Vacuum UV spectroscopy of armor erosion from plasma gun disruption simulation experiments

Extensive simulations of tokamak disruptions have provided a picture of material erosion that is limited by the transfer of energy from the incident plasma to the armor solid surface through a dense vapor shield. Two transmission grating vacuum ultraviolet (VUV) spectrographs were designed and utilized to study the plasma-material interface in plasma gun simulation experiments. Target materials included POCO graphite, ATJ graphite, boron nitride and plasma-sprayed tungsten. Detailed spectra were recorded with a spatial resolution of ca. 0.7 mm resolution on VIKA at Efremov and on 2MK-200 at Troitsk. Time-resolved data with 40–200 ns resolution were then recorded along with the same spatial resolution on 2MK-200. The VIKA plasma gun directly illuminated a target with a high-intensity plasma pulse of 2–100 MJ m−2 with low-energy ions of ca. 100 eV. The 2MK-200 plasma gun illuminated the target via a magnetic cusp that permitted only deuterium to pass with energies of ca. 1 keV, but which produced a fairly low energy density of 2 MJ m−2. Power densities on target ranged from 107 to 108 W cm−2. Emitted spectra were recorded from 15 to 450 Å over a distance from 0 to 7 cm above the armor target surface. The data from both plasma gun facilities demonstrated that the hottest plasma region was sitting several millimeters above the armor tile surface. This apparently constituted the absorption region, which confirmed past computer simulations. Spectra indicated both the species and ionization level that were being ablated from the target, demonstrating impurity content, and showing plasma ablation velocity. Graphite samples clearly showed C V lines as well as impurity lines from O V and O VI. The BN tiles produced textbook examples of B IV and B V, and extensive N IV, V and VI lines. These are being compared with radiation-hydrodynamic calculations.