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

Summary form only given. Rod-pinch diodes utilize a thin annular cathode surrounding a small-diameter anode rod extending through and beyond the plane of the cathode. Analysis of previous data has shown that the diode progresses from initial space-charge-limited (SCL) flow, to magnetically-limited flow, to ion-induced electron beam propagation and, finally, to a tightly focused pinch on the rod tip. Use of the rod-pinch diode for X-ray radiography provides substantial improvement over previously available sources. At low voltage (<1 MV) and short pulse lengths (< 20 ns), it is difficult for the SCL current to exceed the critical current necessary for electron beam self-pinching using cathodes thinner than the anode-cathode gap spacing. Because the SCL current is proportional to the cathode length whereas critical current is independent of this length, we have studied multiple-disk, extended-length cathodes for the purpose of achieving critical current early in the pulse. The experiments were done on the TriMeV facility at 0.8 to 1.2 MV, 20 to 40 kA, and 15 ns pulse length. 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 then continued to operate at critical current for the duration of the pulse. Pinching was observed with a side-viewing pinhole camera, x-ray dose was measured with TLDs, and X-ray source sizes were measured with a 1-m radius rolled edge. Source sizes measured in the forward direction are < 0.6-mm-diam with doses in the 0.4 to 0.7 Rad(Si) range at 1 m from the source.