Inertial Confinement Fusion Driver Research Articles

An efficient computational approach for evaluating radiation flux for laser driven inertial confinement fusion targets

Radiation flux computation on the target is very important for laser driven Inertial Confinement Fusion, and view-factor based equation models (MacFarlane, 2003; Srivastava et al., 2000) are often used to compute this radiation flux on the capsule or samples inside the hohlraum. However, the equation models do not lead to sparse matrices and may involve an intensive solution process when discrete mesh elements become smaller and the number of equations increases.An efficient approach for the computation of radiation flux is proposed in this paper, in which, (1) symmetric and positive definite properties are achieved by transformation, and (2) an efficient Cholesky factorization algorithm is applied to significantly accelerate such equations models solving process.Finally, two targets on a laser facility built in China are considered to validate the computing efficiency of present approach. The results show that the radiation flux computation can be accelerated by a factor of 2.

StarDriver: A Flexible Laser Driver for Inertial Confinement Fusion and High Energy Density Physics

We propose a novel method to minimize laser–plasma instabilities and improve laser–plasma coupling by the use of multi-beam laser architecture with a large system frequency bandwidth and many beamlets per unit solid angle. The StarDriver™, laser driver is constructed from 104 to 105 individual lasers, each delivering nominally 100 J in pulses of ~3–30 ns at a nominal wavelength of ~355 nm with better than 3–5 diffraction-limited performance. The beamlets are individually relatively narrowband to facilitate maximum laser performance, but the ensemble of beamlets span a wide frequency range. Currently available laser media enable Δω/ω ~ 2 % at 355 nm with the possibility of system bandwidths approaching 10 % in the future. The many beamlets of StarDriver™ provide optimal asymptotic smoothing for hydrodynamic instabilities (0–1 %), innovative focusing strategies including zooming, and the large bandwidth enables extremely rapid hydrodynamic smoothing times ~30 fs. The distribution of frequencies among the beamlets allows flexibility for fine control of the seeding of the Rayleigh–Taylor instability. The ultra-broad bandwidth combined with the large total k-spectrum of the laser drive in the plasma corona may enable complete suppression of the most problematic laser–plasma instabilities such as stimulated Brillouin backscatter, stimulated Raman scatter, cross-beam energy transfer, and the two plasmon decay instability. StarDriver™ offers potentially superior flexibility in laser drivers for inertial confinement fusion, enabling almost arbitrary sequencing of wavelength, polarization, focus, and fine control of the spatio-temporal properties of the drive in the corona. The highly modular strategy of StarDriver™ should enable an attractive development pathway as well as maximizing overall system efficiency.

Fast ignition with laser-driven proton and ion beams

Fusion fast ignition (FI) initiated by a laser-driven particle beam promises a path to high-yield and high-gain for inertial fusion energy. FI can readily leverage the proven capability of inertial confinement fusion (ICF) drivers, such as the National Ignition Facility, to assemble DT fusion fuel at the relevant high densities. FI provides a truly alternate route to ignition, independent of the difficulties with achieving the ignition hot spot in conventional ICF. FI by laser-driven ion beams provides attractive alternatives that sidestep the present difficulties with laser-driven electron-beam FI, while leveraging the extensive recent progress in generating ion beams with high-power density on existing laser facilities. Whichever the ion species, the ignition requirements are similar: delivering a power density ≈1022 W cm−3 (∼10 kJ in ≈20 ps within a volume of linear dimension ≈20 µm), to the DT fuel compressed to ∼400 g cm−3 with areal density ∼2 g cm−2. High-current, laser-driven beams of many ion species are promising candidates to deliver such high-power densities. The reason is that high energy, high-power laser drivers can deliver high-power fluxes that can efficiently make ion beams that are born neutralized in ∼fs–ps timescales, making them immune to the charge and current limits of conventional beams. In summary, we find that there are many possible paths to success with FI based on laser-driven ion beams. Although many ion species could be used for ignition, we concentrate here on either protons or C ions, which are technologically convenient species. We review the work to date on FI design studies with those species. We also review the tremendous recent progress in discovering, characterizing and developing many ion-acceleration mechanisms relevant to FI. We also summarize key recent technological advances and methods underwriting that progress. Based on the design studies and on the increased understanding of the physics of laser-driven ion acceleration, we provide laser and ion-generation laser-target design points based on several distinct ion-acceleration mechanisms.

Observation of strong electromagnetic fields around laser-entrance holes of ignition-scale hohlraums in inertial-confinement fusion experiments at the National Ignition Facility

Energy spectra and spectrally resolved one-dimensional fluence images of self-emitted charged-fusion products (14.7 MeV D3He protons) are routinely measured from indirectly driven inertial-confinement fusion (ICF) experiments utilizing ignition-scaled hohlraums at the National Ignition Facility (NIF). A striking and consistent feature of these images is that the fluence of protons leaving the ICF target in the direction of the hohlraum's laser entrance holes (LEHs) is very nonuniform spatially, in contrast to the very uniform fluence of protons leaving through the hohlraum equator. In addition, the measured nonuniformities are unpredictable, and vary greatly from shot to shot. These observations were made separately at the times of shock flash and of compression burn, indicating that the asymmetry persists even at ∼0.5–2.5 ns after the laser has turned off. These phenomena have also been observed in experiments on the OMEGA laser facility with energy-scaled hohlraums, suggesting that the underlying physics is similar. Comprehensive data sets provide compelling evidence that the nonuniformities result from proton deflections due to strong spontaneous electromagnetic fields around the hohlraum LEHs. Although it has not yet been possible to uniquely determine whether the fields are magnetic (B) or electric (E), preliminary analysis indicates that the strength is ∼1 MG if B fields or ∼109 V cm−1 if E fields. These measurements provide important physics insight into the ongoing ignition experiments at the NIF. Understanding the generation, evolution, interaction and dissipation of the self-generated fields may help to answer many physics questions, such as why the electron temperatures measured in the LEH region are anomalously large, and may help to validate hydrodynamic models of plasma dynamics prior to plasma stagnation in the center of the hohlraum.

Symmetry issues in Directly Irradiated Targets

In direct drive Inertial Confinement Fusion (ICF), the typical laser beam to laser beam angle is around 30 ◦ . This fact makes the study of the irradiation symmetry a genuine 3D problem. In this paper we use the three dimensional version of the MULTI hydrocode to assess the symmetry of such ICF implosions. More specifically, we study a shock-ignition proposal for the Laser-Megajoule facility (LMJ) in which two of the equatorial beam cones are used to implode and precompress a spherical capsule (the reference capsule of HiPER project) made of 0.59mg of pure Deuterium-Tritium mixture. The symmetry of this scheme is analysed and optimized to get a design inside the operating limits of LMJ. The studied configuration has been found essentially axial-symmetric, so that the use of 2D hydrocodes would be appropriate for this specific situation.

Time evolution of filamentation and self-generated fields in the coronae of directly driven inertial-confinement fusion capsules

Time-gated radiography with monoenergetic 15-MeV protons, 3-MeV protons, and 4-MeV alpha particles has revealed a rich and complex evolution of electromagnetic field structures in and around imploding, directly driven inertial-confinement fusion (ICF) targets at the OMEGA laser facility. Plastic-shell capsules and solid plastic spheres were imaged during and after irradiation with ICF-relevant laser drive (up to 6 × 1014 W/cm2). Radial filaments appeared while the laser was on; they filled, and were frozen into, the out-flowing corona, persisting until well after the end of the laser drive. Data from specially designed experiments indicate that the filaments were not generated by two-plasmon-decay instabilities or by Rayleigh-Taylor instabilities associated with shell acceleration. Before the onset of visible filamentation, quasi-spherical field structures appeared outside the capsule in the images in a form that suggests outgoing shells of net positive charge. We conjecture that these discrete shells are related to multiple peaks seen previously in the spectra of protons ablated from the targets.

Wire number dependence of the implosion dynamics, stagnation, and radiation output of tungsten wire arrays at Z driver

We report results of the experimental campaign, which studied the initiation, implosion dynamics, and radiation yield of tungsten wire arrays as a function of the wire number. The wire array dimensions and mass were those of interest for the Z-pinch driven Inertial Confinement Fusion (ICF) program. An optimization study of the x-ray emitted peak power, rise time, and full width at half maximum was effectuated by varying the wire number while keeping the total array mass constant and equal to ∼5.8 mg. The driver utilized was the ∼20-MA Z accelerator before refurbishment in its usual short pulse mode of 100 ns. We studied single arrays of 20-mm diameter and 1-cm height. The smaller wire number studied was 30 and the largest 600. It appears that 600 is the highest achievable wire number with present day’s technology. Radial and axial diagnostics were utilized including crystal monochromatic x-ray backlighter. An optimum wire number of ∼375 was observed which was very close to the routinely utilized 300 for the ICF program in Sandia.

Project Icarus: Optimisation of nuclear fusion propulsion for interstellar missions

The Daedalus spacecraft design was a two-stage configuration carrying 50,000 tonnes of DHe 3 propellant. Daedalus was powered by electron driven Inertial Confinement Fusion (ICF) to implode the pellets at a frequency of 250 Hz. The mission was to Barnard's star 5.9 light years away in a duration of around 50 years. This paper is related to the successor Project Icarus, a theoretical engineering design study that began on 30 September 2009 and is a joint initiative between the Tau Zero Foundation and The British Interplanetary Society. In the first part of this paper, we explore ‘flyby’ variations on the Daedalus propellant utilisation for two different mission targets: Barnard's star and Epsilon Eridani, 10.7 light years away. With a fixed propellant mass a number of staged configurations (1–4) are derived for an optimal configuration but then moving to an off-optimal configuration due to the requirement for a high final science payload mass. Some comments are then made on the ICF pellet configuration compared to the typical pellets fielded at the National Ignition Facility (NIF) and those proposed for the Vista and Longshot fusion based propulsion designs. This is a working progress report, which aims to study perturbations of the Daedalus baseline design as part of a trade study. This is a submission of the Project Icarus Study Group.

Study on intensity distribution inside the frequency conversion crystals for continuous phase plate front-located in inertialconfinement fusion driver

This paper studies the intensity distribution inside the frequency conversion crystals when the continuous phase plate (CPP) is placed in 1ω light of final optics assembly for inertial confinement fusion (ICF) driver. Our study shows that the modulation of 1ω light caused by CPP makes the frequency conversion efficiency and the uniformity of intensity field inside the frequency conversion crystals decrease. It leads to the possibility of laser induced damage for frequency conversion crystals to increase. What worth paying special attention to is: the modulation and maximum intensity in the vicinity of entrance and exit surface of frequency conversion crystal is much higher than in other areas, so the possibility of laser induced damage is also relatively greater there. However, if the intensity of 1ω light before the frequency conversion system becomes even greater, for the normal running of final optics assembly the modulation and maximum intensity inside the frequency conversion crystals should be confined within the permitted range.

Ultraintense laser interaction with nanoscale targets: a simple model for layer expansion and ion acceleration

A simple model has been derived for expansion of a thin (up to 100s of nm thickness) target initially of solid density irradiated by an ultraintense laser. In this regime, ion acceleration mechanisms, such as the Break-Out Afterburner (BOA) [1], emerge with the potential for dramatically improved energy, efficiency, and energy spread. Ion beams have been proposed [2] as drivers for fast ignition inertial confinement fusion [3]. Analysis of kinetic simulations of the BOA shows the period of enhanced acceleration occurs between times t1, when the target becomes relativistically transparent to the laser, and t2, when the target becomes classically underdense and the enhanced acceleration terminates. A simple model for target expansion has been derived that contains early, one-dimensional (1D) expansion of the target and three-dimensional (3D) expansion at late times. The model assumes expansion is slab-like at the instantaneous ion sound speed and requires as input target composition, laser intensity, laser spot area, and the efficiency of laser absorption into electron thermal energy.

Multibeam optical pulse generation system based on time division multiplexing

By time division multiplexing (TDM) technique, an all-solid-state all-fiber multibeam optical pulse generation system used for an inertial confinement fusion driver is developed. The continuous wave signal is provided by a single longitudinal mode oscillator. The TDM technique and he high speed electro-optic modulation technique are utilized to realize the train pulse and arbitrary pulse-shaping. The polarization independent acoustooptic modulation technique is adopted to independently output the multibeam. Eight sub-pulses which may be shaped arbitrarily are included in each train pulse, and the time delay between adjacent sub-pulses is 120 ns. A single pulse selection module is used so that each beam turns into an independent output beam and then each beam is amplified and transmitted to output. Finally, it is successfully verified that the train pulse can be produced by the TDM technique, and its sub-pulse can be shaped arbitrarily and independently. The each beam energy is 1.275 μJ.

40 GW Linear Transformer Driver stage for pulse generators of Mega-ampere range

AbstractLinear transformer driver (LTD) technology is actively developed at the Institute of High Current Electronics in Tomsk, Russia. This technology is being examined for use in high current high voltage pulsed accelerators. Recent development of high voltage low inductance capacitors and low inductance switches enabled to achieve ~100 ns rise time of the LTD output pulse. This technique allows one to eliminate intermediate pulse forming sections, used in the present accelerator technology, which would keep the footprint of an LTD accelerator small. LTD based drivers are currently considered for many applications, including future very high current Z-pinch drivers for inertial confinement fusion, medium current drivers with adjustable pulse length for isentropic compression experiments, and finally relatively low current accelerators for radiography and X-pinches. In this article, we present the design and test results for a new LTD stage, that operates at 100 kV charging voltage. Current amplitude up to 850 kA with ~140 ns rise time was obtained on a 0.05 Ω load. Stack of the LTD stages can be easily assembled in series or in parallel, thus providing voltage or current multiplication, respectively. Design of multi-mega-volt and multi-mega-ampere generators becomes straightforward with the LTD technology.

Study on the frontal condition for continuous phase plate in inertial confinement fusion driver

Optimal placement for continuous phase plate （CPP） in terminal optical system of the Inertial Confinement Fusion （ICF） driver has been studied. Based on non-linear perturbation and transmitting theory of high power laser, the optical characteristics in near and far fields as well as third harmonic conversion efficiency after passing throngh the frequency conversion system has been calculated. As a result, the third harmonic conversion efficiency and optical characteristics of the emergent light beam are influenced by the CPP which is placed before the frequency conversion system. But if the diameter of the round focal spot in far field is smaller than 05 mm, the decline of the third harmonic conversion efficiency and the rise of contrast ratio are within permitted range, the shape of focal spot in far field and the encircled energy of focal spot also accord with the design requirement at the same time. The CPP which enables small focal spot in far field, when located in the beam path of fundamental frequency of ICF system to smooth and shape the light beam, will not exert influence on the normal running of ICF system.

Optimization of frequency conversion system in inertial confinement fusion driver for frontally located beam smoothing elements

There are many advantages for frontally located beam smoothing elements in terminal optical system of the inertial confinement fusion （ICF） driver, but it will also exert an influence on the working state of the frequency conversion system at the same time. This paper studies the influence from the viewpoints of system integration and optimization. Based on the non-linear transmission theory of high power laser and adopting coupled-wave differential equations and perturbation theory, we calculate the third harmonic conversion efficiency, optical characteristics in near and far field as well as the shape of focal spot when CPP is laid before the frequency conversion system. Then we put forward the method for the optimization of detuning angle and thickness of the frequency conversion crystal, in order to reduce influence on ICF system when CPP is laid before the frequency conversion system. The third harmonic conversion efficiency, contrast ratio and encircled energy before and after optimization are compared and analyzed. The results show that the optimization of frequency conversion system is very effective in reducing the impact on the beam quality and the third harmonic conversion efficiency when CPP is laid before the frequency conversion system.

Study on the near field modulation and laser induced damage of beam sampling grating

The beam sampling grating (BSG) is one kind of important diffractive optical elements which is used in sampling and diagnosis. In terminal optical system of the inertial confinement fusion (ICF) system, the modulation produced by strong laser in the near field, may cause laser induced damage to the BSG. It makes the element work abnormally. In order to provide the theoretical basis for protecting this kind of optical element from laser induced damage under the strong laser condition, this paper uses the Fourier modal theory to carry on the numerical simulation of the interior modulation for BSG in near field. The results indicate that the interior modulation of BSG is weak, but the optical field distribution is still not even, thus in these areas the damage probability is higher than in other positions. Moreover, through the fabrication error analysis of BSG, the relationship between modulation in near field and fabrication error was obtained. The computed results indicate that the depth error and the duty cycle error of grooves affect the modulation of the light beam in near field, but the influence of groove depth error is larger than the duty cycle error. However, if the groove depth error does not exceed a definite value, its modulation also remains on a lower level. Therefore the BSG can still work in the ICF driver normally.

Fabrication of Pressurized 2 mm Beryllium Targets for ICF Experiments

We have successfully fabricated 2 mm beryllium targets pressurized with a gas mixture of ~20 atm deuterium and ~0.1 atm argon. These targets have been used for indirect drive Inertial Confinement Fusion (ICF) experiments on the Z-pinch machine at Sandia National Laboratories leading to record neutron yields of ~3.5 × 1011 [J.E. Bailey, et al., “Be Capsule Implosions Driven by Dynamic Hohlraum X-rays,” Bull. Am. Phys. Soc. 51, 107 (2006)]. This paper will discuss the process of fabricating such targets from intact shells (Be sputter coated CH mandrels). These processes include laser drilling a ~6 μm diameter fill hole in a shell, removing the CH mandrel by pyrolysis, pressurizing the target with a deuterium/argon gas mixture and sealing the fill hole using UV glue while under pressure. The targets were characterized for gas pressure and deuterium gas permeation half-life by utilizing techniques including mass spectrometry, x-ray fluorescence spectroscopy and controlled shell bursting.

Fabrication of Pressurized 2 mm Beryllium Targets for ICF Experiments

We have successfully fabricated 2 mm beryllium targets pressurized with a gas mixture of ~20 atm deuterium and ~0.1 atm argon. These targets have been used for indirect drive Inertial Confinement Fusion (ICF) experiments on the Z-pinch machine at Sandia National Laboratories leading to record neutron yields of ~3.5 × 1011 [J.E. Bailey, et al., “Be Capsule Implosions Driven by Dynamic Hohlraum X-rays,” Bull. Am. Phys. Soc. 51, 107 (2006)]. This paper will discuss the process of fabricating such targets from intact shells (Be sputter coated CH mandrels). These processes include laser drilling a ~6 μm diameter fill hole in a shell, removing the CH mandrel by pyrolysis, pressurizing the target with a deuterium/argon gas mixture and sealing the fill hole using UV glue while under pressure. The targets were characterized for gas pressure and deuterium gas permeation half-life by utilizing techniques including mass spectrometry, x-ray fluorescence spectroscopy and controlled shell bursting.

Halogens for negative ion beams and ion–ion plasmas

Negative ions have attractive features as drivers for inertial confinement fusion, because they will avoid electron cloud effects, and could be efficiently photodetached to neutrals after the final focus, which could also be beneficial in heating warm dense matter targets. The halogens have large electron affinities, and thus should be able to produce high current densities of relatively robust negative ions. Recent experiments comparing chlorine beams to argon beams using the same source, extraction optics, and diagnostics have demonstrated that Cl − beams can be produced with similar emittance to Ar + beams, and with about 3 4 the current density from the same configuration. The observed effective beam temperature of about 1 3 eV , and the similarity of current densities show that negative halogen beams can meet the current density and emittance requirements of heavy ion fusion. The near equivalence of the Cl − and Cl ++Cl 2 + current densities reaching the Faraday cup after passage through a substantial line density of effluent gas demonstrates that beam losses in the higher vacuum of a heavy ion fusion accelerator should be acceptable. A number of lines of evidence show that negative ion–positive ion plasmas (hereafter ion–ion plasmas), composed primarily of negative and positive ions with a small population of electrons, were produced in the sources near the extractor plane. Since Cl, F, I, and Br should all show similar chemistry, any of these halogens should be suitable as fusion driver beams, and heating thin iodine or bromine foils may produce ion–ion plasmas in the warm dense matter regime.

γ -ray ‘bang-time’ measurements with a gas-Cherenkov detector for inertial-confinement fusion experiments

In a laser driven inertial-confinement fusion experiment, bang time is defined as the time between the laser light impinging the target and the peak of the fusion reactions. Bang time is often used to compare computed predictions to experiment. Large laser facilities, such as NIF and LMJ, which are currently under construction, will produce yields far in excess of any previous inertial-confinement fusion experiment. One of the implications of such high yields is that fusion γ rays, which have branching ratios four orders of magnitude less than that of fusion neutrons, may be used to diagnose bang time. This article describes the first of such γ-ray bang-time measurement made using the OMEGA laser facility at the Laboratory for Laser Energetics, University of Rochester. The diagnostic used for this was a gas Cherenkov detector. The experimental setup, data and error analyses, and suggested improvements are presented.

Hard x-ray imaging for measuring laser absorption spatial profiles on the National Ignition Facility

Hard x-ray (“thin wall”) imaging will be employed on the National Ignition Facility (NIF) to spatially locate laser beam energy deposition regions on the hohlraum walls in indirect drive Inertial Confinement Fusion (ICF) experiments, relevant for ICF symmetry tuning. Based on time resolved imaging of the hard x ray emission of the laser spots, this method will be used to infer hohlraum wall motion due to x ray and laser ablation and any beam refraction caused by plasma density gradients. In optimizing this measurement, issues that have to be addressed are hard x-ray visibility during the entire ignition laser pulse with intensities ranging from 1013to1015W∕cm2, as well as simultaneous visibility of the inner and the outer laser drive cones. In this work we will compare the hard x-ray emission calculated by LASNEX and analytical modeling with thin wall imaging data recorded previously on Omega and during the first hohlraum experiments on NIF. Based on these calculations and comparisons the thin wall imaging will be optimized for ICF/NIF experiments.