S. Cordova

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.

Cygnus Dual Beam Radiography Source

The subcritical experiment (SCE) program was initiated after the 1992 moratorium on underground nuclear testing in support of stockpile stewardship. The dynamic material properties of plutonium are a major topic of exploration for the SCE program. In order to provide for a multilayered containment of plutonium, the SCEs are executed in the Ula underground tunnel complex at the Nevada Test Site (NTS). Cygnus is a new radiographic X-ray source developed for diagnostic support of the SCE Program at NTS. Typically, SCEs have been limited to surface diagnostics. Cygnus radiography was developed to complement the existing surface diagnostics, provide a more extensive spatial view (albeit temporally limited), and provide internal (penetrating) measurements. The Stallion series of SCEs consists of the following four shots listed in chronological order: Vito, Rocco, Mario, and Armando. Armando was the initial experiment for Cygnus radiography. The Rocco, Mario, and Armando tests use identical physics packages, permitting the correlation of Armando radiographic results with surface results from all three shots. The main X-ray source requirements for an SCE involve spot size, intensity, penetration, and duration. To this end Cygnus was designed to satisfy the following specifications: ~1 mm source diameter, 4 Rads dose at a distance of 1 meter, ~2.25 MeV endpoint energy, and < 100 ns pulse length. Two Cygnus sources (Cygnus 1, Cygnus 2) were fielded at NTS providing two views separated in space by 60deg and in time by 2 mus. Cygnus performance as a dual beam radiography source at NTS is highlighted in this paper.

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.

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.

Linear Transformer Driver (LTD) development at Sandia national laboratory

Most of the modern high-current high-voltage pulsed power generators require several stages of pulse conditioning (pulse forming) to convert the multi-microsecond pulses of the Marx generator output to the 40–300 ns pulse required by a number of applications including x-ray radiography, pulsed high current linear accelerators, Z-pinch, Isentropic Compression (ICE), and Inertial Fusion Energy (IFE) drivers. This makes the devices large, cumbersome to operate, and expensive. Sandia, in collaboration with a number of other institutions, is developing a new paradigm in pulsed power technology; the Linear Transformer Driver (LTD) technology. This technological approach can provide very compact devices that can deliver very fast high current and high voltage pulses. The output pulse rise time and width can be easily tailored to the specific application needs. Trains of a large number of high current pulses can be produced with variable inter-pulse separation from nanoseconds to milliseconds. Most importantly, these devices can be rep-rated to frequencies only limited by the capacitor specifications (usually is 10Hz). Their footprint as compared with current day pulsed power accelerators is considerably smaller since LTD do not require large oil and de-ionized water tanks. This makes them ideally fit for applications that require portability. In the present paper we present Sandia Laboratorys broad spectrum of developmental effort to design construct and extensively validate the LTD pulsed power technology.

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

Inductive voltage adder (IVA) for submillimeter radius electron beam

We have already demonstrated the utility of inductive voltage adder accelerators for production of small-size electron beams. In our approach, the inductive voltage adder drives a magnetically immersed foilless diode to produce high-energy (10-20 MeV), high-brightness pencil electron beams. This concept was first demonstrated with the successful experiments which converted the linear induction accelerator RADLAC II into an IVA fitted with a small 1-cm radius cathode magnetically immersed foilless diode (RADLAC II/SMILE). We present validations of extending this idea to mm-scale electron beams using the SABRE and HERMES-III inductive voltage adders as test beds. The SABRE experiments are already completed and have produced 30 kA, 9 MeV electron beams with envelope diameter of 1.5 mm FWHM. The HERMES-III experiments are currently underway.