Impurity issues in material irradiation studies in plasma focus device

The PLANSEE double forged W samples were irradiated at different distances (5, 10, 15 and 20 cm) from the anode top for fixed 50 focus shot using a single shot 10 kJ NX3 plasma focus (PF) device with deuterium as the filling gas. The NX-3 PF device is dri ven by a 100 μF, 20 kJ modular capacitor bank which is capable of delivering peak current in the range of 200 kA to 600 kA depending upon charging voltage. The coaxial electrode assembly of the NX-3 plasma focus head consists of a 140 mm long, cylindrical anode of stainless steel (SS) having diameter of 40 mm and a squirrel cage cathode, consisting of twelve, 12 mm diameter brass rods, uniformly spaced on a coaxial circle of 90 mm diameter. The anode has 110 mm deep cavity of 32 mm diameter to minimize the ablation of anode material. An aluminum double aperture with 1 cm aperture size is used to reduce the anode material from reaching the irradiated PLANSEE double forged W sample surface. The virgin and irradiated samples were analyzed in detail using SEM, EDX, XPS and XRD. The impurity material ablated from anode (Fe and Cr) and aperture (Al) wer e found to be deposited on W substrate surface. This is a serious problem for material irradiation studies as it leads to impurity material deposition on irradiated substrate which should be avoided or minimized to correctly evaluate the effect of fast energetic ions and hot dense decaying plasma on irradiated sample. In order to evaluate that which one, anode or the aperture, is a bigger source of impurity deposition on irradiated substrate another systematic experiment was performed in 3 kJ UNU-ICTP PF facility. Silicon substrates were irradiated using different number of PF shots (1, 5, 10 and 20 shot) at fixed irradiation distance of 6 cm with and without double aperture assembly of bronze (as bronze has greater ablation threshold compared to aluminum). The investigation confirms that the impurities mostly being contributed by contributed by aperture, but only for higher number of irradiation PF shots.