An understanding of the timing and dynamics of hohlraum filling by laser-induced gold wall ablation is critical to the performance of indirectly-driven fusion ignition designs for the National Ignition Facility [E. Moses and C. Wuest, Fusion Science and Technology, 43, 420 (2003)]. Hohlraum wall ablation negatively affects ignition hohlraum performance by (1) reducing laser coupling by increasing backscatter by laser plasma instabilities, e.g., stimulated Brillouin scattering, (2) altering where lagers couple by moving the critical surface away from the walls and changing the refractive index, and (3), in the case of vacuum hohlraums, ablating directly into contact with the ablation layer of the fuel capsule. We report on measurements of gold-filling of hohlraums from a series of OMEGA laser [T.R. Boehly, R.L. McCrory, C.P. Verdon et aL, Fusion Engineering and Design, 44, 35 (1999)] experiments involving vacuum and gas-filled hohlraums. On-axis x-ray imaging of gold self-emission shows delayed filling for gas-filled hohlraums, as expected. In addition, we present data on the hohlraum temperature penalty incurred with the use of a I-atmosphere methane-fill. We discuss data and simulation predictions for I-atmosphere neopentane filled hohlraums driven with a modified laser pulse.

In this work, two-dimensional particle-in-cell simulations are used to examine the electron physics in the rod-pinch diode, a device that can be used to produce a relatively low-energy (a few MeV) radiographic electron source. It is found that with diode parameters for which the electrons’ dominant dynamics are approximated well as a magnetized fluid, the diode produces an electron source with a desired small spot size as the electrons drift to and impinge on the anode tip. However, for a large cathode-to-anode radius ratio, a population of electrons that consists predominantly of electrons emitted from the downstream surface of the cathode is found to propagate in the upstream direction and the diode may perform anomalously as a consequence. A method is proposed for improving the quality of the electron source by suppressing electron emission from the downstream cathode surface to reduce the presence of unmagnetized electrons.

The expectation-maximization (E-M) algorithm [Dempster et al., J. R. Stat. Soc. B 39 (1977) 1–38] is a maximum likelihood technique to estimate the probability density function (PDF) of a set of measurements. A high performance implementation of the E-M algorithm to characterize multidimensional data sets using a PDF parameterized as a Gaussian mixture was developed. The resulting PDFs compare favorably to histogram based techniques—no binning artifacts and less noisy (especially in the tails). The motivation, the mathematical properties and the implementation details will be discussed. The PDF estimator is used extensively in the radiographic chain model [Kwan et al., Comput. Phys. Comm. 142 (2001) 263–269] in simulations which quantify bremsstrahlung X-ray emission from rod-pinch diodes and other devices. In these devices, electrons hit an anode and produce X-ray photons. The PIC code MERLIN [Kwan and Snell, in: Lecture Notes in Physics, Springer, 1985] is used to model the dynamics of a low-energy (up to ∼2.25 MeV) radiographic electron source. The photon production is modeled with the Monte-Carlo transport code MCNP [Briesmeister, ed., MCNP—A General Monte Carlo N-Particle Transport Code, 2000]. The estimator is used to upsample and uniformly weight the PIC electrons to provide a suitable population for the Monte-Carlo calculation that would be computationally prohibitive to generate directly.

To support the design and analysis of x-ray radiographic facilities and experiments at Los Alamos, we have developed an integrated chain model, which is a set of linked physics simulation codes to generate self-consistent synthetic radiographs of experiments [1]. The expectation-maximization (EM) algorithm [2] has recently been used to great advantage to link particle- in-cell (PIC) methods, which model electron propagation to a converter target, and Monte Carlo transport methods, which model bremsstrahlung photon generation and transport through the radiographic object onto a detector. The EM algorithm is a maximum- likelihood technique to estimate the probability density function (PDF) of a set of measurements. A high performance implementation of the EM algorithm to characterize multidimensional data sets using a PDF parameterized as a Gaussian mixture has been developed (GMIX). The resulting PDFs compare favorably to histograms / tallies---no binning artifacts and less noisy (especially in the tails). GMIX is being used extensively in the radiographic chain model to quantify bremsstrahlung x-ray emission from rod-pinch diodes and other devices. In particular, the PIC simulation code MERLIN is used to model the dynamics of the rod-pinch diode in the Los Alamos radiographic machine Cygnus for the linked calculations. The rod-pinch diode used in Cygnus consists of a 9- mm diameter aperture cathode and a 0.75- mm diameter needle anode. A TEM pulse launched at one simulation boundary sets up the voltage required for the electron emission. The electron trajectories are followed self-consistently in the electromagnetic fields as the electron beam pinches at the anode tip with energies up to 2.25 MeV. The Monte Carlo transport code MCNP is used to track the bremsstrahlung photons generated by electrons incident on the anode tip. GMIX is used to “upsample” the PIC electrons to provide a suitably large population for the Monte Carlo calculation (10 7 or greater) that would have been computationally expensive to generate directly. The motivation, the mathematical properties, and the implementation details, and applications of GMIX to plasma and Monte Carlo simulations will be discussed, including: improved coupling of PIC to Monte Carlo simulations, adaptive PIC simulations, and characterization of the Cygnus radiographic parameters such as energy and angular spectra, spot size, and dose.

A realistic open-cycle gas-core nuclear rocket simulation model must be capable of a self-consistent nozzle calculation in conjunction with coupled radiation and neutron transport in three spatial dimensions. As part of the development effort for such a model, five hydrodynamic codes were used to compare with a converging-diverging nozzle experiment. The codes used in the comparison are CHAD, FLUENT, KIVA2, RAMPANT, and VNAP2. Solution accuracy as a function of mesh size is important because, in the near term, a practical three-dimensional simulation model will require rather coarse zoning across the nozzle throat. In the study, four different grids were considered. (1) coarse, radially uniform grid, (2) coarse, radially nonuniform grid, (3) fine, radially uniform grid, and (4) fine, radially nonuniform grid. The study involves code verification, not prediction. In other words, the authors know the solution they want to match, so they can change methods and/or modify an algorithm to best match this class of problem. In this context, it was necessary to use the higher-order methods in both FLUENT and RAMPANT. In addition, KIVA2 required a modification that allows significantly more accurate solutions for a converging-diverging nozzle. From a predictive point of view, code accuracy with no tuning is an important result. The most accurate codes on a coarse grid, CHAD and VNAP2, did not require any tuning. Their main comparison among the codes was the radial dependence of the Mach number across the nozzle throat. All five codes yielded a very similar solution with fine, radially uniform and radially nonuniform grids. However, the codes yielded significantly different solutions with coarse, radially uniform and radially nonuniform grids. For all the codes, radially nonuniform zoning across the throat significantly increased solution accuracy with a coarse mesh. None of the codes agrees in detail with the weak shock located downstream of the nozzle throat, but all the codes indicated the presence of a weak downstream shock.

The next giant leap for mankind will be the human exploration of Mars. Almost certainly within the next thirty years, a human crew will brave the isolation, the radiation, and the lack of gravity to walk on and explore the Red planet. However, because the mission distances and duration will be hundreds of times greater than the lunar missions, a human crew will face much greater obstacles and a higher risk than those experienced during the Apollo program. A single solution to many of these obstacles is to dramatically decrease the mission duration by developing a high performance propulsion system. The gas-core nuclear rocket (GCNR) has the potential to be such a system, We have completed a comparative study of the potential impact that a GCNR could have on a manned Mars mission, The total IMLEO, transit times, and accumulated radiation dose to the crew will be compared with the NASA Design Reference Missions.

Summary form only given. The conversion of an intense relativistic electron beam into X-rays for radiographic imaging is achieved through bremsstrahlung collisions in a target of optimal thickness. This process results in a significant amount of beam energy deposited in a small volume of the target, causing it to vaporize; the resultant expanding plasma can adversely interact with the electron beam in a single- or multiple-pulsing design. In order to simulate the beam-plasma interaction, the ionization state, electron density, and other hydrodynamic variables of the plasma must be known. This paper addresses two quantities crucial to understanding the evolution of the plasma: energy deposition in the target and the ionization state (and electron density) of the expanding plasma.

We have evaluated the usefulness and limitation of a nonintrusive beam radius diagnostic which is based on the measurement of the magnetic moment of a high-current electron beam in an axisymmetric focusing magnetic field, and relates the beam root-mean-square (RMS) radius to the change in magnetic flux through a diamagnetic loop encircling the beam. An analytic formula that gives the RMS radius of the electron beam at a given axial position and a given time is derived and compared with results from a 2-D particle-in-cell code. Our study has established criteria for its validity and optimal applications.

The next giant leap for mankind will be the human exploration of Mars. Almost certainly within the next thirty years, a human crew will brave the isolation, the radiation, and the lack of gravity to walk on and explore the Red planet. However, because the mission distances and duration will be hundreds of times greater than the lunar missions, a human crew will face much greater obstacles and a higher risk than those experienced during the Apollo program. A single solution to many of these obstacles is to dramatically decrease the mission duration by developing a high performance propulsion system. The gas core nuclear rocket (GCNR) has the potential to be such a system. The gas core concept relies on the use of fluid dynamic forces to create and maintain a vortex. The vortex is composed of a fissile material which will achieve criticality and produce high power levels. By radiatively coupling to the surrounding fluids, extremely high temperatures in the propellant and, thus, high specific impulses can be generated. The ship velocities enabled by such performance may allow a 9 month round trip, manned Mars mission to be considered. Alternatively, one might consider slightly longer missions in ships that are heavily shielded against the intense Galactic Cosmic Ray flux to further reduce the radiation dose to the crew. The current status of the research program at the Los Alamos National Laboratory into the gas core nuclear rocket feasibility will be discussed.

Summary form only given. The conversion of an intense relativistic electron beam into X-rays for radiographic imaging is achieved through the bremsstrahlung process of electrons in a target of optimal thickness. To achieve desirable resolution for thick objects, an extremely high-brightness electron beam is used, and a significant amount of beam energy can be deposited in a small area of the target. Vaporization of the target material and expansion of the resultant plasma can occur. In a multi-pulsing design, which will resolve the dynamic behavior of the object, the expanding plasma can have an effect on the quality of subsequent electron beam pulses. The evolution of the plasma was investigated using a two-dimensional Eulerian magnetohydrodynamic code.

Summary form only given. The conversion of an intense relativistic electron beam into X-rays for radiographic imaging is achieved through the bremsstrahlung process of electrons in a tantalum or tungsten target of some optimal thickness. A high-dose radiographic source with small spot size is needed to achieve desirable resolution for thick objects. Consequently, an extremely high brightness electron beam is used and a significant amount of electron beam energy can be deposited in a small area of the target. Vaporization of the target material and plasma generation can result. We describe a computational methodology used to model the beam-target interaction and the evolution of the resultant plasma. Several codes, including particle-in-cell (PIC), Monte Carlo transport, and magnetohydrodynamic (MHD) codes, contribute to simulate different parts of the problem in a linked fashion. Multi-dimensional PIC calculations provide detailed characterization of the beam transmitted to the target foil. Electron-photon Monte Carlo transport codes calculate beam scattering and energy deposition in the target. These energy source conditions are used in 1-D and 2-D MHD codes to model the foil expansion and the evolution of the target plasma issues addressed by the calculations include: the effects of the time dependence of the energy profile deposited in the target; the influence of the external magnetic field on plasma expansion; the influence of the expanding plasma on the guide magnetic field (and, consequently, on the beam quality); radiation effects; and multi-dimensional effects.