D. Droemer

Design of a radiographic integrated test stand (RITS) based on a voltage adder, to drive a diode immersed in an high magnetic field

Advanced radiography capabilities can be provided by diodes in which a <1 mm diameter cathode is immersed in a /spl sim/60 T magnetic field and pulsed to /spl sim/16 MV. An electron current of /spl sim/50 kA is constrained by the magnetic field to /spl sim/1 mm at the anode, and produces V 1 krad at 1 m. A 16 MV, 50 ns flat-top voltage adder has been designed that is optimum to develop such diodes. The adder approach is chosen to give a relatively short risetime and low prepulse. It also has the advantage of being re-configurable to provide two 8 MV pulses. The pulser uses standard Marxes, intermediate stores, and water PFLs. Novel features include an oil prepulse switch, an induction cell that is fed from one point, and a blocking network to couple two pulses to one cell. Based on detailed simulations, a design has been completed through detailed drawings and prototype hardware will be tested next year.

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.

Intense electron beam sources for flash radiography

High intensity pulsed electron beams are used to create bremsstrahlung x-ray sources for flash radiographic interrogation of dynamic experiments. Typical industrial sources operate below 200 GW/cm2 intensities, while experimental requirements can demand above 50 TW/cm2. Recent developments in pulsed power-driven high intensity electron beam systems have significantly increased these operating regimes, demonstrating 20 TW/cm2, and computations predict successful extrapolation to higher intensities. Detailed studies of electron beam configurations, both theoretical and experimental, and the prognosis for each to increase to the required levels is discussed.

Inductive voltage adder driven X-ray sources for hydrodynamic radiography

Inductive voltage adder (IVA) accelerators were developed to provide high-current (100s of kA) power pulses at high voltage (up to 20 MV) using robust modular components. This architecture simultaneously resolves problems found in conventional pulsed and linear induction accelerators. A variety of high-brightness pulsed X-ray radiographic sources are needed from submegavolt to 16-MeV endpoints with greater source brightness (dose/spot/sup 2/) than presently available. We are applying IVA systems to produce very intense (up to 75 TW/cm/sup 2/) electron beams for these flash radiographic applications. The accelerator electromagnetic pulse is converted to a directed electron beam at the end of a self-magnetically insulated vacuum transmission line. The cantilevered cathode threading the accelerator cavities terminates in a small (l-mm diameter) needle, producing the electron beam which is transported to a grounded Bremsstrahlung converter within a strong (/spl sim/50 T) axial magnetic field. These systems produce mm-sized stable electron beams, yielding very intense X-ray sources. Detailed simulations of the electron beam generation, transport, and target interaction are presented along with scaling laws for the radiation production and X-ray spot size. Experimental studies confirm these simulations and show this reliable, compact, and inexpensive technology scales to 1000-R doses a meter from a mm-diameter source in 50 ns.

Rod pinch radiography source optimization at 2.3 MV

Rod pinch diodes have shown considerable promise as high-brightness flash X-ray sources for penetrating dynamic radiography for a variety of DOE Defense Programs applications. The rod pinch diode uses a small diameter (0.4 - 2 mm) anode rod extended through a cathode aperture. When properly configured, the electron beam from the aperture edge can self-insulate and pinch onto the tip of the rod creating an intense, small X-ray source. Experiments have been performed on Sandias SABRE accelerator (2.3MV, 40 /spl Omega/, 60 ns) to optimize the source by maximizing the figure of merit (dose/spot diameter/sup 2/) and minimizing the diode impedance droop. Many diode parameters have been examined including rod diameter, rod length, rod material, cathode aperture diameter, and cathode thickness. The best configuration tested so far uses a 0.5 mm diameter gold rod, a 6 mm rod extension beyond the cathode aperture (diameter = 8 mm), to produce a world record 3.5 rad (filtered dose) at 1 m from a 0.85 mm x-ray spot.

Performance of the Cygnus x-ray source

Cygnus is a radiographic x-ray source developed for support of the Sub-Critical Experiments Program at the Nevada Test Site. Major requirements for this application are: a dramatically reduced spot size as compared to both Government Laboratory and existing commercial alternatives, layout flexibility, and reliability. Cygnus incorporates proven pulsed power technology (Marx Generator, Pulse Forming Line, Water Transmission Line, and Inductive Voltage Adder sub-components) to drive a high voltage vacuum diode. In the case of Cygnus, a relatively new approach (the rod pinch diode [1]) is employed to achieve a small source diameter. Design specifications are: 2.25 MeV endpoint energy, &#60; 1 mm source diameter, and >3 rads dose at 1 meter. The pulsed power and system architecture design plan has been previously presented [2]. The first set of Cygnus shots were geared to verification of electrical parameters and, therefore, used a large area diode configuration offering increased shot rate as compared to that of the rod pinch diode. In this paper we present results of initial rod pinch operation in terms of electrical and radiation parameters.

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

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.