Publications

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Paraxial diodes have been a stronghold for high-brightness, flash x-ray radiography. In its traditional configuration, an electron beam impinges onto an anode foil, entering a gas-filled transport cell. Within the cell, the beam is focused into a small spot onto a high-Z target to generate x-rays for the radiographic utility. Simulations using Lsp, a particle-in-cell code, have shown that within the gas-filled focusing cell the electron beam spot location sweeps axially during the course of the beam pulse. The result is a larger radiographic spot than is desirable. Lsp has also shown that replacing the gas-filled cell with a fully ionized plasma on the order of 1016 cm-3 will prevent the spot from significant beam sweeping, thus resulting in a smaller, more stable radiographic spot size. Sandia National Laboratories (SNL) is developing a plasma-filled focusing cell for future paraxial diode experiments. A z-discharge in a hydrogen fill is used to generate a uniform, highly ionized plasma. Laser interferometry is the key diagnostic to determine electron density in a light lab setting and during future paraxial diode shots on SNL's RITS-3 accelerator. A time-resolved spot diagnostic will also be implemented during diode shots to measure the change in spot size during the course of the pulse. © 2004 IEEE.

Flash x-ray radiography has undergone a transformation in recent years with the resurgence of interest in compact, high intensity pulsed-power-driven electron beam sources. The radiographic requirements and the choice of a consistent x-ray source determine the accelerator parameters, which can be met by demonstrated Induction Voltage Adder technologies. This paper reviews the state of the art and the recent advances which have improved performance by over an order of magnitude in beam brightness and radiographic utility.

The radiographic integrated test stand (RITS-3) is a 5-MV, 160-kA, 70-ns inductive voltage adder accelerator at Sandia National Laboratories used to develop critical understanding of x-ray sources and flash radiographic drivers. On RITS-3 three pulse forming lines (PFLs) are used to drive three inductive voltage adder cavities. Each PFL contains a fast-pulse-charged, self-breakdown annular water switch that is used for initial pulse shaping and timing. Low loss in the switches combined with good synchronization is required for efficient operation of the accelerator. Switch maintenance is closely monitored over time to determine the effects of wear on switch breakdown performance.

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.

RITS (Radiographic Integrated Test Stand) is planned to be a 12-cell, 16-MV, 150-kA, 70-ns induction voltage adder. A three-cell, 4-MV, 150-kA, 70-ns version (RITS-3) is operating routinely at its specified level at Sandia. Its over-all performance will be described. Advances have been made in understanding and modeling many of the pulsed power features of RITS and several fundamental accelerator design guidelines have been developed. We summerize these. We omit discussion of vacuum power flow and symmetrization, which are the subject of other detailed papers. Subjects include: performance and redesign of the input oil-water diaphram of the pulse forming line (PFL); water switch losses; prepulse measurements at the cell; high voltages breakdowns; and impacts on the induction cell risetime due to the current-symmetrizing azimuthal oil line and the vacuum injection to the magnetically insulated output tranmission line.

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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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