Recent experiments at Sandia National Laboratories have demonstrated an electron beam diode X-ray source capable of producing > 350 rad at one meter with 1.7 mm FWHM X-ray source distribution, with a 50 ns pulse-width and X-ray photon endpoint energy spectrum in the 6-7 MeV range. The diode operates at current densities of ≈ 1 MA/cm2. The intense electron beam rapidly (≈ 5 ns) heats the X-ray conversion anode/target, liberating material in the form of low density ion emission early in the pulse and high density plasma later. This environment gives rise to beam/plasma collective effects which dominate the diode and beam characteristics, affecting the radiation properties (dose and spot-size). A review of the diode operation, the measured source characteristics and the simulation methods and diagnostics used to guide its optimization is given.
Minimization of the radiographic spot size and maximization of the radiation dose is a continuing long-range goal for development of electron beam driven X-ray radiography sources. In collaboration with members of the Atomic Weapons Establishment(AWE), Aldermaston UK, the Advanced Radiographic Technologies Dept. 1645 is conducting research on the development of X-ray sources for flash core-punch radiography. The Hydrodynamics Dept. at AWE has defined a near term radiographic source requirement for scaled core-punch experiments to be 250 rads{at}m with a 2.75 mm source spot-size. As part of this collaborative effort, Dept. 1645 is investigating the potential of the Self-Magnetic-Pinched (SMP) diode as a source for core-punch radiography. Recent experiments conducted on the RITS-6 accelerator [1,2] demonstrated the potential of the SMP diode by meeting and exceeding the near term radiographic requirements established by AWE. During the demonstration experiments, RITS-6 was configured with a low-impedance (40 {Omega}) Magnetically Insulated Transmission Line (MITL), which provided a 75-ns, 180-kA, 7.5-MeV forward going electrical pulse to the diode. The use of a low-impedance MITL enabled greater power coupling to the SMP diode and thus allowed for increased radiation output. In addition to reconfiguring the driver (accelerator), geometric changes to the diode were also performed which allowed for an increase in dose production without sacrificing the time integrated spot characteristics. The combination of changes to both the pulsed power driver and the diode significantly increased the source x-ray intensity.
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.
A 1 MV linear transformer driver (LTD), capable of driving a radiographic diode load, has been built and tested. A circuit model of this accelerator has been developed using the BERTHA circuit simulation code. Simulations are compared to data from power-flow experiments utilizing a large area electron-beam diode load. Results show that the simulation model performs well in modeling the baseline operation of the accelerator. In addition, the circuit model has been used to predict several possible fault modes. Simulations of switch prefires, main capacitor failure, vacuum insulator flashover, and core saturation have been used to estimate the probability of inducing further failures and the impact on the load voltage and current.
A 1 MV linear transformer driver (LTD) is being tested with a large area e-beam diode load at Sandia National Laboratories (SNL). The experiments will be utilized to determine the repeatability of the output pulse and the reliability of the components. The 1 MV accelerator is being used to determine the feasibility of designing a 6 MV LTD for radiography experiments. The peak voltage, risetime, and pulse width as well as the cavity timing jitter are analyzed to determine the repeatability of the output pulse.
Spectroscopic investigations in the visible and near UV are underway to study plasmas present in X-ray radiography diodes during the time of the electron beam propagation. These studies are being performed on the RITS-3 accelerator (5.25 MV and 120 kA) at Sandia National Laboratories using several diode configurations. The proper characterization of the plasmas occurring during the time of the X-ray pulse can lead to a greater understanding of diode behavior and X-ray spot size evolution. By studying these plasmas along with the use of selective dopants, insights into such phenomena as impedance collapse, thermal and non-thermal species behavior, charge and current neutralization, anode and cathode plasma formation and propagation, and beam/foil interactions, can be obtained. Information from line and continuum emission and absorption can give key plasma parameters such as temperatures, densities, charge states, and expansion velocities. This information is important for proper modeling and future predictive capabilities for the design and improvement of flash X-ray radiography diodes. Diagnostics include a gated, intensified multichannel plate camera combined with a 1 meter Czerny-Turner monochromator with a multi-fiber spectral input, allowing for both temporal and spatial resolution. Recent results are presented.
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.
There has been considerable work in recent years in the development of high-brightness, high-dose flash x-ray radiographic sources. Spot size is one of several parameters that helps characterize source performance and provides a figure of merit to assess the suitability of various sources to specific experimental requirements. Time-integrated spot-size measurements using radiographic film and a high-Z rolled-edge object have been used for several years with great success. The Advanced Radiographic Technologies program thrust to improve diode performance requires extending both modeling and experimental measurements into the transient time domain. A new Time Resolved Spot Detector (TRSD) is under development to provide this information. In this paper we report the initial results of the performance of a 148-element scintillating fiber array that is fiber-optically coupled to a gated streak camera. Spatial and temporal resolution results are discussed and the data obtained from the Sandia National Laboratories (SNL) RITS-3 (Radiographic Integrated Test Stand) accelerator are presented.
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.
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.
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.