Michael Berninger

Simulation of the Cygnus Rod-Pinch Diode Using the Radiographic Chain Model

Radiographic chain model is used to self-consistently model the diode with a two-dimensional particle-in-cell code (Merlin) which links to an electron-photon Monte Carlo transport code (MCNP) and obtain the spectrum under three different situations. These are: steady state spectrum using a voltage pulse of 2.5 MV; time integrated spectrum using an experimental voltage pulse; inclusion of reflexing electrons at the anode in our particle-in-cell simulation. Detailed electron dynamics are obtained in this study. The investigations conclude that the time integrated bremsstrahlung spectrum is significantly softer than that of the steady state. The simulations included reflexing electrons around the tapered and un-tapered rods. The dynamics and impact on the bremsstrahlung spectrum are presented.

Comparison of Triature Doppler Velocimetry and Visar

Triature Photon Doppler Velocimetry (TDV) is an adaptation of Photonic Doppler Velocimetry (PDV) that rejects common-mode data noise after splitting PDV three ways, with each signal 120° out of phase from each other. Testing has demonstrated that the TDV also improves temporal resolution from the typical five-nanoseconds of PDV to a subnanosecond range. This paper compares the temporal response of TDV with that of PDV and VISAR [velocity interferometer system for any reflector] in an experiment with a subnanosecond (~120-picosecond rise time) shock source. Laboratory tests were performed using a high-power laser on targets of copper and aluminum. A fast VISAR with a single-point PDV and a prototype TDV were used. A special probe that combined PDV, TDV, and fast VISAR made simultaneous velocity measurements. Breakout velocities of 1.3 km/second on copper and 2.5 km/second on aluminum were observed, where TDV resolved rise times of ~200 ps. This resolution was better than that of a fast VISAR, which can achieve ~500 ps temporal resolution. Test methods and results are presented.

X-ray pinhole camera measurements

The development of the rod pinch diode has lead to high resolution radiography used on contained explosive experiments. The rod pinch diodes use a small diameter anode rod, which extends through a cathode aperture. Electrons borne off the aperture edge can self-insulate and pinch onto the tip of the rod, creating an intense, small x-ray source. This source is utilized as the primary diagnostic on numerous experiments that include high-value, single-shot events. In such applications there is an emphasis on machine reliability, x-ray reproducibility, and x-ray quality. We have observed that an additional pinch occurs at the interface near the anode rod and the rod holder. This suggests that there are stray electrons emitted from the surfaces of the surrounding area. In this paper we present results of x-ray measurements using a pinhole camera. The camera geometry used is an upstream view 30° with respect to the diode centerline. This diagnostic will be employed to: (1) diagnose x-ray reproducibility and quality, and (2) investigate the effect of different diode configurations.

Using a relativistic electron beam to generate warm dense matter for equation of state studies

Experimental equation-of-state (EOS) data are difficult to obtain for warm dense matter (WDM)-ionized materials at near-solid densities and temperatures ranging from a few to tens of electron volts-due to the difficulty in preparing suitable plasmas without significant density gradients and transient phenomena. We propose that the Dual Axis Radiographic Hydrodynamic Test (DARHT) facility can be used to create a temporally stationary and spatially uniform WDM. DARHT has an 18 MeV electron beam with 2 kA of current and a programmable pulse length of 20 ns to 200 ns. This poster describes how Monte Carlo n-Particle (MCNP) radiation transport and LASNEX hydrodynamics codes were used to demonstrate that the DARHT beam is favorable for avoiding the problems that have hindered past attempts to constrain WDM properties.

The physics and validation of the Cygnus radiographic source for Armando

Summary form only given. Conventional simulation techniques for radiographic systems use approximations that poorly represent the dynamics of the electron beam that generates photons via the bremsstrahlung process. The radiographic chain model more accurately accounts for the electron dynamics by linking electron distributions generated in electromagnetic particle-in-cell (PIC) simulations in a self-consistent way to the Monte Carlo particle transport code MCNP. Based on the electron dynamics from PIC simulations, MCNP simulates the emission of bremsstrahlung photons due to the electron collisions with a dense target in the radiographic source and then calculates the photon transport through the imaging object onto the detectors, which are simulated with detector response functions. This integrated radiographic simulation capability has been applied in understanding the Cygnus source physics, and is validated in conjunction with the performance of the Cygnus radiographic machine, which is being used in a dual-axis configuration in the sub-critical experiment Armando. We used the two-dimensional, time-dependent, fully electromagnetic relativistic PIC code MERLIN to simulate the Cygnus rod-pinch diode. By employing the methodology of the chain model, we characterized the effect of the rod-pinch diode operating parameters (e.g., voltage, current, anode-cathode aspect ratio, anode material and radius) in the Cygnus radiographic-machine parameters, such as energy-and angle-dependent photon spectra, spot size, and dose. The calculations were validated in juxtaposition with radiographic experimental data on step wedges, rolled edges, and static objects. We present the physics principle of the rod-pinch driven radiographic source and the detailed characterization of the performance of the Cygnus source for Armando.

A physics model of lutetium oxyorthosilicate detectors: theory and experimental validation

Detector physics is an important element in the simulation of X-ray radiography. In conjunction with the radiographic chain model (RCM) developed at Los Alamos National Laboratory (LANL), we have built a high-fidelity model of the Lu2SiO5:Ce3+ (LSO) detector system for use with the Cygnus rod-pinch X-ray source. In the RCM, the two-dimensional (2D) fully electromagnetic and relativistic particle-in-cell (PIC) code MERLIN is used to model the Cygnus electron diode. The electron distributions from PIC calculations are used in the Monte Carlo N-Particle (MCNP) code to model the generation of the X-rays via the bremsstrahlung process and subsequent transport through dense objects to detectors. Radiographs are calculated in conjunction with empirically measured scintillation efficiencies for light yields. To model detector blur, MCNP calculates the point-spread functions (PSF) of X-ray scattering in the LSO. Two length scales in the PSFs can account for correlated short-range (< 0.4 mm) and long-range (uncorrelated) blur. By employing a detector model methodology, we can examine detector parameters such as the detector quantum efficiency (DQE), blur, and photon statistics. The calculations are validated in juxtaposition with experimental radiographic data on step wedges, rolled edges, and static objects. In this paper, we focus on characterizing the detector performance.