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.
High-power pulse generators have been used to produce dense plasmas by the explosion of thin polymer fibers. Sufficiently high ion energies have been achieved to produce large neutron yields. Neutron production is attributed mainly to the reaction d(d $ n) 3 He and neutrons are observed when either fibers containing the natural abundance of deuterium or nearly fully deuterated fibers are used. Results are given which show the variation of the neutron yield with initial fiber diameter and with deuterium content.Recent studies 1 * 2 of exploded-wire plasmas have indicated that values of nr in excess of 10 12 sec/ cm 3 have been achieved in the high-power discharges (10 12 W) generated by the Gamble I and II devices. 3 Electron temperatures deduced from an analysis of the plasma radiation were in the range of 1 to 10 keV. A calculation of the electron-ion energy equipartition time for these discharges suggests that the ions and the electrons were equally energetic. Here we report on an experiment which shows that the ions are indeed sufficiently energetic to induce the d-d fusion reaction when the plasma contains deuterium.Deuterium-bearing plasmas were produced by exploding polyethylene fibers in a vacuum using the Gamble II generator. The electrical properties of these plasmas are similar to those produced by the explosion of fine metal wires.
The laser wavefront analyzer (LWA) consists of a polarized laser beam pulse that traverses an imploding z-pinch, and a microlens array that focuses the laser beam into a large number (104) of very tiny spots. LWA image analysis determines the refractive bending angles (due to density gradients) and Faraday rotation angles (due to the magnetic field-density integral) throughout the plasma cross section. Electron density and current distributions are derived from LWA data in an imploding gas-puff z-pinch plasma.
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