Inertial Confinement Fusion Capsule Research Articles

Planar hydrodynamic instability computations and experiments with rugby-shaped hohlraums at the Omega laser

Implosion of inertial confinement fusion (ICF) capsule is very sensitive to the growth of sphericity perturbations. The control of the feeding of such perturbations and their transport (‘feedthrough’) through the ablator is a key point to reach ignition. Since 2002 [1, 2], experiments have been designed and performed on the Omega laser facility in order to study these phenomena in planar geometry. A new ‘rugby shaped’ hohlraum was used [3, 4]. We present experimental results and comparisons with numerical simulations.

Gamma background calculation for the HEXRI diagnostic at the National Ignition Facility

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is nearing completion and the commencement of experiment campaigns leading to ignition. Different parameters of an implosion, such as the neutron yield and images of the target, will be inferred from the measurement of emitted neutrons, gammas, and x-rays using different diagnostics. The High-Energy X-Ray Imager (HEXRI) is one of these diagnostics and will be used on cryogenic Deuterium-Tritium implosions at NIF to provide data on the size and shape of the imploding inertial confinement fusion (ICF) capsules. These data are fundamental for understanding the reasons for ignition failure. Following an implosion, the background from neutrons and photons can be very large and can affect the quality of HEXRI images in terms of lowering the signal-to-background ratio below the threshold of 1. In this work we present Monte Carlo simulations of the n-gamma background from the components present inside the target chamber, i.e. the cryogenic target positioner, the Laser Plasma Interaction collimators, the neutron and HEXRI pinholes, and the hohlraum. We found that the ratio of the self-emission x-ray signal to n-gamma background signal stays well above 1 in all cases, allowing a good HEXRI image quality.

Robustness studies of ignition targets for the National Ignition Facility in two dimensions

Inertial confinement fusion capsules are critically dependent on the integrity of their hot spots to ignite. At the time of ignition, only a certain fractional perturbation of the nominally spherical hot spot boundary can be tolerated and the capsule still achieve ignition. The degree to which the expected hot spot perturbation in any given capsule design is less than this maximum tolerable perturbation is a measure of the ignition margin or robustness of that design. Moreover, since there will inevitably be uncertainties in the initial character and implosion dynamics of any given capsule, all of which can contribute to the eventual hot spot perturbation, quantifying the robustness of that capsule against a range of parameter variations is an important consideration in the capsule design. Here, the robustness of the 300eV indirect drive target design for the National Ignition Facility [Lindl et al., Phys. Plasmas 11, 339 (2004)] is studied in the parameter space of inner ice roughness, implosion velocity, and capsule scale. A suite of 2000 two-dimensional simulations, run with the radiation hydrodynamics code LASNEX, is used as the data base for the study. For each scale, an ignition region in the two remaining variables is identified and the ignition cliff is mapped. In accordance with the theoretical arguments of Levedahl and Lindl [Nucl. Fusion 37, 165 (1997)] and Kishony and Shvarts [Phys. Plasmas 8, 4925 (2001)], the location of this cliff is fitted to a power law of the capsule implosion velocity and scale. It is found that the cliff can be quite well represented in this power law form, and, using this scaling law, an assessment of the overall (one- and two-dimensional) ignition margin of the design can be made. The effect on the ignition margin of an increase or decrease in the density of the target fill gas is also assessed.

Charged particle stopping power effects on ignition: Some results from an exact calculation

A completely rigorous first-principles calculation of the charged particle stopping power has recently been performed by Brown, Preston, and Singleton (BPS). This calculation is exact to leading and next-to-leading order in the plasma number density, including an exact treatment of two-body quantum scattering. The BPS calculation is therefore extremely accurate in the plasma regime realized during the ignition and burn of an inertial confinement fusion capsule. For deuterium-tritium fusion, the 3.5MeV alpha particle range tends to be 20%–30% longer than most models in the literature have predicted, and the energy deposition into the ions tends to be smaller. Preliminary numerical simulations indicate that this increases the ρR required to achieve ignition.

Overview of the Dynamic Hohlraum X-Ray Source at Sandia National Laboratories

Progress in understanding the physics of dynamic hohlraums is reviewed for a system that is capable of generating 10 TW of axial radiation for high-temperature ( 200 eV) radiation-flow experiments and inertial confinement fusion capsule implosions. Two-dimensional magneto-hydrodynamic simulation comparisons with data show the need to include wire initiation physics and subsequent discrete-wire dynamics in the simulations if a predictive capability is to be achieved.

An overview of EBIT data needed for experiments on laser-producedplasmas

Data produced by an electron beam ion trap (EBIT) can be crucial for interpreting spectra from plasmas created by long-pulse lasers. Four example spectra are considered: (i) the spectra from the hot gold plasma in the laser deposition region of a hot hohlraum, (ii) the He-β spectra from an argon-doped imploding core of an inertial confinement fusion capsule, (iii) the polarization in spectral lines produced by hot electrons generated by laser-plasma parametric instabilities, and (iv) the spectra of the underdense plasma from an aerogel X-ray source. The EBIT data needed for these cases are: (i) the line positions for 3 → 2 transitions in open M shell gold ions, (ii) the Li-like satellite lines for the K-β transition in open L shell argon ions, (iii) the polarization of suitable X-ray lines at 30 keV, and (iv) the precise wavelengths of K lines of highly charged Si ions and the precise wavelengths of L and K lines of highly charged Ge ions.PACS Nos.: 52.25.Os, 52.38.–4, 52.38.Bv, 52.57.–z, 52.57.Fg, 52.59.Px, 52.70.–m 52.70.La

High-gain direct-drive inertial confinement fusion for the Laser Mégajoule: recent progress

Recent progress in high-gain direct-drive inertial confinement fusion with the laser Mégajoule is reviewed. A new baseline direct-drive target design is presented which implodes with a two-cones irradiation pattern of indirect-drive beam configuration and zooming. Perturbation amplitudes and correlated growth rates of hydrodynamic instabilities in the compressed core of a directly driven inertial confinement fusion capsule are analyzed in planar and spherical geometries, with and without heat conduction, in the unsteady state regime of the deceleration. Shock propagation in heterogeneous media is addressed in the context of first shock. The neutron and photon emissions of high-gain direct-drive target are characterized. Numerical interpretations of directly driven homothetic cryogenic D2 target implosion experiments on the Omega facility are presented.

Nonlinear evolution of localized perturbations in the deceleration-phase Rayleigh-Taylor instability of an inertial confinement fusion capsule

The evolution of axially symmetric localized perturbations in the deceleration-phase Rayleigh-Taylor instability of an inertial confinement fusion shell capsule is studied by two-dimensional simulations. Large amplitude divot-like mass perturbations were tracked into the deeply nonlinear regime. Dense fluid spikes penetrating the hot spot as well as light material bubbles rising into the dense shell have been studied, using both a full physics model and a simplified (classical) model neglecting thermal conductivity and fusion reactions. The stabilizing effect of ablation (due to electron thermal conductivity and fusion alpha-particle transport) is found to be more pronounced for spikes than for bubbles. For small width perturbations, bubbles grow faster than spikes, contrary to classical model results.

Simulations investigating the effect of a deuterium-tritium-ice coating on the motion of the gold cone surface in a re-entrant cone-guided fast ignition inertial confinement fusion capsule

One- and two-dimensional multigroup radiation hydrodynamics simulations have been performed to investigate the motion of the gold plasma generated at the surface of the embedded gold cone in a re-entrant cone-guided inertial confinement fusion capsule. The effect of deuterium-tritium (DT) ice layers, and other possible tampers, of varying thickness, upon the motion of the gold cone plasma has been investigated. The effect of the x-ray drive spectrum incident upon the ice layer is also explored. Ice is shown to tamp the expansion of the gold cone, and whilst denser materials are shown to be more effective in this role, ice does not pollute the ignition region with intermediate-Z ions, which, though preferable to gold contamination, also tend to inhibit the attainment of high fuel-ion temperatures.

A planar-geometry platform for the experimental investigation of Be jets

Initial experiments were performed at the OMEGA laser [J. Soures et al., Phys. Plasmas 3, 2108 (1996)] to investigate the physics associated with inertial confinement fusion capsule fill tubes and holes. These experiments were performed in planar geometry and examined the hydrodynamics of a 6.7:1 aspect ratio fill-hole. X-ray radiographs at 310eV show a jet has formed due to the interaction between the temperature drive and the beryllium (Be) washer.

Effect of higher z dopants on implosion dynamics: X-ray spectroscopy

Low- z dopants such as argon are used to characterize the plasma properties in imploding inertial confinement fusion (ICF) capsules. Higher z dopants will be used as the temperature of the capsules is increased especially as we approach ignition on the National Ignition Facility (NIF). The presence of the higher z dopants also affects the dynamics of the implosion as it increases the electron density, which increases the radiation losses from the plasma allowing the plasma to compress to a smaller volume. In the present study, we examine the effects at higher dopant densities. While a normal glass capsule loses energy to radiation during compression, the high- z shells will confine the radiation even as equilibrium burn is approached. We have fielded thin 1-mm diameter glass shells filled with varying amounts of xenon and krypton gas to study the progression from non-equilibrium to equilibrium burn as the dopant gas concentration is increased. The shells used for these experiments also contained 3He to measure the proton spectrum from the D 3He reaction providing information about the target temperature and the density-radius product, ρR. Here we present results using the X-ray measurements, as well as discuss some of the issues and the progress we made.

Fast ignition of a compressed inertial confinement fusion hemispherical capsule by two proton beams

A hemispherical conically guided indirectly driven inertial confinement fusion capsule has been considered. The fast ignition of the precompressed capsule driven by one or two laser-accelerated proton beams has been numerically investigated. The energy distribution of the protons is Gaussian with a mean energy of 12MeV and a full width at half maximum of 1MeV. A new scheme that uses two laser-accelerated proton beams is proposed. It is found that the energy deposition of 1kJ provided by a first proton beam generates a low-density cylindrical channel and launches a forward shock. A second proton beam, delayed by a few tens of ps and driving the energy of 6kJ, crosses the low-density channel and heats the dense shocked region where the ignition of the deuterium-tritium nuclear fuel is achieved. For the considered capsule, this new two-beam configuration reduces the ignition energy threshold to 7kJ.

Hydrodynamic instabilities in ablative tamped flows

Perturbation amplitudes and correlated growth rates of hydrodynamic instabilities in the compressed core of a directly driven inertial confinement fusion capsule are analyzed in planar and spherical geometries, with and without heat conduction. It is found that the maximum Lagrangian perturbation amplitude is always located inside the hot spot and not at the ablation front when heat conduction is switched on. A reduction of the Rayleigh-Taylor growth rate compared to the classical one is also shown. The growth rate is varying in k (against k1∕2 classically), due to the unsteady state regime of the deceleration.

High-brightness, high-spatial-resolution, 6.151keV x-ray imaging of inertial confinement fusion capsule implosion and complex hydrodynamics experiments on Sandia’s Z accelerator (invited)

When used for the production of an x-ray imaging backlighter source on Sandia National Laboratories’ 20MA, 100ns rise-time Z accelerator [M. K. Matzen et al., Phys. Plasmas 12, 055503 (2005)], the terawatt-class, multikilojoule, 526.57nm Z-Beamlet laser (ZBL) [P. K. Rambo et al., Appl. Opt. 44, 2421 (2005)], in conjunction with the 6.151keV, Mn–Heα curved-crystal imager [D. B. Sinars et al., Rev. Sci. Instrum. 75, 3672 (2004)], is capable of providing a high quality x radiograph per Z shot for various high-energy-density physics experiments. Enhancements to this imaging system during 2005 have led to the capture of inertial confinement fusion capsule implosion and complex hydrodynamics images of significantly higher quality. The three main improvements, all leading effectively to enhanced image plane brightness, were bringing the source inside the Rowland circle to approximately double the collection solid angle, replacing direct exposure film with Fuji BAS-TR2025 image plate (read with a Fuji BAS-5000 scanner), and generating a 0.3–0.6ns, ∼200J prepulse 2ns before the 1.0ns, ∼1kJ main pulse to more than double the 6.151keV flux produced compared with a single 1kJ pulse. It appears that the 20±5μm imaging resolution is limited by the 25μm scanning resolution of the BAS-5000 unit, and to this end, a higher resolution scanner will replace it. ZBL is presently undergoing modifications to provide two temporally separated images (“two-frame”) per Z shot for this system before the accelerator closes down in summer 2006 for the Z-refurbished (ZR) upgrade. In 2008, after ZR, it is anticipated that the high-energy petawatt (HEPW) addition to ZBL will be completed, possibly allowing high-energy 11.2224 and 15.7751keV Kα1 curved-crystal imaging to be performed. With an ongoing several-year project to develop a highly sensitive multiframe ultrafast digital x-ray camera (MUDXC), it is expected that two-frame HEPW 11 and 16keV imaging and four-frame ZBL 6.151keV curved-crystal imaging will be possible. MUDXC will be based on the technology of highly cooled silicon and germanium photodiode arrays and ultrafast, radiation-hardened integrated circuitry.

Effect of experimentally observed hydrogenic fractionation on inertial confinement fusion ignition target performance

The need of cryogenic hydrogenic fuels in inertial confinement fusion (ICF) ignition targets has been long been established. Efficient implosion of such targets has mandated keeping the adiabat of the main fuel layer at low levels to ensure drive energies are kept at reasonable minima. The use of cryogenic fuels helps meet this requirement and has therefore become the standard in most ICF ignition designs. To date most theoretical ICF ignition target designs have assumed a homogeneous layer of deuterium-tritium (DT) fuel kept slightly below the triple point. However, recent work has indicated that, as cryogenic fuel layers are formed inside an ICF capsule, isotopic dissociation of the tritium (T), deuterium (D), and DT takes place leading to a “fractionation” of the final ice layer. This paper will numerically investigate the effects that various scenarios of fractionation have on hot-spot formation, ignition, and burn in ICF ignition target designs.

Diagnosing radiation drive asymmetry and absorbed energy in ignition Hohlraums using gas-filled capsules

The shape of the x-ray self-emission image from an imploded inertial confinement fusion capsule has been used to diagnose Hohlraum radiation drive asymmetry [L. J. Suter et al., Phys. Rev. Lett. 73, 2328 (1994); A. Hauer et al., Rev. Sci. Instrum. 66, 672 (1995); A. Hauer et al., Phys. Plasmas 2, 2488 (1995); N. D. Delameter et al., Phys. Plasmas 3, 2022 (1996)]. Timing of neutron yield from the implosion can also be used to diagnose the absorbed radiation drive energy. We propose a set of beryllium capsules filled with gas, rather than layered cryogenic ice, to diagnose asymmetry and absorbed energy in ignition Hohlraums. Capsules with a range of initial shell thickness have a corresponding range of implosion times, allowing sampling of drive conditions throughout an ignition pulse. For example a 12μm thick shell implodes during the foot of the ignition pulse, sampling the drive up to about 10ns. For such capsules, the foot pulse is a high-adiabat drive and leads to implosions that are rather robust against instability and mixing. In these capsules, the best time to record images for diagnosing asymmetry may be several hundred picoseconds after peak x-ray brightness, when the image distortion is increasing rapidly.

High energy x-ray imager for inertial confinement fusion at the National Ignition Facility

X-ray imaging is a fundamental diagnostic tool for inertial confinement fusion (ICF) research and provides data on the size and the shape of the core in implosions. We report on the feasibility and performance analyses of an ignition x-ray imager to be used on cryogenic deuterium-tritium implosions at the National Ignition Facility. The system is intended to provide time-integrated, broadband, moderate-energy x-ray core images of imploding inertial confinement fusion capsules. It is optimized with respect to spatial-resolution, signal-to-background, and signal-to-noise ratios, taking into account the extreme operating conditions expected at NIF due to high expected neutrons yields, gamma rays, and x rays from laser-plasma interactions.

Compact X-pinch based point x-ray source for phase contrast imaging of inertial confinement fusion capsules

Results from experiments performed to characterize plastic capsules containing foam layers are presented. A compact X-pinch pulser with a footprint <1m2 having a peak current of 80kA and a rise time of 50ns was used. Various wire materials including tungsten, molybdenum, and aluminum were employed. Results with plastic capsules (1mm diameter, 20μm thick wall with 80μm foam inside the capsule) show phase contrast effects at the edges of the wall due to the foam, which mimics the ice inside the shell. The sharpness of the image reveals a source less than 2μm in size and x-ray diodes show a pulse length of ∼10ns. The small source size allows high-resolution phase contrast imaging of capsules. The x-ray pulse from an X-pinch is sufficiently short to avoid the motional blurring due to cryogenic system vibrations, which is not possible with low flux sources.

Hohlraum X-ray deposition in indirect-drive ICF ablator materials

Accurate measurements of shock timing and ablator x-ray burnthrough will be essential for the successful ignition of an indirect-drive inertial confinement fusion (ICF) capsule. In previous work [1], measurements of ablator shock velocities, shock temperatures, and preheat temperatures were made using a 280nm Streaked Optical Pyrometer (SOP) [2]. The x-ray fluxes were supplied by hohlraums driven by the University of Rochester Omega Laser [3]. More recent ablator experiments at Omega have extended the previous work by using an absolutely calibrated 600-800 nm SOP [4] together with a line-imaging velocity interferometer [5] similar to the diagnostic proposed for accurate National Ignition Facility (NIF) ignition shock timing measurements [6]. Important new information has been obtained relating to ablator surface movement prior to shock breakout, ablator preheat temperature, and preheat effects on the anvil and window components of the shock timing diagnostic system.

A potential X-pinch based point X-ray source for phase contrast imaging of inertial confinement fusion capsules

Phase contrast x-ray imaging is necessary to monitor and validate the DT ice layer inside a cryoignition target. Presently available sources require a minute long exposure for imaging, thus contributing to blurring of images at the edges. An x-pinch is a bright, pulsed x-ray source that produces pulses of x-rays duration < 1 ns thus eliminating such blurring. Our preliminary results show that with an optimized detection system an x-pinch generated image could be used to locate the ice layer with the accuracy demanded by the NIF specifications.