Theoretical modeling and experimental characterization of a rod-pinch diode

Abstract

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.

Coupled particle-in-cell and Monte Carlo transport modeling of intense radiographic sources

“…Ions present in the A-K gap introduce a bipolar ͑BP͒ flow condition which alters the SCL current as I BP Ӎ1.84I SCL for nonrelativistic electron energies and I BP Ӎ 2 I SCL /4 for relativistic electron energies ͑See Ref. 6; these results are for a one-dimensional planar diode.͒ The ion current fraction is proportional to 5,6 …”

“…5,6 The space-charged-limited phase describes the initial turn on and rise of the electron current in the diode anode-cathode ͑A-K͒ gap. As the current flowing in the gap increases to the point at which the self-electric and self-magnetic field effects on the electron flow become comparable, the diode is described as being in a weakly pinched phase.…”

“…Testing and development of the rod-pinch and immersed-B diodes is presently underway on a number of other pulsed power generators including the Gamble II generator 5,8 at the Naval Research Laboratory, the TriMeV generator 12 at Bechtel-Nevada, the Asterix generator 13 at the Center d'Etudes de Gramat ͑CEG͒ in France, and several generators 14 -16 at the Atomic Weapons Establishment ͑AWE͒ in the United Kingdom.…”

Advances in High Intensity e-Beam Diode Development for Flash X-Ray Radiography

“…where α is an empirical scaling factor ≈ 2 and is associated with the increased current due to the presence of ion space-charge [2,4,5] and is dependent on geometry and the beam and ion space-charge profile (assuming a radially uniform space-charge limited distribution of ions results in α ≈ 1.6 [5]). A typical diode current and voltage profile is given in Fig.…”

Cygnus performance in subcritical experiments

“…The major Cygnus elements are given in Figure 1: Marx Generator, Pulse Forming Line (PFL), Water Transmission Line (WTL), Inductive Voltage Adder (IVA), and Rod Pinch Diode [3] - [7]. The grid barrier isolates the downstream zero room, which contains the experimental package and x-ray cameras, from Cygnus.…”