Showing papers in "Laser and Particle Beams in 1994"

Numerical simulation of mixing by Rayleigh-Taylor and Richtmyer-Meshkov instabilities

Abstract: Rayleigh-Taylor (RT) and Richtmyer–Meshkov (RM) instabilities at the pusher–fuel interface in inertial confinement fusion (ICF) targets may significantly degrade thermonuclear burn. Present-day supercomputers may be used to understand the fundamental instability mechanisms and to model the effect of the ensuing mixing on the performance of the ICF target. Direct three-dimensional numerical simulation is used to investigate turbulent mixing due to RT and RM instability in simple situations. A two-dimensional turbulence model is used to assess the effect of small-scale turbulent mixing in the axisymmetric implosion of an idealized ICF target.

Radiation transport and atomic physics modeling in high-energy-density laser-produced plasmas

Abstract: The radiation hydrodynamics in laser-produced high-energy-density plasmas has been successfully simulated by means of the MULTI hydrocode It is used in combination with the SNOP atomic physics code, which uses a steady-state screened hydrogenic explicit ion model and which generates non-LTE opacity tables for MULTI After a brief general review of the modeling of the radiation hydrodynamics in laser-produced plasmas, the underlying physical models of MULTI and SNOP are described in detail, with particular emphasis on atomic physics Examples of simulations of the radiation transport in laser plasmas are presented They include a laser-irradiated gold foil and a radiatively heated carbon foil

LISP: Laser impulse space propulsion

Abstract: It is not often that a new form of transportation suddenly appears and replaces what was hitherto regarded as mankind's only realistic option. In space and upper atmosphere transportation, chemical rockets have held center stage for over half a century. Tsiokolvsky's ideas led to Wernher von Braun's V2, which in turn led to the Soyuz, Apollo, and Ariane programs and the Space Shuttle. But recently theoretical and computational studies as well as a few initial experiments have pointed to a new option: laser impulse space propulsion (LISP). This may offer a more efficient and less ecologically damaging means of putting payloads into orbit. The world high-power laser community is well suited to following and aiding developments in LISP, though most practical research is still at an embryonic level. Obviously an effort of the size required to develop a laser-driven low-earth-orbit (LEO) launcher would require a multinational commitment. LISP could then be regarded as a parallel challenge to those of achieving ICF rriicrofusion yield and of improving X-ray lasers, especially in the “water window.” Any physicist or engineer involved with the latter projects would find many points in common with the former. It therefore seems appropriate to briefly review the progress made in LISP and also to communicate some recent results from high-power laser-matter experiments that have lead to conceptual designs.

Simulation codes for light-ion diode modeling

Abstract: The computational tools used in the investigation of light-ion diode physics at Sandia National Laboratories are described. Applied-B ion diodes are used to generate intense beams of ions and focus these beams onto targets as part of Sandia's inertial confinement fusion program. Computer codes are used to simulate the energy storage and pulse forming sections of the accelerator and the power flow and coupling into the diode where the ion beam is generated. Other codes are used to calculate the applied magnetic field diffusion in the diode region, the electromagnetic fluctuations in the anode-cathode gap, the subsequent beam divergence, the beam propagation, and response of various beam diagnostics. These codes are described and some typical results are shown.

Detection of mercury in air by time-resolved laser-induced breakdown spectroscopy technique

Abstract: For several years the laser-induced breakdown spectroscopy (LIBS) technique has been applied successfully to the problem of detecting small traces of pollutants in gases. The possible application of this method for the individuation of mercury in air is discussed. The laboratory prototype of the detection system is described in detail, and the sensitivity of the system for the diagnostics of small traces of mercury is determined. The reduced dimensions of the experimental apparatus and its relatively low cost make the LIBS method competitive with other laser-based methods for in situ analysis.

Fokker–Planck modeling of electron transport in laser-produced plasmas

Abstract: Fokker-Planck (FP) codes have become useful tools for modeling nonlocal heat-transport phenomena in laser-produced plasmas. Several possible simplifications to the FP equation as well as different numerical techniques available for its solution are investigated. The most robust and efficient approach is found to involve the diffusive approximation of the FP equation and the alternating-direction-implicit method of solution. The SPARK FP code, which has been developed along these principles, is described in detail. It incorporates fluid ions and solves for transport on either a two-dimensional Eulerian grid or a one-dimensional Lagrangian grid. Sample simulation results are presented, together with a discussion of possible improvements to the code.

Volume ignition targets for heavy-ion inertial fusion

Abstract: Inertial confinement fusion (ICF) targets can be imploded by heavy-ion beams (HIBs) in order to obtain a highly compressed fuel microsphere. The hydrodynamic efficiency of the compression can be optimized by tuning the ablation process in order to produce the total evaporation of the pusher material by the end of the implosion. Such pusherless compressions produce very highly compressed targets for relatively short confinement times. However, these times are long enough for a fusion burst to take place, and burnup fractions of 30% and higher can be obtained if the volume ignition requirements are met. Numerical simulations demonstrate that targets of 1-mg DT driven by a few MJ can yield energy gains of over 70. Although direct drive is used in these simulations, the main conclusions about volume ignition are also applicable to indirect drive.

Ultrashort powerful laser matter interaction: Physical problems, models, and computations

Abstract: The different physical models, computational methods, and results describing the properties of a plasma produced by an ultrashort laser pulse interacting with a solid target at different laser intensities are presented The basic issues affecting the laser-plasma interaction are considered, including: the different absorption mechanisms; the formation of a transient, nonequilibrium, asymmetric electron distribution function; the energy losses; the role of instabilities and the ponderomotive force; ion expansion and acceleration; and surface wave propagation along the boundary of the plasma The article concludes with a discussion of the range of applicability of the different approximations and future perspectives for the field

Computational modeling for X-ray lasers

Abstract: Various problems involved in the simulation of current X-ray laser experiments are discussed. The methods are based on laser-plasma simulation techniques with simultaneous calculation of the ionization dynamics, and are particularly appropriate for collisional and recombination pumped systems.

Radially convergent 30–100-μs e-beam-pumped Xe and Ne lasers

Abstract: The design scheme of a radially convergent 30-, 70-, or 100-μs e-beam-pumped laser with 9 or 18 1 excited volume is described. Amplitude, temporal, and spectral characteristics of the laser output, as well as laser thresholds in (Ar–Xe, He–Ar–Xe, He–Ne–Ar, and He– Ne–Ar–H2) gas mixtures have been investigated. The distribution of the specific deposited energy over the laser chamber cross section has been calculated by the Monte Carlo method. The small signal gain and unsaturable loss values of the active medium were measured for a Penning plasma Ne laser at λ = 0.585 μm.

A high-contrast all-glass ps-terawatt CPA laser system

Abstract: We report on a compact, all-glass laser using a fiberless CPA technique as a potential system for the generation of extreme high-contrast picosecond pulses in the terawatt power range. New exciting experiments in the field of laser-matter interaction at extremely high intensities require both a high output energy and a very high peak to background intensity ratio (contrast). At present the system delivers 1.5 ps-pulses of 1.5-J energy with an intensity contrast of 4.109.

New methods of X-ray spectroscopy of laser-produced plasmas with one-dimensional spatial resolution

Abstract: Two novel high-dispersion, high-resolution spectroscopic methods are described. The properties of the vertical dispersion variants of the double-crystal spectrometer and the Johann spectrometer with a cylindrically bent crystal are discussed and compared with those of standard spectroscopic schemes. Preliminary experimental data demonstrate the good luminosity and extreme spectral and 1-D spatial resolution of these methods, which should prove useful in high-precision X-ray spectroscopic measurements of laser-produced plasmas.

Three-dimensional α-particle imaging of laser-driven implosions

Abstract: A technique is demonstrated for optimally generating three-dimensional reconstructions of images formed using a minimal quantity of data. The results are illustrated using thermonuclear α-particles from laser-driven implosions. The images are generated with a maximum entropy deconvolution algorithm from sets of three or four penumbral imaging cameras. It is demonstrated that this approach provides superior resolution and reveals structures not visible from the corresponding two-dimensional reconstructions of the constituent data. This technique can be successfully applied even when the total number of particles recorded in the image is less than 1000.

Hydrodynamic instability and target design

Abstract: The development of hydrodynamic instability (HDI) in laser targets is studied by means of 2D numerical code “ATLANT.” The scaling of HDI development for the condition of “DELFIN-1” laser setup experiments is derived. It is shown that there is the possibility to improve the shell compression condition for the case of long-wave perturbation of laser flux (determined by particular target irradiation geometry) by using the shell with “relief.”

Computer modeling of ICF target chamber phenomena

Abstract: The target chamber of an inertial confinement fusion (ICF) power plant or high-yield test facility must be designed to absorb the target produced Xrays and ions and survive the resulting effects. The target chamber conditions must be restored in fractions of a second for high repetition rate power applications. Computer modeling of these phenomena is essential because equivalent conditions cannot be produced in laboratory experiments prior to the first ignition of high-yield ICF targets. Choices of models are dictated by specific reactor design strategies. The two major strategies, gas protection and sacrificial first surfaces, are used as a guide to our discussion. Physical models for ion, electron, and X-ray deposition are discussed, along with physical and numerical modeling of the resulting phase changes intarget chamber structures. The hydrodynamics and radiative transfer in the target chamber vapors and plasmas are central topics.

ELBA (electron beams in accelerators) particle simulation code

Abstract: ELBA is a three-dimensional, particle-in-cell, simulation code that has been developed to study the propagation and transport of relativistic charged particle beams. The code is particularly suited to the simulation of relativistic electron beams propagating through collisionless or slightly collisional plasmas or through external electric or magnetic fields. Particle motion is followed via a coordinate “window” in the laboratory frame that moves at the speed of light. This scheme allows us to model only the immediate vicinity of the beam. Because no information can move in the forward direction in these coordinates, particle and field data can be handled in a simple way that allows for very large scale simulations. A mapping scheme has been implemented that, with corrections to Maxwell's equations, allows the inclusion of bends in the simulation system.

Spectroscopic analysis of short-pulse laser-produced plasmas

Abstract: Experimental spectra of hot dense plasmas of aluminium produced by the interaction of a subpicosecond laser with solid targets at 1016 and 5 × 1017 W/cm2 are analyzed and discussed. A detailed analysis of the K-shell spectra is given through time-dependent calculations of atomic physics postprocessed to Fokker-Planck calculations of the laser-matter interaction. The non-Maxwellian character of the electron distribution function is shown. An evaluation of the electronic density and of the ion temperature 7i will be presented through Stark line broadening calculations. An X-ray spectrum from a Tantalum target also will be presented along with a preliminary interpretation.

Progress in inertial confinement fusion physics at Centre d'Etudes de Limeil-Valenton

Abstract: The laser program developed at the Centre d'Etudes de Limeil-Valenton, Saint-Georges, France (CEL-V) is concentrated on a systematic investigation of indirect drive fusion; by comparison with direct drive, this process is expected to provide the required irradiation uniformity with relaxed constraints on laser beam quality. The main concerns are radiative transfer and preheat, hydrodynamic instabilities, and high-density X-ray driven implosions. Ablative implosion experiments have been conducted with the two beams at the Phebus facility (5 kJ, 1.3 ns, 0.35 μm). Symmetry was proved to be controlled by the casing structure, following scaling laws describing hohlraum physics. A compressed DT density ∼100 ρ0 (ρ0 liquid DT density) has been deduced from activation measurements. Different aspects of the soft X-ray transfer processes, and particularly of the ablation of a low-Z material, which drives the capsule implosion, are dealt with in detailed investigations. Reported here are results on X-ray reemission and penetration in several materials, and on induced hydrodynamics of accelerated foils. The laser energy required to reach fuel ignition conditions has been evaluated from numerical simulations as well as from analytical models, taking into account hohlraum physics, capsule implosion, hot spot formation, and burn propagation. Several crucial parameters have been drawn, the most important being the radiation temperature. A target gain in the order of 10 appears achievable with a 2-MJ laser.