wavelength separation of the Stark multiplet. The implementation and application of beam emission spectroscopy as a quantitative diagnostic tool on the Joint European Torus (JET) experiment is reviewed.

http://iopscience.iop.org/article/10.1088/0741-3335/35/10/003/pdf

Beam emission spectroscopy as a comprehensive plasma diagnostic tool

Recent developments and results of the JET CXRS diagnostic are reported. The measurements of radial profiles of ion temperatures and densities are based on CXR spectra of fully stripped ions of either carbon or beryllium. Considerable effort has been expended in ensuring consistency between radial profiles of low Z impurity densities and those from other diagnostics. The contributions of the main light impurities are used to reconstruct radial profiles of Zeff which can be compared with Abel‐inverted signals from visible bremsstrahlung or soft x‐ray emission. Active Balmer‐Alpha spectroscopy (ABAS) is being introduced as a diagnostic tool providing data on local magnetic fields, neutral beam densities, and dilution factors. The effects of collision‐energy‐dependent CXR cross sections on observed CXR spectra are calculated. Corrections for the values of deduced ion temperatures, toroidal velocities, and impurity densities are discussed for the case of plasmas with high ion temperatures and high toroidal rotation velocities. Some recent results of the JET 1989 operation illustrating the CXRS diagnostic potential are given.

Visible charge exchange spectroscopy at JET

An analytical procedure is presented which enables a fast estimate of collision-energy-dependent cross-section effects on thermal charge exchange spectra. The model is based both on experimental evidence and numerical simulations showing that the observed charge exchange (CX) spectra are essentially Gaussian in their shape. The collision-energy-dependent emission rate leads effectively to a lineshift (apparent velocity), usually to a reduction in linewidth (apparent temperature), and to a change in the effective emission rate averaged over the entire thermal velocity distribution function. It is demonstrated that the cross-section effect can be treated analytically introducing an approximated emission rate factor which retains the characteristics of a Maxwellian velocity distribution function using an exponential expression with only linear and quadratic velocity terms in its exponent. An algebraic deconvolution procedure is described, which enables the reconstruction of true temperature, velocity and intensities from measured CX spectra. Examples taken from a recent JET experimental campaign are used to illustrate the cross-section effects on low-Z impurity CX spectra for a comprehensive variety of neutral beams (deuterium, tritium or helium), target densities, temperatures and toroidal rotation speeds. An overview is given of representative correction factors established for high-power, high-temperature plasmas, as well as for plasmas with combined neutral beam and radiofrequency heating, and for the case of locked modes.

http://iopscience.iop.org/article/10.1088/0741-3335/37/2/002/pdf

Analytical approximation of cross-section effects on charge exchange spectra observed in hot fusion plasmas

Observations are reported of resolved components of deuterium Balmer emission in neutral beam heated JET plasmas. The features are interpreted as Doppler-shifted emission from deuterium in the neutral beams spectrally resolved into components by the nu *B motional Stark electric field. The mechanisms of excitation of the deuterium are elucidated and the emission modelled. Attention is drawn to the diagnostic potential of the observations for measurement of local magnetic fields, deuteron density and effective plasma ion charge.

Observations of motional Stark features in the Balmer spectrum of deuterium in the JET plasma

The first experimental results are reported of anisotropic slowing-down features observed in JET helium beam fuelling experiments. Two independent observation ports, one with a view perpendicular to the magnetic field in the centre of the plasma and a second multichord viewing arrangement, approximately tangential to the toroidal field, provide radially and temporally resolved information on the velocity distribution function comprising the populations of both fast and thermalized alpha particles. The fuelling process is characterized by a change-over from a distinctly non-Maxwellian distribution function to a dominantly Maxwellian distribution and also by a broadening of the deduced fast ion density radial profile. The fast particle component in the observed composite charge exchange spectrum is found to be in excellent agreement with predictions are based on anisotropic velocity distribution functions obtained from the analytical solution of the neutral injection Fokker-Planck equation. Signal-to-noise levels in the measurement of fast alpha particle in the JET helium fuelling campaign are extrapolated to thermonuclear-fusion alpha particle density levels expected for the D-T phase of JET. It is shown that beam penetration and not competing continuum radiation is a major constraint, and that acceptable (hydrogen or helium) neutral beam power and energy requirements promise a feasible CX alpha particle diagnosis in the core of next-step devices such as ITER.

https://iopscience.iop.org/article/10.1088/0741-3335/35/7/002/pdf

W. Mandl

Papers

Beam emission spectroscopy as a comprehensive plasma diagnostic tool

Visible charge exchange spectroscopy at JET

Analytical approximation of cross-section effects on charge exchange spectra observed in hot fusion plasmas

Observations of motional Stark features in the Balmer spectrum of deuterium in the JET plasma

Observation of alpha particle slowing-down spectra in JET helium beam fuelling and heating experiments