NUMERICAL MODELING AND EXPERIMENTAL SIMULATION OF VAPOR SHIELD FORMATION AND DIVERTOR MATERIAL EROSION FOR ITER TYPICAL PLASMA DISRUPTIONS

The high divertor heat load during a tokamak plasma disruption results in sudden evaporation of a thin layer of divertor plate material, which acts as vapor shield and protects the target from further excessive evaporation. Formation and effectiveness of the vapor shield are theoretically modeled and experimentally investigated at the 2MK-200 facility under conditions simulating the thermal quench phase of ITER tokamak plasma disruptions. In the optical wavelength range C II, C III, C IV emission lines for graphite, Cu I, Cu II lines for copper and continuum radiation for tungsten samples are observed in the target plasma. The plasma expands along the magnetic field lines with velocities of (4 +/- 1) X 10(6) cm/s for graphite and 10(5) cm/s for copper. Modeling was done with a radiation hydrodynamics code in one-dimensional planar geometry. The multifrequency radiation transport is treated in flux limited diffusion and in forward reverse transport approximation. In these first modeling studies the overall shielding efficiency for carbon and tungsten defined as ratio of the incident energy and the vaporization energy for power densities of 10 MW/cm(2) exceeds a factor of 30. The vapor shield is established within 2 mu s, the power fraction to the target after 10 mu s is below 3% and reaches in the stationary state after about 20 mu s a value of around 1.5%.