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.

/pdf/plasma-material-interactions-in-current-tokamaks-and-their-1a6iwv89x6.pdf

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

Polycrystalline Nb samples were bombarded by 3–15‐keV proton beams at normal incidence. The energy and angular distributions of backscattered positively and negatively charged particles were measured quantitatively. By means of the charged fractions measured previously, the energy and angular distributions of the total number of backscattered particles including the neutrals were obtained. Integration of the distributions over all energies and angles yield the total reflection coefficients. These are considerably lower than existing theoretical values.

Total backscattering yields of 3-15-keV protons from polycrystalline Nb

The plasma wall interactions for two extreme cases, the ‘vacuum model’ and the ‘cold gas blanket’ are outlined. As a first step for understanding the plasma wall interactions the elementary interaction processes at the first wall are identified. These are energetic ion and neutral particle trapping and release, ion and neutral backscattering, ion sputtering, desorption by ions, photons and electrons and evaporation. These processes have only recently been started to be investigated in the parameter range of interest for fusion research. The few measured data and their extrapolation into regions not yet investigated are reviewed.

Plasma-Wall Interactions

Inhomogeneities in Tokamak peripheral plasmas due to localized recycling of neutrals (plasma fuelling) are considered. A dense cloud of cold charged particles arises in the vicinity of such a localized particle source, for example a gas-puffing valve, as the plasma density exceeds a certain critical level. A dependence of the critical density on such parameters as dimensions of the device, heat flux from the discharge core and diffusivity of charged particles at the edge is found. The results of calculation are compared with experimental data.

The possible nature of the localized recycling effect on the plasma edge in Tokamaks

In laser welding with high laser intensities a hole is formed in the material, the so-called keyhole. In this keyhole a plasma is detected, which is characterized by high charge densities and high pressure as well as being influenced by the boundary of the keyhole. The authors describe the transport of the laser power into the material by the plasma for a CO2 laser and iron. They set up a system of stationary transport equations for the ions, electrons and non-excited neutrals, which contain convective transport and heat flux. The boundary conditions take into account the evaporation of neutral atoms from the hot keyhole surface and the transition from the quasineutral presheath plasma to the sheath plasma. With the incident laser power as input parameter the authors obtain the radial variation of the hydrodynamic variables and quantities like the degree of ionization, the electron temperature and the fraction of absorbed laser power.

http://iopscience.iop.org/article/10.1088/0022-3727/26/11/035/pdf

A transport theoretical model of the keyhole plasma in penetration laser welding

Starting from the Boltzmann equation for the neutral gas in a plasma, an integral equation for the density of the neutral gas is derived. Inelastic collisions are taken into account only as charge exchange and ionisation. The integral equation is solved numerically. It is shown that the reflection mechanism of neutral particles from the wall modifies the density profile. The distribution function of the charge exchange neutrals deviates from a Maxwellian, and for fast particles (v>vth) we obtain f0 approximately v-1exp(-(v2/vi)). The total particle and energy fluxes on the wall are also calculated.

/pdf/a-kinetic-model-for-the-neutral-gas-between-plasma-and-wall-1y36ybekvt.pdf

A kinetic model for the neutral gas between plasma and wall

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A kinetic model for the neutral gas between plasma and wall