A high-impedance, high-resolution, 1-MV radiography diode

Summary form only given. A new high-impedance (~120 ohm), space-charge-limited (SCL) diode has been designed and tested on an ~1-MV, 50-ns FWHM Marx which has a characteristic impedance of ~120 ohms. The diode produces an ~1-mm diameter X-ray spot suitable for high-resolution radiography. Diodes driven with this class of Marx typically use one or more cylindrical washer-like cathodes that surrounds a tapered tungsten anode. The SCL electron beam flows along the electric field lines and spreads out symmetrically from the cathode´s edge to produce a radiation source whose axial length (~2 cm) is approximately twice the AK gap spacing. This usually limits the ability to obtain a small radiographic source diameter and limits the useful off-axis angle of view. The present diode uses the same tapered tungsten anode; however the cathode is a sharp edged cylinder that is coaxial with the anode but axially located downstream of the anode tip. The SCL electron beam concentrates at the tapered tungsten tip producing a smaller-diameter radiographic source with shorter axial length. Impedance collapse is also more benign than exhibited with the conventional cathode. Results from PIC simulations agree with experimental measurements and suggest that the diode behavior is dominated by the production of low-atomic-number ions from the irradiated anode tip. These ions increase the electron current by almost a factor-of-four rather than the usual bipolar factor-of-two because of the cylindrical geometry and the large ratio of the cathode to anode radius. We speculate that the time-dependent production of ions starting from the anode tip helps to keep electrons confined near that tip. The geometry also helps to decouple expanding anode plasma from the cathode