It is demonstrated that measurements of radial electric currents excited from biased electrodes in TUMAN 3 and other tokamaks provide a crucial test in validating different models for L-H transitions. The results are assessed from the viewpoint of a previously developed theory, which is briefly described. There is evidence from the voltage-current characteristics in TUMAN 3 which corroborates features of this theory. A spontaneously occurring Ohmic H-mode is switched off when a substantial positive biasing voltage is applied to the electrode

Bombardment of solid surfaces by energetic plasma particles may heat them to temperatures at which melting and intense vaporization begin. The incident charge carriers, depending upon their masses and initial energies, penetrate to different depths in the vapour layer, thus setting up there, together with the resident thermal charge carriers, electric fields. Three effects are considered: the electrostatic sheath at the solid surface, the electric field distribution in the vapour layer and the electrostatic sheath forming at the vapour-plasma interface. A self-consistent model is proposed for the transition of an initially negatively biased (with respect to the plasma potential) sheath to a double layer at the vapour-plasma interface; the respective threshold conditions are defined quantitatively. The field strength in the vapour layer and the potential drop at the vapour-plasma interface are functions of the ionization state of the vapour phase and of the electron fluxes available on the two sides of the sheath. The resulting field strengths and potential drops may significantly affect the particle energies and particle fluxes, thus reducing the total energy flux incident at the surface.

Plasma propagation along the magnetic field lines after pellet evaporation is simulated. The character of the plasma expansion depends strongly on the ambient plasma parameters. For medium plasma parameters, Te < 1 keV, ne = 3 × 1019 m-3, the ablatant expansion, calculated with respect to the ambient temperature, is supersonic. Shock formation at the plasma front is observed. The results of the numerical simulation are in reasonable agreement with the self-similar solution obtained for the isothermal plasma. For reactor type parameters, the ablatant heating is a much slower process, and plasma expansion is subsonic

The time evolution of radiating vapour shields over eroding solid surfaces and the resulting erosion rates are modelled by one dimensional (1-D) and 1½-D resistive MHD codes. Graphite or carbonized divertor plates subjected to high energy deuterium plasma particles during disruptions or giant ELMs are considered. The energy flux range assumed corresponds to ITER conditions. Various physical phenomena having a primary effect on the erosion rate, such as collisional interaction of the energy carriers with the target (solid surface or vapour particles), electrostatic shielding, magnetohydrodynamic interaction and radiant energy transport, are investigated in detail. In the 1-D and 1½-D approximations used and for the energy input parameter range considered (Q0 = 1011 W/m2) ablation rates of the order of 1028 m-2·s-1 were obtained. It is shown that processes, such as lateral expansion, lateral drift, radiation losses through the lateral surfaces of the scrape-off layer (SOL) and the Hall effect, may notably change the predicted erosion rates and warrant a more elaborate, at least 2-D, treatment of the problem.

As a result of plasma-wall contact during hard disruptions, intense vaporization of the divertor plates is initiated. The evolving vapour layer intercepts a fraction of the incident energy carriers, thus shielding the plates. The plasma particles of different energies and masses moving along the skewed magnetic field lines penetrate the vapour layer to different depths, thus causing charge separation there and the onset of an electrostatic field. It is shown in a one dimensional approximation that, owing to the constraints imposed on the electric field and on the current components at the plate surface, a rather intense lateral drift motion of the vapour evolves in the scrape-off layer (SOL). The analysis shows that drift velocities of the order of 103 to 104 m/s are to be expected, which may notably impair the shielding characteristics of the vapour layers and increase the erosion rates of the divertor plates