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Melting of the surface of tungsten exposed to a pulsed electron beam has been simulated numerically. Comparison of the experimentally measured at BETA facility time dependence of the radius of the molten region with the calculated data has shown that the surface cooling caused by evaporation has a significant effect on the temperature distribution and melting of the material at sufficiently high densities of the surface heating power. This result validates the created theoretical model of the tungsten melting and evaporation in exposure to a pulsed electron beam. The studied mechanism of the limitation of the surface temperature is different from the well-studied vapor shielding. The presented model is a step to correct interpretation of the erosion caused by the melt motion and splashing in exposure to the ITER-relevant pulsed heating by electron beam.

A number of experimental studies of melt motion and droplet ejection caused by pulsed plasma load include the measurements of the shape of surface after the solidification of target. The measured shape may be different from the one during the heating stage because of melt motion. In present paper the evolution of this perturbations is treated as capillary waves on the melt surface. The dispersion relation for capillary waves taking into account viscosity and limited depth of liquid was used. The numerical estimations for the melt surface behavior are done for tungsten samples irradiated at BETA facility.

Damage of divertor tungsten tiles as a result of transient plasma processes is one of the key problems of the ITER tokamak. Even if the temperature of the divertor tiles is maintained below the melting threshold, their thermal deformation, accumulation of residual stresses and cracking will occur. Cracking reduces the resistance of the metal to subsequent loads, therefore, the study of this process is important. In materials science, non-destructive methods for measuring mechanical stresses are very limited. In this work, a new method is proposed that allows non-contact investigation of stresses in polycrystalline tungsten plates by their bending. A corresponding diagnostic system and theoretical model are implemented. A number of experiments were carried out using this diagnostic system at the BETA facility at the Budker Institute to study the dynamics of tungsten plates bending with different thickness, subjected to pulsed heating of different intensities. The experimental results showed the qualitatively expected process of the plate bending, as well as good agreement with the theoretical model. This proved that the method described in the work is promising for use in order to study the residual stresses in a pulse-heated metal sample.

The review presents experimental studies of plasma-surface interaction and materials behavior under plasma loads done in the multiple-mirror trap of the GOL-3 facility. In the experiments for the PSI, the energy density in the extracted plasma stream varies from 0.5 to 30 MJ/m2. Parameters of near-surface plasma measured by a set of diagnostics are reviewed. Surface patterns of targets exposed to the plasma are analyzed. The erosion depth depends on the energy loads—it rises from 0 to 600 μm at 0.5 and 30 MJ/m2, correspondingly. Cracking and evolution of graphite and tungsten surface morphology are discussed. The enthalpy of brittle destruction of graphite (10 kJ/g), which determines the threshold of bulk damage of targets irradiated with a charged-particle flux with large penetration depth, was determined. Comparison of different facilities for PSI studies are presented. Heat flux play a key role to the target surface erosion.

Experiments on the effect of fast heat loads on the surface of tungsten were carried out on the BETA facility at the Budker Institute. Tungsten samples were uniformly heated by an electron beam with a heat flux factor below the melting threshold. During and shortly after exposure, the 2D surface temperature distribution was measured, as well as the temperature history on selected surface areas. Active diagnostics using the scattering of CW laser light on a surface exposed by the electron beam allowed us to monitor the damage dynamics. At the heating stage, an increase in the surface roughness occurred, caused by inhomogeneous elastic and plastic deformations of the heated layer. As the cooling progressed, the residual plastic deformations remained. Simultaneously with the modification of the surface, bending of samples with a thickness of 3-4 mm occurred. The bending dynamics of the sample was measured by the intensity of a converging laser beam reflected from the back surface of the sample, polished to a mirror state. The residual sag due to bending increases with the heat load similarly as residual roughness of the front surface of the sample. These data, together with simultaneously measured temperature dynamics and the spatial heating profile, can provide an experimental basis for the numerical calculation of the residual stresses in the sample. The data obtained in situ were compared with those measured outside the vacuum chamber with X-ray diffraction, optical profiler, and optical interferometer. At the stage of cooling, after a sufficient intensity of heating, the second stage of damage took place — the cracking of the surface layer. The time before the start of this relatively fast process usually exceeded the time to achieve a DBTT by 1–4 orders of magnitude.

Hihg-power flows of plasma cause mechanical destruction of materials via non-uniform deformation and stresses. The possibility of synchrotron radiation scattering diagnostics of deformation is discussed. The experiments at the new scattering station “Plasma” on VEPP-4M demonstrated the measurements of change in the diffraction angle during pulsed heating.

A mathematical model of surface cracking under pulsed heat load was developed. The model correctly describes a smooth brittle–ductile transition. The elastic deformation is described in a thin-heated-layer approximation. The plastic deformation is described with the Hollomon equation. The time dependence of the deformation and stresses is described for one heating–cooling cycle for a material without initial plastic deformation. The model can be applied to tungsten manufactured according to ITER specifications. The model shows that the stability of stress-relieved tungsten deteriorates when the base temperature increases. This proved to be a result of the close ultimate tensile and yield strengths. For a heat load of arbitrary magnitude a stability criterion was obtained in the form of condition on the relation of the ultimate tensile and yield strengths.

Two-pulse modification of the Thomson scattering system that is used at GOL-3 for studies of fast large-amplitude local density fluctuations is described. Two 1054 nm 20 J pulses are generated with two master oscillators that share the same power amplification chain. First experiments indicate existence of shots with a significant change of electron density at adjacent radial locations at ∼100 ns time scale.

The Thomson scattering technique has been employed for the observation of turbulent waves excited by a relativistic electron beam. The frequency and k-spectra of the Langmuir waves are measured both in the excited and damped regions. The waves interact directly with the beam electrons inside a narrow section of k-sppace amid the region occupied by the non-resonant waves, which concentrate near k ∼ ωpe/c. Both resonant and non-resonant waves hold equal (to within an order of magnitude) amounts of energy. Incoherent scattering of the second harmonic of a Nd : glass laser is used to measure the time history of the electron distribution function including the super thermal tails. The typical beam and plasma parameters are as follows: (ne ∼ 1015 cm−3, nb/ne ∼ 10−4, B = 1–4 T, tb = 100–200 ns). The peculiarity of these experiments is the presence of a strong magnetic field (ωBec/ωpe VTe ⪢ 1, but cope ⪢ ωBe) which, together with plasma non-isothermality (Te ⪢ Ti), sets conditions typical for laboratory and space plasmas. At present, the processes under these conditions have not been adequately investigated, either experimentally or theoretically, especially for our case of developed Langmuir turbulence, when the spatial and temporal scales far exceed those for a single caviton. This work is centred on the study of developed Langmuir turbulence in a magnetized plasma.