Rod-pinch diode development for short-pulse radiography using extended-length cathodes

Rod-pinch diodes utilize a small-diameter anode rod extending through and beyond the plane of a thin annular cathode. At low voltage (< 1 MV) and short pulse duration (< 20 ns), it is difficult for the space-charge-limited current (SCL) to exceed the critical current necessary for electron beam self-pinching using cathodes thinner than the anode-cathode gap spacing. Because the SCL is proportional to the cathode length and the critical current is independent of it, we have studied multiple-disk, extended-length cathodes for the purpose of achieving critical current early in the pulse. Experiments were conducted on the TriMeV facility at 0.8 to 1.2 MV and 20 to 40 kA with a 15-ns pulse duration. The cathode radius was typically 4 mm with cathode lengths varying from 3 mm to 3 cm. The anode radius was 0.25 mm. As expected, the longer cathodes achieved critical current earlier in the pulse and continued to operate at critical current for the pulse duration. Source diameters measured in the forward direction were about 0.5 mm with doses 1 m from the source in the 0.3- to 0.5-Roentgen range.