I. Molina

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.

Magnetic insulation, power flow, and pulse power results on RITS-3

RTFS (Radiographic Integrated Test Stand) is an induction voltage adder designed by Sandia and PSD to provide 16-MV, 150-kA electron beams and other capabilities. Previous publications have reported on tests of a single pulse forming line and adder cell, including initial results of the effects of various degrees of non-uniform injection of current into the adder bore on magnetic insulation and power flow in the downstream MITL. Now RITS-3 has been constructed, consisting of three pfls driven by a common intermediate store; three induction cells, one driven by each pfl; a three-stage, 4-MV, 150-kA vacuum voltage adder; and an output MITL and diode. Here we report on (1) simulations of the three-stage adder using the MRC 3-D particle-in-cell code LSP that address the effects of injected current non-uniformities on magnetic insulation and power-flow both upstream and downstream in a multi-cell adder; (2) experimental results compared with simulations; and (3) initial performance of the RITS-3 pulse power.

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.

Measurement of the electron–ion‐hose instability growth rate

The growth rate of the ion‐hose instability has been measured for a 2.5 MeV, 1 kA, 1 μsec electron beam following plasma channels of O2, N2, and H2 (in the ion‐focused regime). The amplitude of transverse oscillations of a given beam segment was seen to grow, saturate, and damp as the segment traveled. The offset amplitude at saturation (dm) was seen to be an exponential function of the beam pulse duration (t): dm=dm,0 exp(2πGLt/tc), where tc is the time required for one channel oscillation and GL is the growth rate (for beam oscillations less than the channel radius). With beam radius equal to channel radius (rb=rc), and channel density equal to half the beam density, GL=0.75±0.15. Here GL was seen to scale with the square root of channel ion mass when data from channels of O2, N2, and H2 were compared. Also, GL was seen to increase as rb was increased (with the initial beam emittance kept the same). A fivefold decrease in growth rate was observed for t>tc and dm>rc. The decrease in growth rate may be du...

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.

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.

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.