Hohlraum Targets Research Articles

Comparative properties of the interior and blowoff plasmas in a dynamic hohlraum

A Dynamic Hohlraum (DH) is formed when arrays of tungsten wires driven by a high-current pulse implode and compress a cylindrical foam target. The resulting radiation is confined by the wire plasma and forms an intense, ∼200–250eV Planckian x-ray source. The internal radiation can be used for indirect drive inertial confinement fusion. The radiation emitted from the ends can be employed for radiation flow and material interaction studies. This external radiation is accompanied by an expanding blowoff plasma. We have diagnosed this blowoff plasma using K-shell spectra of Mg tracer layers placed at the ends of some of the Dynamic Hohlraum targets. A similar diagnosis of the interior hohlraum has been carried out using Al and Mg tracers placed at 2mm depth from the ends. It is found that the blowoff plasma is about 20–25% as dense as that of the interior hohlraum, and that its presence does not significantly affect the outward flow of the nearly Planckian radiation field generated in the hohlraum interior. However, the electron temperature of the blowoff region, at ∼120eV, is only about half that of the interior hohlraum plasma.

Time-resolved soft x-ray imaging diagnostic for use at the NIF and OMEGA lasers

The soft x-ray imager (SXRI) built for the first experiments at the National Ignition Facility (NIF) has four soft x-ray channels and one hard x-ray channel. The SXRI is a snout that mounts to a four strip gated imager. This produces four soft x-ray images per strip, which can be separated in time by ∼60ps. Each soft x-ray channel consists of a mirror plus a filter. The diagnostic was used to study x-ray burnthrough of hot Hohlraum targets at the NIF and OMEGA lasers. The SXRI snout design and issues involved in selecting the desired soft x-ray channels are discussed.

Proposed design and mass production of wire arrays and targets for a Z-pinch IFE power plant

This paper summarizes the results of a detailed study reported in Ref. [1]. A concept for a 1000 MW(e) Z-Pinch IFE power plant has ten reactor chambers. A Load, consisting of a wire array surrounding a cryogenic dynamic hohlraum target with a 3-GJ yield, must be produced, inserted into a Replaceable Transmission Lines (RTL), transported to one of the chambers, and shot every second. The conceptual design of the load facilitates automatic mass manufacturing and insertion at 1 Hz at minimum cost. A sequence of operations necessary to (1) fabricate each wire array on a holding and insertion tool, (2) manufacture, evacuate, fill with DT, freeze, layer, and assemble each target with helium gas and liquid hydrogen, (3) insert a wire array and a target under vacuum into an RTL, and (4) remove their debris, is described. Detailed cost estimates derived for the complete load production and solid debris removal cycle in a commercial-scale facility are summarized.

Thomson-scattering measurements of high electron temperature hohlraum plasmas for laser-plasma interaction studies

Accurate measurements of the plasma conditions in laser-produced high-temperature plasmas have been achieved using the recently activated 4ω Thomson-scattering diagnostic at the Omega Laser Facility, Soures et al., Laser Part. Beams 11 (1993). These diagnostic measurements were performed in a new hohlraum target platform that will be used to study laser-plasma interaction in a strongly damped regime comparable to those occurring in indirect drive inertial confinement fusion plasmas. The Thomson-scattering spectra show the collective ion-acoustic features that fit the theory for two ion species plasmas allowing us to accurately and independently determine both the electron and ion temperatures. The electron temperature was found to range from 2to4keV as the total heater beam energy deposited into the hohlraum was increased from 8to17kJ. The results are compared to 2D hydrodynamic simulations using flux limited diffusion and nonlocal heat flux models. The target platform presented provides a novel test bed to investigate laser-plasma interaction physics in the strongly damped backscatter regime.

Using laser entrance hole shields to increase coupling efficiency in indirect drive ignition targets for the National Ignition Facility

Coupling efficiency, the ratio of the capsule absorbed energy to the driver energy, is a key parameter in ignition target designs. The hohlraum originally proposed for the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, S228 (2004)] coupled ∼11% of the absorbed laser energy to the capsule as x rays. Described here is a second generation of the hohlraum target which has a higher coupling efficiency, ∼16%. Because the ignition capsule’s ability to withstand three-dimensional effects increases rapidly with absorbed energy, the additional energy can significantly increase the likelihood of ignition. The new target includes laser entrance hole (LEH) shields as a principal method for increasing coupling efficiency while controlling symmetry in indirect-drive inertial confinement fusion. The LEH shields are high Z disks placed inside the hohlraum on the symmetry axis to block the capsule’s view of the relatively cold LEHs. The LEH shields can reduce the amount of laser energy required to drive a target to a given temperature via two mechanisms: (1) keeping the temperature high near the capsule pole by putting a barrier between the capsule and the pole; (2) because the capsule pole does not have a view of the cold LEHs, good symmetry requires a shorter hohlraum with less wall area. Current integrated simulations of this class of target couple 140kJ of x rays to a capsule out of 865kJ of absorbed laser energy and produce ∼10MJ of yield. In the current designs, which continue to be optimized, the addition of the LEH shields saves ∼95kJ of energy (about 10%) over hohlraums without LEH shields.

Power Plant and Fusion Chamber Considerations for Fast Ignition

Using a simple inertial fusion energy (IFE) power plant economic model, it is demonstrated that there are several potential advantages of an IFE power plant based upon fast ignition targets compared with one based upon central ignition targets. The fast ignition version can have a lower cost of electricity (COE) at the same output power, and a smaller fast ignition plant can have the same COE as a larger central ignition plant. This paper also considers the chamber issues raised by using fast ignition targets. Some direct-drive chamber concepts must be larger for cone-focus fast ignition targets because of the increase in the X-ray output. On the other hand, the use of fast ignition hohlraum targets may allow the use of thick-liquid-wall chambers, bringing the benefits of a smaller chamber and containment building, smaller amounts of hazardous waste, and a faster and cheaper development path. However, many technology issues need resolution before these benefits can become a reality.

Physics of Inertial Fusion: Beam Plasma Interaction, Hydrodynamics, Hot Dense Matter, Stafano Atzeni and Jürgen Meyer-ter-Vehn

Physics of Inertial Fusion: Beam Plasma Interaction, Hydrodynamics, Hot Dense Matter, Stafano Atzeni and Jürgen Meyer-ter-Vehn, Clarendon Press, 2004, 458 pages, ISBN: 0198562640This book has several remarkable highlights summarized on laser produced plasmas and particle beam driven fusion energy. In contrast to the usual books, the nuclear fusion reactions are presented with very detailed experience, including pycnonuclear reactions, spin polarization, and mentioning the 25 orders of magnitude less probable weak force pp-reaction than DT. There is a rather comprehensive collection of the conditions of confinement, spherical implosion, ignition burn, and gain. Hydrodynamics is based on a one fluid model, not Schlüter's space charge quasi-neutral two-fluid model nor the genuine two-fluid treatment. Hohlraum targets are covered and the fast ignition (FI) contains the forte of the authors' own achievements though the entire problems shown experimentally or theoretically (Mulser et al., 2004) are not discussed nor the new aspects on FI known before finishing the book (Hora, 2004). The 36 pages for the entire physics of (laser- and particle-) beam-target interaction are used to sketch at least the most important aspects.

Krypton K-shell X-ray spectra recorded by the HENEX spectrometer

High-resolution X-ray spectra were recorded by the High-Energy Electronic X-ray (HENEX) spectrometer from a variety of targets irradiated by the Omega laser at the Laboratory for Laser Energetics. The HENEX spectrometer utilizes four reflection crystals covering the 1–20 keV energy range and one quartz(1 0 −1 1) transmission crystal (Laué geometry) covering the 11–40 keV range. The time-integrated spectral images were recorded on five CMOS X-ray detectors. In the spectra recorded from krypton-filled gasbag and hohlraum targets, the helium-like K-shell transitions n = 1 – 2 , 1–3, and 1–4 appeared in the 13–17 keV energy range. A number of additional spectral features were observed at energies lower than the helium-like n = 1 – 3 and n = 1 – 4 transitions. Based on computational simulations of the spectra using the FLYCHK/FLYSPEC codes, which included opacity effects, these additional features are identified to be inner-shell transitions from the Li-like through N-like krypton charge states. The comparisons of the calculated and observed spectra indicate that these transitions are characteristic of the plasma conditions immediately after the laser pulse when the krypton density is 2×10 18 cm −3 and the electron temperature is in the range 2.8–3.2 keV. These spectral features represent a new diagnostic for the charge state distribution, the density and electron temperature, and the plasma opacity. Laboratory experiments indicate that it is feasible to record K-shell spectra from gold and higher Z targets in the >60 keV energy range using a Ge(2 2 0) transmission crystal.

Laser coupling to reduced-scale hohlraum targets at the Early Light Program of the National Ignition Facility

A platform for analysis of material properties under extreme conditions, where a sample is bathed in radiation with a high temperature, is under development. Depositing maximum laser energy into a small, high-Z enclosure produces this hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, there is a unique wavelength dependence of the Raman backscattered light that is consistent with Brillouin backscatter of Raman forward scatter [A. B. Langdon and D. E. Hinkel, Phys. Rev. Lett. 89, 015003 (2002)]. Finally, novel diagnostic capabilities indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization orthogonal to that of the incident light.

Evidence for High-Efficiency Laser-Heated Hohlraum Performance at 527 nm

A series of experiments conducted on the HELEN laser system [M. J. Norman, Appl. Opt.4120023497], into thermal x-ray generation from hohlraum targets using 527 nm (2omega) wavelength laser light, has shown that it is possible to exceed radiation temperatures previously thought limited by high levels of superthermal or hot electron production or stimulated backscatter. This Letter questions whether the assumptions traditionally applied to hohlraum design with respect to hot plasma filling and the use of 2omega light are too conservative.

Indirectly driven target design for fast ignition with proton beams

The heavy ion beam (HIB) indirectly driven compression of a small amount of DT fuel, and its fast ignition by a beam of laser accelerated protons (PB) is considered (Roth M. et al 2001 Phys. Rev. Lett. 86 436). First, a representative working point (0.8 mg of DT at 400 g cm−3) has been derived analytically from the energy balance of an hypothetical power plant. Second, through self-consistent two-dimensional radiation hydrodynamic simulations with the MULTI code (Ramis R. et al 1988 Comput. Phys. Commun. 49 475) we define and optimize a HIB-driven hohlraum target to produce the above conditions; also, with ignition simulations, we determine the thermonuclear yield (≃83 MJ) and the minimum PB energy (≃33 kJ) needed to ignite the asymmetrically compressed fuel. The target is found to be a factor 5 less sensitive to implosion asymmetries than a conventional central spot one. This allows us to reduce the hohlraum size (≃6 mm), improve the driver coupling, reduce the compression energy to the 0.8–1.2 MJ range, and have enough clearance for the installation of the PB converter foil at distances (3–4 mm) appropriate for an efficient ignition.

Trends in radiation production from dynamic-hohlraums driven by single and nested wire arrays

The axial radiation developed primarily from the interior of an imploding dynamic hohlraum (DH) target within a Z pinch using a single array with a large number of tungsten wires is compared with that generated using a standard [Phys. Plasmas 9, 3573 (2002)] nested (outer plus inner) array on Z. Measurements indicate that a single-array with a mass (2.5 mg) near that of the combined mass of the nested-array maximizes the DH axial power. At this mass, the DH utilizing the standard nested array generates (23±15)% more axial power than that of a single array. Measurements over a range of single-array masses (2–6 mg) show a decrease in radiation power for masses above 3.5 mg. Two-dimensional radiation magnetohydrodynamic simulations, which have successfully described radial emission from targetless implosions for both single and nested-array configurations [Phys. Plasmas 6, 2178 (1999)], however, do not follow the observed trends well. This lack of tracking implies that current 2D models, which take into account the development of the magnetic Rayleigh–Taylor (RT) instability in the r–z plane, require improvements in order to provide a self-consistent description of the implosion dynamics and radiation production for DH experiments.

On Stimulated Scattering of Laser Light in Inertial Fusion Energy Targets

Propagation of a laser light through regions of an underdense plasma is an active research topic in laser fusion. In particular, a large effort has been invested in studies of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), which can reflect laser energy and produce energetic particles to preheat a fusion energy target. Experiments, theory, and simulations agree on a complex interplay between various laser-plasma instabilities. By particle-in-cell simulations of an underdense electron plasma, apart from the standard SRS, a strong backscattering was found near the electron plasma frequency at densities beyond the quarter critical. This novel instability, recognized in recent experiments as stimulated laser scattering on a trapped electron-acoustic mode (SEAS), is absent from a classical theory of laser-parametric instabilities. A parametric excitation of SEAS instability is explained by a three-wave resonant decay of the incident laser light into a standing backscattered wave and a slow trapped electron-acoustic wave ([omega] < [omega]p). Large SEAS pulsations, eventually suppressed by relativistic heating of electrons, are observed in these simulations. This phenomenon seems relevant to future hohlraum target and fast ignition experiments.

Progress toward heavy-ion IFE

Successful development of heavy-ion fusion (HIF) will require scientific and technology advances in areas of targets, drivers and chambers. Design work on heavy-ion targets indicates that high gain (60–130) may be possible with a ∼3–6 MJ driver depending on the ability to focus the beams to small spot sizes. Significant improvements have been made on key components of heavy-ion drivers, including sources, injectors, insulators and ferromagnetic materials for long-pulse induction accelerator cells, solid-state pulsers, and superconducting quadrupole magnets. The leading chamber concept for HIF is the thick-liquid-wall HYLIFE-II design, which uses an array of flibe jets to protect chamber structures from X-ray, debris, and neutron damage. Significant progress has been made in demonstrating the ability to create and control the types of flow needed to form the protective liquid blanket. Progress has also been made on neutron shielding for the final focus magnet arrays with predicted lifetimes now exceeding the life of the power plant. Safety analyses have been completed for the HYLIFE-II design using state-of-the-art codes. Work also continues on target fabrication and injection for HIF. A target injector experiment capable of >5 Hz operation has been designed and construction will start in 2002. Methods for mass-production of hohlraum targets are being evaluated with small-scale experiments and analyses. Progress in these areas will be reviewed.

Dynamics and characteristics of a 215-eV dynamic-hohlraum x-ray source on Z

A radiation source has been developed on the 20-MA Z facility that produces a high-power x-ray pulse, generated in the axial direction primarily from the interior of a collapsing dynamic hohlraum (DH). The hohlraum is created from a solid cylindrical CH2 target centered within an imploding tungsten wire-array Z pinch. Analyses and interpretation of measurements made of the x-ray generation within and radiated from the hohlraum target have been done using radiation-magnetohydrodynamic-code simulations in the r-z plane that take account of the magnetic Rayleigh–Taylor (RT) instability. These analyses suggest that a significantly reduced RT seed (relative to that used to explain targetless Z-pinch data on Z) is required to explain the observations. Although some quantitative and qualitative agreement with the measurements is obtained with the reduced RT seed, differences remain. Initial attempts to include into the simulations a precursor plasma, arising from wire material driven ahead of the main implosion, did not ameliorate the differences. Modification of the simulated W/CH2 interface may be required to properly explain the measured axial radiation pulse. This pulse, which exits a 4.5-mm2 hole centered above the target, begins ∼5 ns prior to stagnation (as defined by peak radial radiation power). The 5-ns interval leading to stagnation represents the duration when the imploding tungsten plasma acts as a hohlraum wall, trapping radiation within the interior of the foam target. The hohlraum radiation exiting the hole at 6 degrees to the z-axis reaches a maximum intensity of 3.1±0.6 TW/str (associated with an average hohlraum temperature of 215±10 eV), 1.4±0.4 ns prior to stagnation. (The uncertainties represent rms shot-to-shot variations.) This radiation pulse, characterized here, is useful for performing radiation-transport experiments with drive temperatures in excess of 200 eV.

Supersonic jet and shock interactions

Supersonic fluid flow and the interaction of strong shock waves to produce jets of material are ubiquitous features of inertial confinement fusion (ICF), astrophysics, and other fields of high energy-density science. The availability of large laser systems provides an opportunity to investigate such hydrodynamic systems in the laboratory, and to test their modeling by radiation hydrocodes. We describe experiments to investigate the propagation of a structured shock front within a radiation-driven target assembly, the formation of a supersonic jet of material, and the subsequent interaction of this jet with an ambient medium in which a second, ablatively driven shock wave is propagating. The density distribution within the jet, the Kelvin–Helmholz roll-up at the tip of the jet, and the jet’s interaction with the counterpropagating shock are investigated by x-ray backlighting. The experiments were designed and modeled using radiation hydrocodes developed by Los Alamos National Laboratory, AWE, and Lawrence Livermore National Laboratory. The same hydrocodes are being used to model a large number of other ICF and high energy-density physics experiments. Excellent agreement between the different simulations and the experimental data is obtained, but only when the full geometry of the experiment, including both laser-heated hohlraum targets (driving the jet and counter-propagating shock), is included. The experiments were carried out at the University of Rochester’s Omega laser [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)].

Laser Compression and Ignition of Z-Pinch Magnetized Dense Fusion Targets

Abstract With thin wire multimegampere shear flow stabilized fast z-pinch discharges, magnetic fields of hundreds of megagauss can be reached in the vicinity of the discharge channel. Then, if by laser-ablation-propulsion pieces of solid DT are simultaneously shot onto the discharge channel from several sides, the DT is compressed upon impact to high densities, with the magnetic field acting as a cushion to make the compression isentropic. The highly compressed and magnetized DT target can then be ignited at one point by a pulsed laser beam launching a thermonuclear detonation wave propagating along the discharge channel. Estimates indicate thermonuclear gains large in comparison to hohlraum targets.

Numerical simulation of plasma implosion due to radiation heating of the inlet-hole walls of a hohlraum target

Numerical simulation of plasma implosion due to radiation heating of the inlet-hole walls of a hohlraum target

Computer simulations of laser hot spots and implosion symmetry kiniform phase plate experiments on Nova

LASNEX computer code simulations have been performed for radiation symmetry experiments on the Nova laser with vacuum and gas-filled hohlraum targets [R. L. Kauffman et al., Phys. Plasmas 5, 1927 (1998)]. In previous experiments with unsmoothed laser beams, the symmetry was substantially shifted by deflection of the laser beams. In these experiments, laser beams have been smoothed with Kiniform Phase Plates in an attempt to remove deflection of the beams. The experiments have shown that this smoothing significantly improves the agreement with LASNEX calculations of implosion symmetry. The images of laser produced hot spots on the inside of the hohlraum case have been found to differ from LASNEX calculations, suggesting that some beam deflection or self-focusing may still be present or that emission from interpenetrating plasmas is an important component of the images. The measured neutron yields are in good agreement with simulations for vacuum hohlraums but are far different for gas-filled hohlraums.

Pulse Power Compression by Cutting a Dense Z-Pinch with a Laser Beam

Abstract A thin cut made through a z-pinch by an intense laser beam can become a magnetically insulated diode crossed by an intense ion beam. For larger cuts, the gap is crossed by an intense relativistic electron beam, stopped by magnetic bremsstrahlung resulting in a pointlike intense x-ray source. In either case, the impedance of the pinch discharge is increased, with the power delivered rising in the same pro-portion. A magnetically insulated cut is advantageous for three reasons: First, with the ion current com-parable to the Alfvèn ion current, the pinch instabilities are reduced. Second, with the energy deposit-ed into fast ions, a non-Maxwellian velocity distribution is established increasing〈σ ν〉 value for nuclear fusion reactions taking place in the pinch discharge. Third, in a high density z-pinch plasma, the intense ion beam can launch a thermonuclear detonation wave propagating along the pinch discharge channel. For larger cuts the soft x-rays produced by magnetic bremsstrahlung can be used to drive a thermonuclear hohlraum target. Finally, the proposed pulse power compression scheme permits to use a cheap low power d.c. source charging a magnetic storage coil delivering the magnetically stored energy to the pinch discharge load by an exploding wire opening switch.