S. Portillo

Design, Simulation, and Fault Analysis of a 6.5-MV LTD for Flash X-Ray Radiography

The design of a 6.5-MV linear transformer driver (LTD) for flash-radiography experiments is presented. The design is based on a previously tested 1-MV LTD and is predicted to be capable of producing diode voltages of 6.5 MV for a 50-Omega radiographic-diode load. Several fault modes are identified, and circuit simulations are used to determine their effect on the output pulse and other components. For all the identified fault modes, the peak load voltage is reduced by less than 5%

Status of the 10 MV, 120 kA RITS-6 Inductive Voltage Adder

The six-cell RITS-6 accelerator is an upgrade of the existing RITS-3 accelerator and is next in the sequence of Sandia IVA accelerators built to investigate/validate critical accelerator and radiographic diode issues for scaling to the Radiographic Integrated Test Stand (RITS) (nominally 16 MV, 156 kA, and 70 ns). In the RITS-6 upgrade to RITS-3 the number of cells/cavities, PFLs, laser triggered gas switches and intermediate stores is being doubled. A rebuilt single 61-nF Marx generator will charge the two intermediate storage capacitors. The RITS-3 experiments have demonstrated a MITL configuration matched to the PFL/induction cell impedance and a higher impedance MITL. RITS-6 is designed to utilize the higher impedance MITL providing a 10.5-MV, 123-kA output. The three years of pulsed power performance data from RITS-3 will be summarized and the design improvements being incorporated into RITS-6 will be outlined. The predicted output voltage and current for RITS-6 as a function of diode impedance will be shown. Particle-in-cell simulations of the vacuum power flow from the cell to the load for a range of diode impedances from matched to ~ 40 Ohms will be shown and compared with the re-trapped parapotential flow predictions. The status of the component fabrication and system integration will be given. Another potential upgrade under consideration is RITS-62. In this case the RITS-6 Marx, intermediate stores, gas switches, and PFLs would be duplicated and a tee would replace the elbow that now connects a single PFL to a cell thereby allowing two PFLs to be connected to one cell. The output of RITS-62 matched to the cell/PFL impedance would then be 8 MV, 312 kA or 25.6 ohms. The predicted operating curves for RITS-62 with other non-matched MITLs will be shown. The power delivered to a radiographic diode can be maximized by the correct choice of MITL impedance given the cell/PFL and radiographic diode impedances. If the radiated output for a given diode has a stronger than linear voltage dependence this dependence can also be included in the correct choice of MITL impedance. The optimizations and trade-offs will be shown for RITS-6 and RITS-62 for diode impedances characteristic of radiographic diodes.

Reliability Assessment of a 1 MV LTD

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.

Circuit Simulations of a 1 MV LTD for Radiography

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.

Design of a high impedance MITL for RITS-3

RITS-3 is the three-cell, 4 MV, 156 kA, 70 ns embodiment of the full twelve-cell 16 MV Radiographic Integrated Test Stand (RITS) (Ian D. Smith et al., 2000). The well-instrumented RITS-3 experiments (David L. Johnson et al., 2002) now underway at Sandia are intended to investigate/validate critical design issues for scaling to RITS. These experiments use a magnetically-insulated transmission line (MITL) in which the increment in the operating impedance of the MITL from cell to cell is equal to the impedance of the individual pulse forming line (pfl)/induction cell (8 ohms). The matched load voltage that is obtained in this configuration is 4.0 MV and occurs when the load impedance equals the sum of the PFL impedances (24 ohms). This paper discusses the design of a higher impedance MITL intended to increase the RITS-3 output voltage from 4 MV to 5.25 MV for the same pulse forming line charging voltage. The fundamental operating impedance increment for the MITL steps is increased to 14.25 ohms providing a matched 5.25 MV, 123 kA, 70 ns output pulse. Particle-in-cell simulations (LSP) of the MITL power flow from the cell to the load predict a nominal output in agreement with the design value. The cathode (boundary) current and the vacuum flow (sheath) current from the simulations scale roughly as predicted by parapotential flow theory (M.Y. Wang et al., 1978). The increased cell voltage and core flux swing are well within the RITS-3 design levels. When the load impedance is <42.75 ohms the MITL behavior is divided into three separate phases. A vacuum precursor, a magnetic insulation phase when the operating impedance is determined by parapotential flow, and an over insulation phase where the impedance is determined by the load. The over insulation wave moves back up the MITL toward the source with a velocity of 0.3-0.6 the speed of light. The diode voltage is less than 5.25 MV and the boundary and sheath currents are observed to change significantly from those for a matched diode.

Characterization of the rod-pinch diode x-ray source on Cygnus

The rod-pinch diode[1] is a self-magnetically insulated electron beam diode that is capable of producing a very bright source of hard x-rays. As fielded on the Cygnus accelerator[2], the diode operates at an impedance of 50 Ohms and produces short pulse ( ∼50 ns) bremsstrahlung radiation with a 2 MeV photon endpoint energy and dose of 4 rad measured at one meter, with an x-ray spot size ∼ 1mm. The source can be used to image through ∼ 40 g/cm2 of material with spatial resolution of order 300 µm. Recently, a series of experiments on Cygnus have been conducted to better characterize the diodes operation and x-ray output. In particular, the x-ray spectral content, source spot-size, and shot-to-shot reproducibility have been diagnosed. The intent of these experiments is to enable improvements that may extend the diodes radiographic utility. An array of diagnostics have been utilized which include, end-on and side view x-ray pin hole imaging, time resolved and time integrated spot size measurements, step wedges, x-ray p-i-n diodes, and diode/MITL current measurements. High fidelity, PIC/Monte-Carlo simulations have also been conducted to help analyze the data. An overview of these experiments, simulations, and the conclusions from analysis is presented.

Time-Resolved Spot Size Measurements From Various Radiographic Diodes on the RITS-3 Accelerator

Sandia National Laboratories is leading an intensive research effort into fielding and diagnosing electron-beam flash radiographic X-ray sources. Several X-ray sources are presently being studied, including the self-magnetic pinched diode, the immersed Bz diode, and the plasma-filled flat cathode (paraxial) diode. These studies are being carried out on RITS-3, an inductive voltage adder accelerator capable of delivering 140 kA at 5 MV with a radiation pulse of 70-ns full width at half maximum. The interactions of the electron beam with plasmas created at the anode and/or cathode, for the self-pinched and Bz diode or in the plasma cell for the paraxial diode, can greatly effect the temporal behavior of the radiation spot size. Measuring the dynamic behavior of the beam size and coupling this with theoretical models of the beam plasma interactions can lead to improvements that can be made in these sources. A time-resolved spot size diagnostic (TRSD) has been developed and fielded on RITS-3. This diagnostic consists of a linear array of scintillating fibers, shadowed by a tungsten rolled edge. The scintillating array is optically coupled to a streak camera, and the output is recorded on a charge-coupled device. This paper presents a description of this second-generation TRSD as well as data on the time history behavior of the spot sizes for these three diodes

Demonstration of the self-magnetic-pinch diode as an X-ray source for flash core-punch radiography.

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.

Advances in pulsed power modeling and experimentation on the RITS accelerator

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 summarize 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 transmission line.

Studies into the Time Resolved Source Diameter of a Self Magnetic Pinch Radiographic Diode

The self magnetic pinch diode is being considered as a possible source for future high voltage (10MV) flash radiographic systems being developed. Previous studies at both the Atomic Weapons Establishment (AWE) Aldermaston in the UK and Sandia National Labs (SNL) in the US have investigated the time integrated dose and source diameter produced by the diode. Presented here are results from investigations into the dynamic time history of the source diameter. Experimental investigations have been carried out at both AWE and SNL with the image of the source projected onto fast decay scintilators viewed by nanosecond gating and streak cameras. Experimental measurements are compared with results from 2D and 3D electromagnetic particle in cell codes.