Publications

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The immersed-B{sub z} diode is being developed as a high-brightness, flash x-ray radiography source. This diode is a foil-less electron-beam diode with a long, thin, needle-like cathode inserted into the bore of a solenoid. The solenoidal magnetic field guides the electron beam emitted from the cathode to the anode while maintaining a small beam radius. The electron beam strikes a thin, high-atomic-number anode and produces bremsstrahlung. We report on an extensive series of experiments where an immersed-B{sub z} diode was fielded on the RITS-3 pulsed power accelerator, a 3-cell inductive voltage generator that produced peak voltages between 4 and 5 MV, {approx}140 kA of total current, and power pulse widths of {approx}50 ns. The diode is a high impedance device that, for these parameters, nominally conducts {approx}30 kA of electron beam current. Diode operating characteristics are presented and two broadly characterized operating regimes are identified: a nominal operating regime where the total diode current is characterized as classically bipolar and an anomalous impedance collapse regime where the total diode current is in excess of the bipolar limit and up to the full accelerator current. The operating regimes are approximately separated by cathode diameters greater than {approx}3 mm for the nominal regime and less than {approx} 3 mm for the anomalous impedance collapse regime. This report represents a compilation of data taken on RITS-3. Results from key parameter variations are presented in the main body of the report and include cathode diameter, anode-cathode gap, and anode material. Results from supporting parameter variations are presented in the appendices and include magnetic field strength, prepulse, pressure and accelerator variations.

The propagation of a 30 kA, 3.5 Mev electron beam which was focused into gas and plasma-filled cells was discussed. Gas cells which were used for X-ray radiography were produced using pulsed-power accelerators, onto a high atomic number target to generate bremsstrahlung radiation. The effectiveness of beam focusing using neutral gas, partially ionized gas, and fully ionized (plasma-filled) cells was investigated using numerical simulation. It was observed in an optimized gas cell that an initial plasma density approaching 1016 cm-3 was sufficient to prevent significant net currents and the subsequent beam sweep.

An important issue in designing a higher-power version of the Z machine at Sandia National Laboratories is electron current loss in the vacuum section, which consists of four radial transmission lines and a convolute (current-adder). There is evidence from experiments on Z that 1-2MA of current out of about 20MA is lost in the vacuum section before reaching the wire-array load [1]. Calculations using the LSP [2] and QUICKSILVER [3] particle-in-cell codes have shown much less current loss [4,5,6]. The current loss in the calculations is due to sheath-current loss in the region of the convolute, and is associated with the magnetic nulls which are intrinsic to the current splitting in the convolute Detailed 2-D calculations for the radial MITLs show that, in the region between the insulator stack and a radius of about 20cm (over which the radial-line vacuum impedance increases slowly from 2Ω to 3Ω), excess electron sheath current is mostly retrapped to the cathode electrode. The electron sheath current is given approximately by Mendel's force-balance expression [7] applied locally, and as a result, the sheath current decreases as Zv-2, where Zv is the vacuum impedance. Between a radius of 20cm and the convolute, where the radial-line vacuum impedance increases more sharply (to 6Ω at 10cm) there is significant "launching" of sheath current. The sheath behavior in this region is qualitatively similar to that predicted using a "constant flow impedance" model, but in the simulations the sheath is unstable and breaks up into vortices.

As part of a continuous research effort into advanced flash radiographic sources using intense electron beams, Sandia National Laboratories (SNL) has been investigating coupling vacuum power flow into various high power diodes. Of key importance is the issue of the re-trapping of electrons from the sheath current of a magnetically insulated vacuum transmission line (MITL) into the diode load. Results of electron re-trapping studies on a large area diode (LAD) on the RITS-3 accelerator are presented here. RITS-3 is a 4.5 MV, 160 kA inductive voltage adder pulsed power accelerator. Results show that re-trapping of the sheath current does occur and compares favorably with particle in cell (PIC) predictions of the LSP modeling code.

Plasmas are ubiquitous in the high-power electron beam diodes used for radiographic applications. In rod pinch and immersed Bz diodes they are found adjacent to the cathode and anode electrodes, and are suspected of affecting the diodes' impedance characteristics as well as the radiographic spot size. In paraxial diodes, preionized plasmas or beam-formed plasmas are also found in the gas focusing section. A common feature of the plasmas adjacent to the electrodes is that their densities can range from 10 12-1017 cm-3, and their velocity is on the order of 107 cm/s. Researchers from the Naval Research Laboratory have developed a high-sensitivity two-color interferometer that is presently being tested on Gamble II for future use on the Sandia RITS accelerator operating with a Bz diode. This diagnostic is capable of resolving a line-integrated electron density of 2×1012 cm-2, a density that might be capable of even observing the electron beam directly. This paper will present an overview of laser-based and spectroscopic diagnostics that could be used to measure plasmas found in radiographic diodes with spatial and temporal resolutions on the order of 1-5 mm and 5 ns, respectively. Plans for the use of this diagnostic on a preionized plasma cell of a paraxial diode on the Sandia RITS experiment will be discussed.

Composite-rod-pinch loads on Asterix consisting of hollow aluminum tubes supporting either 1-cm-long, 1-mm-diam blunt-end or tapered gold slugs, or 1.5- to 2-mm-diam gold spheres are characterized. Composite-slug loads have slightly-lower doses than the 1.6- or 2-mm-diam standard rod pinches reported elsewhere and smaller spot sizes, leading to higher measured radiographic figures-of-merit (FOM). The FOM for the gold-sphere loads is substantially-smaller than for the slug loads.

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SNL is developing intense sources for flash x-ray radiography. The goals of the experiments presented here were to assess power flow issues and to help benchmark the LSP particle-in-cell code used to design the experiment. Comparisons between LSP simulations and experimental data are presented.

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High-brightness flash x-ray sources are needed for penetrating dynamic radiography for a variety of applications. Various bremsstrahlung source experiments have been conducted on the TriMeV accelerator (3MV, 60 {Omega}, 20 ns) to determine the best diode and focusing configuration in the 2-3 MV range. Three classes of candidate diodes were examined: gas cell focusing, magnetically immersed, and rod pinch. The best result for the gas cell diode was 6 rad at 1 meter from the source with a 5 mm diameter x-ray spot. Using a 0.5 mm diameter cathode immersed in a 17 T solenoidal magnetic field, the best shot produced 4.1 rad with a 2.9 mm spot. The rod pinch diode demonstrated very reproducible radiographic spots between 0.75 and 0.8 mm in diameter, producing 1.2 rad. This represents a factor of eight improvement in the TriMeV flash radiographic capability above the original gas cell diode to a figure of merit (dose/spot diameter) > 1.8 rad/mm. These results clearly show the rod pinch diode to be the choice x-ray source for flash radiography at 2-3 M V endpoint.

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