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The rod-pinch diode consists of an annular cathode and a small-diameter anode rod that extends through the hole in the cathode. With high-atomic-number material at the tip of the anode rod, the diode provides a small-area, high-yield x-ray source for pulsed radiography. The diode is operated in positive polarity at peak voltages of 1 to 2 MV with peak total electrical currents of 30–70 kA. Anode rod diameters as small as 0.5 mm are used. When electrode plasma motion is properly included, analysis shows that the diode impedance is determined by space-charge-limited current scaling at low voltage and self-magnetically limited critical current scaling at high voltage. As the current approaches the critical current, the electron beam pinches. When anode plasma forms and ions are produced, a strong pinch occurs at the tip of the rod with current densities exceeding 106 A/cm2. Under these conditions, pinch propagation speeds as high as 0.8 cm/ns are observed along a rod extending well beyond the cathode. Even faster pinch propagation is observed when the rod is replaced with a hollow tube whose wall thickness is much less than an electron range, although the propagation mechanism may be different. The diode displays well-behaved electrical characteristics for aspect ratios of cathode to anode radii that are less than 16. New physics understanding and important properties of the rod-pinch diode are described, and a theoretical diode current model is developed and shown to agree with the experimental results. Results from numerical simulations are consistent with this understanding and support the important role that ions play. In particular, it is shown that, as the ratio of the cathode radius to the anode radius increases, both the Langmuir–Blodgett space-charge-limited current and the magnetically limited critical current increase above previously predicted values.

FIG. 5. Wave power (linear scale) vs axial distance. The launched wave amplitude is near the saturation level which is marked by the dashed line. V 0 = 55.7 V, I Q = 0.50 mA, /= 45.0 MHz, £ dc = 1.88 V/cm, and P s = 5.36 mW. The applied static voltage V H = 506 V.static electric field on beam trapping in a TWT. For weak fields the wave power can be enhanced while for stronger fields beam detrapping occurs and the enhancement diminishes. Space charge can play an important role in causing the beam to be detrapped. The wave enhancement has been found to be strongly dependent on the rf input drive level. In particular, appreciable wave en-hancement of launched large-amplitude waves has been observed.We wish to thank Professor N. M. Kroll for useful discussions.