Evaluation of the rod-pinch diode as a high-resolution source for flash radiography at 2 to 4 MV

The ASTERIX generator is used to obtain the first evaluation of the rod-pinch electron-beam diode as an intense source of X-rays for high-resolution, pulsed (30- to 40-ns FWHM) radiography at voltages of 2 to 4 MV and currents between 50 and 100 kA. At these levels, the rod pinch exhibits standard diode electrical behavior for land 2-mm diam anodes. The impedance is more sensitive to the ratio of anode-to-cathode radius and to the Marx charge voltage for the 1-mm diam anode. Shots with a 0.5-mm diam anode exhibit rapid impedance collapse. Electrical modeling of a limited number of shots using a physics-based diode model reproduces the measured current. For peak voltages /spl ges/ 4 MV, doses of 20 rad(Si) at 1 m are obtained with a 2-mm diam anode and 16 rad(Si) with a 1-mm diam anode, consistent with the dose/charge scaling with voltage to the 1.3 to 1.6 power. The radiation exhibits a large anisotropy, with 2.7 to 4.5 times more dose off-axis than on-axis. The source diam scales with the anode diam, is independent of voltage, and ranges from 1.8 to 3.1 mm (LANL definition). The largest figure of merit for these non-optimized shots is 4.6 rad(Si)/mm/sup 2/ at a peak voltage of 4.3 MV. A composite diode with a large diam carbon-rod anode followed by a smaller-diam tungsten-tip converter shows promise for applications where a small central source feature is desired.