Hohlraum Targets Research Articles

Hot, dense, millimeter-scale, high-Z plasmas for laser-plasma interactions studies.

We have designed and produced hot, millimeter-scale, high-Z plasmas of interest for National Ignition Facility hohlraum target design. Using a high-Z gas fill produces electron temperatures in the 3.5-6-keV range, the highest temperatures measured to date for high-density (10(21) e/cm(3)) laser-heated plasmas, and much higher than the 3 keV found for low-Z (neopentane) fills. These measurements are in good agreement with the target design calculations, and the L-shell spectroscopic approach used to estimate the electron temperature has certain advantages over traditional K-shell approaches.

Neutron penumbral aperture images of inertial confinement fusion targets achieve 60 μm resolution (abstract)

Images formed by 14 MeV neutrons emitted from the core of inertial confinement fusion targets demonstrate 60 μm resolution for the penumbral aperture imaging system used in our 1988 experiments. Hohlraums containing deuterium–tritium filled capsules were irradiated at the Nova Laser Facility and produced images with detector limited resolution. Neutron yields ranged from 5×1010 to 2×1011. The results imply that convolution of the aperture point spread function with the neutron emitting region of the target core is less than 40 μm. Images produced by the indirect-drive hohlraum targets are compared with previously reported images of high-yield (1×1013) exploding-pusher targets that show 150 μm (full width half maximum) diameters for the emitting region.

Hohlraum targets driven by cluster ion beams for inertial confinement fusion

Correlated ion stopping of charged debris resulting from the fragmentation of energetic cluster ions focussed on Au foam converters is considered for driving indirectly a thermonuclear capsule. The 1D simulation of converter time evolution demonstrates a very high conversion efficiency of projectile energy into hard X-ray photons building up a very hot ( T r>300 eV) thermal bath. Intense and energetic cluster ion beams thus demonstrate considerable potential as a novel driver for inertial confinement fusion.

Recent progress on fusion nuclear technology in inertial fusion

Abstract Inertial fusion energy (IFE) ignition and gain facilities based on lasers, the national ignition facility (NIF/USA) and the laser megajoule (LMJ/France), will be operating in the first years (2002–2010) of the next century. The conceptual design of another ignition and gain facility, the heavy ion design of ignition facility (HIDIF), is under way in an European initiative. NIF's target emission spectra show the differences in the energy fraction of debris and X-rays between direct and indirect driven (hohlraum) targets and the importance of the angular dependence in hohlraum emissions. Emerging neutrons could limit the availability to the chamber area because of activation and produce damage in the final optics; however, the NIF/LMJ designs do not envision significant effects in routine or accident conditions. Damage from debris and X-rays to the chamber wall has been extensively analyzed, concluding on the use/choice of coating materials (B4C, B, C). Removal depths in the wall/chamber have been estimated, together with the materials deposition in areas such as final optical assembly (FOA), target positioner, and debris shields. Differences in NIF and LMJ chambers include essentially the concrete shielding sandwiches in the Al layers in the case of LMJ. The use of present technology of composite materials, instead of Al, will give a better contact dose rate in some frequent access areas of the facilities. After previous reactor concepts reported in the early 90's, additional work has been done on LIBRA-SP, HYLIFE-II and KOYO designs, establishing new approaches to protection and material consideration, together with systems such as the target injector (HYLIFE-II). The choice and disposition of materials in KOYO (SiC + HT-9), LIBRA-SP (HT-9) and HYLIFE-II (SS304) is presented for handling and disposal considerations, with clear consequences in the classification of areas for shallow land burial (SLB). The importance of uncertainties in nuclear data will be discussed, and its influence in early conclusions. The effect of neutron spectrum in accidental release emissions by activation of materials is also critical in the characterization of the components of the reactor, and will be discussed.

Inertial fusion activities in Spain

A method is proposed to exploit the aneutronic p- 11B fusion reaction by means of igniting a heat detonation wave that expands across the fuel from a small heated region. The tritium seeding effect of DT, 6LiD, 7LiD, p 11B, 10BD, 3HeD fuels has been analysed, showing that the impact of initial tritium is not so important in aneutronic reactions. The 2D ARWEN code includes a flux limited multigroup radiation diffusion solver with grey acceleration, which is being used to design and analyse X-ray driven experiments on Richtmyer–Meshkov instability, and on jet production with conical hollow charges. Shock wave propagation experiments, using Al foils, performed at LULI have been analysed using simulations with the SARA-1D multigroup radiation code. A small level of preheating caused by the absorption of X-rays with energies close to the K-edge of aluminium has been shown. Integrated simulations of NIF-type hohlraum targets based on latest revision of MULTI2D are presented. A new model for the Rayleigh–Taylor instability of steady ablation fronts which overcomes past inconsistencies and reproduces a wide set of reported results has been developed. DENIM's LTE and non-LTE atomic physics codes, JIMENA, ANALOP, CARMEN, and M3R, have been compared with others codes and with recent available experiments. Results reviewed here have been presented in the Workshop on Kinetic NLTE codes held in August 1996, and the WORKOP-IV for LTE plasmas held in Madrid May 1997. New capabilities in the inventory ACAB code are: (i) pulsed irradiation, applied to NIF scenario; (ii) capability to estimate the effect of the cross-sections uncertainties in the accuracy of activation calculations, applied to study the long-term activation of natural elements, with neutron spectrum from HYLIFE-II vessel; (iii) sequential charged-particle reactions, analysing the effect in the activation of Flibe, under a HYLIFE-II scenario for maintenance and decommissioning purposes. Analysis of deep penetrations in the KOYO reactor, affecting damage of final optics, has been performed. Irradiation damage of high-Z materials, using Molecular Dynamics and Defect Kinetic MonteCarlo Diffusion models, has been studied in a microscopic base. New reviewed results are being produced for primary damage parameters of SiC, as low activation material, and first calculations on basic metals to be extended to binary/ternary alloys (steels).

Hohlraum Symmetry Experiments with Multiple Beam Cones on the Omega Laser Facility

Symmetry experiments have been performed on the Omega laser facility using cylindrical hohlraum targets with as many as 40thinspthinspbeams arranged into multiple beam cones. These experiments constitute a first step in the development of {open_quotes}beam phasing{close_quotes} in which beams are arranged into multiple beam cones, forming multiple rings of beam spots on the inner surface of a cylindrical hohlraum, and demonstrate the ability to model hohlraums incorporating multiple beam cones and to tune the time-integrated capsule flux asymmetry by adjustment of the beam pointing. {copyright} {ital 1998} {ital The American Physical Society}

Integrated numerical simulation of indirect laser-driven implosion for ICF

A NIF-type hohlraum target (Lindl 1995; CEA 1995) is analyzed in an integrated calculation that uses last revision (4.2) of MULTI2D code (Ramis et al. 1992). The numerical simulation includes: Lagrangian mesh, tracking of laser beams, inverse bremsstrahlung absorption, 3D radiation transport, radiation-induced hydrodynamics of casing and window panes, and radiatively driven capsule implosion. Electron heat conduction (Spitzer with flux limiter), tabulated Equations of State (Sesame), and LTE opacities are also included. Detailed results on hydrodynamic evolution of the system, compression symmetry, and energy balance are also provided.

Prospects of inertial confinement fusion

The present status of inertial confinement fusion (ICF) is briefly reviewed, emphasizing the National Ignition Facility (NIF) project in the US and the Megajoule project in France. Critical aspects of target performance such as symmetry and stability of capsule implosions and interaction physics in hohlraum targets are discussed. The advantages of heavy-ion beam drivers and corresponding research programs are pointed out with reference to the long-term prospects for ICF power production. The new concept of the fast ignition of precompressed fuel by petawatt, picosecond laser pulses is also covered. The laser plasma group at the Max-Planck-Institute for Quantum Optics (MPQ) is one of the European institutes funded by EURATOM for an ICF keep-in-touch activity, and we highlight results obtained at MPQ relevant to the recent progress of ICF.

Hohlraum Radiation Drive Measurements on the Omega Laser

Time-resolved drive measurements with thin-walled hohlraum targets on Omega [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)] are presented and compared with two-dimensional hydrodynamical simulations. For the first time, radiation fluxes are measured through the laser entrance hole instead of through a diagnostic side hole. We find improved agreement between time dependent experiments and simulations using this new technique. In addition, the drive history obtained in this manner correlates well with the drive onto the capsule at target center.

Reduction of Radiation Asymmetry with Radiation Shields in Indirectly Driven Inertial Confinement Fusion Hohlraum Targets

Using a relatively simple static model and allowing a number of additional radiation shields in an axially symmetric hohlraum having two converters, a systematic process of reducing the asymmetry of the radiation field on a fusion capsule is presented. As a result of this procedure, a hohlraum target is obtained that shows a high degree of symmetrization even in a very early stage of irradiation. The sensitivity of the symmetry to the form and the position of each hohlraum component is investigated. To increase the reliability of the results, an enhanced model of radiation reemission in a hohlraum target, including reemission of the converter, is developed. Using this enhanced model it is found that the obtained hohlraum configuration is still valid, while the simple reemission model leads to incorrect results in special cases. It is also shown that the detailed configuration of a hohlraum target, especially of the radiation shields, depends considerably on the temperature distribution of the converter surface, but it is always possible to achieve a high degree of symmetry with radiation shields.

Dynamic Effects on Symmetrization in Indirectly Driven Inertial Confinement Fusion Hohlraum Targets

A simple dynamic model is developed for the investigation of the hohlraum symmetrization by estimating the time changes of the optical geometry of the components under the influence of hohlraum radiation. The expansion of the converters heated by ion beams is also included. By performing dynamic simulations of the hohlraum target, it is found that the change in hohlraum geometry due to hydrodynamic expansion has a crucial effect on symmetrization. It is also found that the symmetry is now strongly dependent on time, and the optimal condition can only be satisfied for a limited time interval. An improved version of the hohlraum target is discussed, which may considerably increase the optimal time interval. This concept includes the suppression of the dynamic expansion by using low-Z gas in the hohlraum, the reduction of the optical expansion of the converter by using low-Z material, and modification of the shield configuration.

Time-dependent, x-ray spectral unfolds and brightness temperatures for intense Li+ ion beam-driven hohlraums

X-ray-producing hohlraums are being studied as indirect drives for inertial confinement fusion targets. In a 1994 target series on the PBFAII accelerator, cylindrical hohlraum targets were heated by an intense Li+ ion beam and viewed by an array of 13 time-resolved, filtered x-ray detectors (XRDs). The unfold operator (UFO) code and its suite of auxiliary functions were used extensively in obtaining time-resolved x-ray spectra and radiation temperatures from this diagnostic. The UFO was also used to obtain fitted response functions from calibration data, to simulate data from blackbody x-ray spectra of interest, to determine the suitability of various unfolding parameters (e.g., energy domain, energy partition, smoothing conditions, and basis functions), to interpolate the XRD signal traces, and to unfold experimental data. The simulation capabilities of the code were useful in understanding an anomalous feature in the unfolded spectra at low photon energies (⩽100 eV). Uncertainties in the differential and energy-integrated unfolded spectra were estimated from uncertainties in the data. The time–history of the radiation temperature agreed well with independent calculations of the wall temperature in the hohlraum.

One-dimensional time resolved soft x-ray imaging of colliding plasmas in a laser heated cavity

We describe x-ray streak camera measurements of wall motion and plasma filling in hohlraum targets heated by the AWE HELEN laser. An x-ray streak camera using a transmission mode photocathode on a thin plastic substrate (1000 Å Parylene-N) was coupled to a 15° incidence gold mirror to define a spectral channel response of width 45 eV full width at half-maximum centered around 120 eV. A 20 μm diam pinhole was used to image the hohlraum interior onto the photocathode slit of the streak camera, via the gold reflector. Plasma expansion from the laser hot spots, and the indirectly heated wall, was recorded. The experimental data are compared with simulations using the AWE Lagrangian hydrocode NYM.

Hohlraum Targets Driven by Cluster Ion Beams for Inertial Confinement Fusion

Correlated ion stopping of charged debris resulting from the fragmentation of energetic cluster ions focussed on Au foam converters is considered for driving indirectly a thermonuclear capsule. The 1D simulation of converter time evolution demonstrates a very high conversion efficiency of projectile energy into hard X-ray photons building up a very hot (Tr > 300 eV) thermal bath. Intense and energetic cluster ion beams thus demonstrate considerable potential as a novel driver for inertial confinement fusion.

Measurements of Electron Temperature by Spectroscopy in Hohlraum Targets.

We report on the use of x-ray spectroscopy of mid- $Z$ dopants to measure the electron temperature of hohlraum targets. The hohlraums are gas-filled Au cylinders and, when irradiated with 20 kJ of $0.35\ensuremath{\mu}\mathrm{m}$ laser light, they become mm-sized plasmas bathed in a radiation field. The peak temperatures achieved by the target are 3.7 keV. In addition to being the first electron temperature measurements of the hohlraum itself, these measurements enable an investigation of the thermal energy and electron conduction of the target.

ITEP conception of a heavy ion fusion facility

A conceptual scheme is presented which combines self-consistently a hohlraum target design at the input energy E dr = 10 MJ with a scheme for a charge-symmetric heavy ion driver capable of producing the pulse power profile and the focal spot size required for target ignition and high energy gain. The physical issues underlying the key aspects of the target performance are put together into a coherent theoretical scheme which allows the target parameters to be rescaled to other values of E dr. Special attention is paid to the method of pulse shaping in the final stages of beam compression by taking advantage of four different platinum isotopes employed in the driver scheme. It is found that a heavy ion fusion facility designed along the same lines for a considerably lower input energy E dr ≈ 2 MJ would require a power pulse of 2.5–3 ns, which appears to be prohibitively short for present accelerator technology.

Energetics and symmetry of hohlraum targets driven by ion beam pulses with simple time shape

The energetics of hohlraum targets for inertial fusion are studied by means of one-dimensional radiation hydrodynamics simulations, assuming that a pulse of thermal X-rays with a simple time shape is fed into the cavity. A fusion yield E fus = 160–250 MJ is released by a capsule with fuel mass m DT = 3.3 mg, driven by a two-step pulse. The required input energy is E x ≈ 3.4 MJ for a hohlraum area ratio a = 9 and 6.6 MJ for a = 20, corresponding to gains of G x = E fus/ E x = 50–73 and 25–35 respectively. Higher gains are obtained by three-step pulses. Targets with m DT = 0.4 mg require better-shaped pulses, with at least three steps. Driven by E x = 0.85−1.7 MJ, they release E fus = 8–10 MJ. Symmetry aspects of axially symmetric hohlraums driven by heavy ion beams are studied by a viewfactor code, employing wall motion and re-emissivities provided by the one-dimensional hydro-simulations. The dependence of the capsule irradiation asymmetry on the hohlraum aspect ratio, area ratio and fill density is analyzed. Reductions of wall motion and converter expansion, and the use of shields appear necessary to allow for the use of a moderate area ratio a ≈ 10–15.

Integrated two-dimensional simulations of hohlraum targets

We report on integrated simulations of hohlraum targets based on the new code MULT12D for two-dimensional radiation hydrodynamics. Targets for heavy ion beam fusion with two-sided irradiation are discussed; l = 4 asymmetry is controlled by low-albedo rings in the hohlraum wall. Targets designed for laser-driven shock wave experiments are also presented. MULT12D in its present version is still restricted to the use of simplified (power-law-type) equations of state and opacities, so the results are of semi-quantitative nature. We focus on hohlraum dynamics and aspects of radiation field symmetrization.

Equation of state studies at SILP by laser-driven shock waves

The experimental progress of laser equation of state (EOS) studies at Shanghai Institute of Laser Plasma (SILP) is discussed in this paper. With a unique focal system, the uniformity of the laser illumination on the target surface is improved and a laser-driven shock wave with good spatial planarity is obtained. With an inclined aluminum target plane, the stability of shock waves are studied, and the corresponding thickness range of the target of laser-driven shock waves propagating steadily are given. The shock adiabats of Cu, Fe, SiO2are experimentally measured. The pressure in the material is heightened remarkably with the flyer increasing pressure, and the effect of the increasing pressure is observed. Also, the high-pressure shock wave is produced and recorded in the experimentation of indirect laser-driven shock waves with the hohlraum target.

Isoelectronic x-ray spectroscopy to determine electron temperatures in long-scale-length inertial-confinement-fusion plasmas.

We have successfully employed isoelectronic line ratios to measure the electron temperature in gas-filled Hohlraum targets and gas bags shot with the Nova laser. These targets produce millimeter-scale-length plasmas with electron density ${\mathit{N}}_{\mathit{e}}$\ensuremath{\sim}${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and electron temperature ${\mathit{T}}_{\mathit{e}}$\ensuremath{\sim}3 keV. The Hohlraum targets can also produce radiation temperature exceeding 200 eV. Isoelectronic line ratios are well suited to this measurement because they are relatively insensitive to radiation field effects in Hohlraum targets, opacity, transients, and variations in electron density compared to conventional line ratios. We survey the properties of isoelectronic line ratios formed from ratios of n-to-1 resonance transitions in heliumlike Cr to the same transitions in Ti and compare with conventional ratios of n-to-1 transitions in hydrogenlike Ti to the corresponding transitions in heliumlike Ti, concentrating on plasma parameter ranges of interest to the Nova experiments. We also consider the same ratios using K and Cl. Atomic kinetics are treated using collisional-radiative models and experimental data are analyzed with the aid of radiation-hydrodynamics calculations. When we apply isoelectronic techniques to the Nova experimental data, we find that the targets have electron temperatures of at least 3 keV. \textcopyright{} 1996 The American Physical Society.