Inertial Confinement Fusion Driver Research Articles

The impact of low-mode symmetry on inertial fusion energy output in the burning plasma state

Indirect Drive Inertial Confinement Fusion Experiments on the National Ignition Facility (NIF) have achieved a burning plasma state with neutron yields exceeding 170 kJ, roughly 3 times the prior record and a necessary stage for igniting plasmas. The results are achieved despite multiple sources of degradations that lead to high variability in performance. Results shown here, for the first time, include an empirical correction factor for mode-2 asymmetry in the burning plasma regime in addition to previously determined corrections for radiative mix and mode-1. Analysis shows that including these three corrections alone accounts for the measured fusion performance variability in the two highest performing experimental campaigns on the NIF to within error. Here we quantify the performance sensitivity to mode-2 symmetry in the burning plasma regime and apply the results, in the form of an empirical correction to a 1D performance model. Furthermore, we find the sensitivity to mode-2 determined through a series of integrated 2D radiation hydrodynamic simulations to be consistent with the experimentally determined sensitivity only when including alpha-heating.

First indirect drive inertial confinement fusion campaign at Laser Megajoule

The first indirect drive Inertial Confinement Fusion (ICF) experiments on the Laser Megajoule facility were carried out with approximately 150 kJ of laser energy distributed on 48 beams (12 quads) arranged in two cones. The target consisted of a gold vacuum rugby-shaped hohlraum and a plastic capsule located at its center, filled with deuterium gas fuel. The arrangement of the 12 quads is such that the laser irradiation on the wall generated a three-dimensional (3D) x-ray flux around the capsule creating 3D deformations on the imploding plastic shell. This constraint forced the design of a robust target (relatively thin ablator, around 40 μm) driven by a short laser pulse (3 ns) that delivered about 1011 neutrons. Full-integrated 3D radiation hydrodynamics simulations allowed both the target definition and the data interpretation (mainly radiation temperature, x-ray images, and neutron yield). 3D calculations and experiments compare well.

Exploration of cross-beam energy transfer mitigation constraints for designing an ignition-scale direct-drive inertial confinement fusion driver

The compression of direct-drive inertial confinement fusion (ICF) targets is strongly impacted by cross-beam energy transfer (CBET), a laser-plasma instability that limits ablation pressure by redirecting laser energy outward and that is projected to be mitigated by laser bandwidth. Here, we explore various CBET mitigation constraints to guide the design of future ICF facilities. First, we find that the flat, Gaussian, and Lorentzian spectral shapes have similar CBET mitigation properties, and a flat shape with nine spectral lines is a good surrogate for what can be obtained with other spectral shapes. Then, we conduct a comprehensive study across energy scales and ignition designs. 3D hydrodynamic simulations are used to derive an analytical model for the expected CBET mitigation as a function of laser and plasma parameters. From this model, we study the bandwidth requirements of conventional and shock ignition designs across four different energy scales and find that they require between 0.5 and 3±0.2% relative bandwidth. Best mitigation is achieved when the beam radius over critical radius Rb/Rc is kept low during the drive while the plasma temperature is kept high. In a steady state, we find that the bandwidth required to mitigate 85% of CBET scales as (Rb/Rc)2.15Ln−0.58I0.7, where Ln is the density scale length, and I the laser intensity. Finally, we find that the chamber beam port layout does not influence CBET mitigation. In the case of a driver using many monochromatic beamlets, we find that ∼10 beamlets per port is required, with diminishing returns above ∼20.

Suppressing parametric instabilities in direct-drive inertial-confinement-fusion plasmas using broadband laser light

It has long been recognized that broadband laser light has the potential to control parametric instabilities in inertial-confinement-fusion (ICF) plasmas. Here, we use results from laser-plasma-interaction simulations to estimate the bandwidth requirements for mitigating the three predominant classes of instabilities in direct-drive ICF implosions: cross-beam energy transfer (CBET), two-plasmon decay (TPD), and stimulated Raman scattering (SRS). We find that for frequency-tripled, Nd:glass laser light, a bandwidth of 8.5 THz can significantly increase laser absorption by suppressing CBET, while ∼13 THz is needed to mitigate absolute TPD and SRS on an ignition-scale platform. None of the glass lasers used in contemporary ICF experiments, however, possess a bandwidth greater than 1 THz and reaching larger values requires the use of an auxiliary broadening technique such as optical parametric amplification or stimulated-rotational-Raman scattering. An arguably superior approach is the adoption of an argon-fluoride (ArF) laser as an ICF driver. Besides having a broad bandwidth of ∼10 THz, the ArF laser also possesses the shortest wavelength (193 nm) that can scale to the high energy/power required for ICF—a feature that helps to mitigate parametric instabilities even further. We show that these native properties of ArF laser light are sufficient to eliminate nearly all CBET scattering in a direct-drive target and also raise absolute TPD and SRS thresholds well above those for broadband glass lasers. The effective control of parametric instabilities with broad bandwidth is potentially a “game changer” in ICF because it would enable higher laser intensities and ablation pressures in future target designs.

Cable Coupling Effect of Compound Chamber Environment in Laser Inertial Confinement Fusion Driver

Cable Coupling Effect of Compound Chamber Environment in Laser Inertial Confinement Fusion Driver

Experimental Evidence of Harnessed Expansion of a High-Z Plasma Using the Hollow Wall Design for Indirect Drive Inertial Confinement Fusion.

The effectiveness of a dome-shaped wall covered by a thin gold foil (hollow wall) [M. Vandenboomgaerde etal., Phys. Plasmas 25, 012713 (2018)PHPAEN1070-664X10.1063/1.5008669] in holding back the high-Z plasma expansion in a gas-filled hohlraum is demonstrated for the first time in experiments reproducing the irradiation conditions of indirect drive at the ignition scale. The setup exploits a 1D geometry enabling record of the complete history of the gold expansion for 8ns by imaging its emission in multiple x-ray energy ranges featuring either the absorption zones or the thermal emission regions. The measured expansion dynamics is well reproduced by numerical simulations. This novel wall design could now be tailored for the megajoule scale to enable the propagation of the inner beams up to the equator in low gas-filled hohlraum thus allowing the fine-tuning of the irradiation symmetry on the timescale required for ignition.

Development of low-coherence high-power laser drivers for inertial confinement fusion

The use of low-coherence light is expected to be one of the effective ways to suppress or even eliminate the laser–plasma instabilities that arise in attempts to achieve inertial confinement fusion. In this paper, a review of low-coherence high-power laser drivers and related key techniques is first presented. Work at typical low-coherence laser facilities, including Gekko XII, PHEBUS, Pharos III, and Kanal-2 is described. The many key techniques that are used in the research and development of low-coherence laser drivers are described and analyzed, including low-coherence source generation, amplification, harmonic conversion, and beam smoothing of low-coherence light. Then, recent progress achieved by our group in research on a broadband low-coherence laser driver is presented. During the development of our low-coherence high-power laser facility, we have proposed and implemented many key techniques for working with low-coherence light, including source generation, efficient amplification and propagation, harmonic conversion, beam smoothing, and precise beam control. Based on a series of technological breakthroughs, a kilojoule low-coherence laser driver named Kunwu with a coherence time of only 300 fs has been built, and the first round of physical experiments has been completed. This high-power laser facility provides not only a demonstration and verification platform for key techniques and system integration of a low-coherence laser driver, but also a new type of experimental platform for research into, for example, high-energy-density physics and, in particular, laser–plasma interactions.

An efficient radiation analysis approach through compressive model for laser driven inertial confinement fusion

Radiation symmetry evaluation is critical to the laser driven Inertial Confinement Fusion (ICF), which is usually obtained through a view-factor equation model. The model is nonlinear, and its equation number can be very large when the size of discrete mesh element is very small to achieve a prescribed accuracy, which may lead to an intensive equation solving process. In this paper, an efficient radiation symmetry analysis approach is presented through equation model compression, in which, (1) the radiation symmetry analysis is carried out for two kinds of geometric shapes: cylindrical capsule and spherical capsule. For the structure of cylindrical capsule, the Legendre–Fourier basis and Zernike basis are applied to represent the radiation flux of the capsule. And for the structure of spherical capsule, the spherical harmonics is employed to sparsely represent the radiation flux on the capsule and spherical cavity. Then the nonlinear energy equilibrium equations are transformed into the equations with sparse coefficients, which means there are many redundant equations, (2) Only a few equations are uniformly selected to recover such sparse coefficients with a Latin hypercube sampling scheme, (3) A Modified Conjugate Gradient-Iteration Hard Thresholding (MCG-IHT) algorithm is then given to rapidly solve such sparse coefficients equations with as few as possible iterations. Finally, the presented approach is validated with two experiment targets for Shenguang III-YX and Shenguang II laser facilities in China. The equation model is reduced much more, only a few of equations are required to achieve the radiation flux with comparable accuracy within much less of computation time. And the solution is much more efficient as the size of discrete mesh element decreases, in which, almost 1.1% computation time is required to obtain the comparable result.

Development of a broad bandwidth 193 nanometer laser driver for inertial confinement fusion

The U.S. Naval Research Laboratory (NRL) has built a 137 J electron beam pumped argon fluoride (ArF) laser operating at 193 nm utilizing the Electra facility. This paper highlights initial ArF oscillator measurements in the context of significant benefits of ArF laser characterization for inertial confinement fusion target performance and development of a kinetics code with predictive capabilities. Other aspects of NRL's ArF laser development program, including large scale electron beam pumped ArF amplifier development and ArF optical transport, are discussed. Initial measurements were made utilizing a quadruple pass oscillator configuration constructed in the Electra facility with a nominal 100 cm2 aperture. Laser yields between 80 and 100 J, time-dependent laser intensity, and amplified spontaneous emission (ASE) are reported over a range of gas pressure (0.8–1.4 atmospheres). Higher laser yields, 137 J, were attained in a non-optimized dual ArF oscillator configuration wherein the optical axis was rotated to conform to the measured electron beam deposition in the laser gas mixture.

Development of a gated X-ray imager with multiple views and spectral selectivity for observing plasma evolution in hohlraums.

In the study of indirect drive Inertial Confinement Fusion (ICF), X-ray images through the laser entrance hole of the hohlraums can provide key information. In this paper, we present the advances on the development of a gated X-ray imager with multiple views and spectral selectivity. A diagnostic snout which contains four X-ray planar mirrors has been designed and coupled to a gated X-ray detector which includes an air box and has a spatial resolution better than 35 µm and a temporal resolution better than 70 ps. The two inner mirrors in the diagnostic snout operate at M band (1.6-3.5 keV), while the two outer mirrors operate at N band (0.5-1 keV). The four mirrors define four observation angles. Each mirror can reflect 4 X-ray images to the four microstrips of the gated X-ray detector with almost the same observation angle. The imager enables us to observe the spatial and temporal evolution of plasma in the hohlraums from four different observation angles based on both N band and M band images. The imager has been used in the ICF experiments conducted at Shenguang laser facility in China, and the obtained X-ray images of Au cylindrical hohlraum are presented and discussed in this paper.

Using polarization control plate to suppress transverse stimulated Raman scattering in large-aperture KDP crystal

AbstractTransverse stimulated Raman scattering (TSRS) is strongly generated in the third-harmonic-generation crystal potassium dihydrogen phosphate (KDP) and can even damage the KDP crystal in inertial confinement fusion drivers. In this work, a method to suppress TSRS is proposed in which the polarization control plate (PCP) is moved to a new position in the existing optical path. The proposed method can suppress TSRS significantly and doubles the laser threshold intensity in KDP crystal when the order of the PCP is 16. This result is attributed to the reduction of the gain length for the Stokes radiation. The proposed method may also be used to suppress other nonlinear effects, including transverse stimulated Brillouin scattering in large-aperture optical components.

A Compact Fusion–Fission Hybrid Reactor

Up today, two hyper research projects to achieve nuclear fusion energy exist; inertial confinement fusion (ICF) driven by laser, called national ignition facility (NIF) and magnetic confinement fusion the international thermonuclear experimental reactor (ITER) project. In reaching the required temperature and pressure, to ignite nuclear fusion reactor, is technologically complex and economically expensive. Thus, a breakthrough and a short cut, other alternative methods should be considered. Pulsed power ICF driver with repetitive pulse operation, mainly dense plasma focus (DPF) machines for high yield fusion neutrons could be taken as drivers for the fission blanket operation. The setup can be a cost-effective and efficient. In this article, we consider a set of two medium energy sizes DPF to produce simultaneously dense plasma columns, operating as thermonuclear plasma driver, to pierce the pellet target for external nuclear fusion reactions. These DPFs produce sufficient fast neutrons for the fission process in the neutral uranium or thorium and/or weak enriched uranium blanket. The drive systems and the concept for delivering thermonuclear plasma to pellets target in the magnetic free zone of central region will be presented. The feasibility of such fusion–fission hybrid reactor will be discussed.

Minimizing scatter-losses during pre-heat for magneto-inertial fusion targets

The size, temporal and spatial shape, and energy content of a laser pulse for the pre-heat phase of magneto-inertial fusion affect the ability to penetrate the window of the laser-entrance-hole and to heat the fuel behind it. High laser intensities and dense targets are subject to laser-plasma-instabilities (LPI), which can lead to an effective loss of pre-heat energy or to pronounced heating of areas that should stay unexposed. While this problem has been the subject of many studies over the last decades, the investigated parameters were typically geared towards traditional laser driven Inertial Confinement Fusion (ICF) with densities either at 10% and above or at 1% and below the laser's critical density, electron temperatures of 3–5 keV, and laser powers near (or in excess of) 1 × 1015 W/cm2. In contrast, Magnetized Liner Inertial Fusion (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010) and Slutz and Vesey, Phys. Rev. Lett. 108, 025003 (2012)] currently operates at 5% of the laser's critical density using much thicker windows (1.5–3.5 μm) than the sub-micron thick windows of traditional ICF hohlraum targets. This article describes the Pecos target area at Sandia National Laboratories using the Z-Beamlet Laser Facility [Rambo et al., Appl. Opt. 44(12), 2421 (2005)] as a platform to study laser induced pre-heat for magneto-inertial fusion targets, and the related progress for Sandia's MagLIF program. Forward and backward scattered light were measured and minimized at larger spatial scales with lower densities, temperatures, and powers compared to LPI studies available in literature.

Zinc Oxide–Coated Poly(HIPE) Annular Liners to Advance Laser Indirect Drive Inertial Confinement Fusion

Laser indirect drive is hindered, in part, by two problems: “wall motion” resulting from ablation of the hohlraum inner wall and “preheat” of the fuel capsule. To mitigate wall motion and preheat, a mid-Z–coated high internal phase emulsion, poly(HIPE) foam liner (5.7-mm diameter, 150 μm thick, 2.8 mm long, 33 mg/cm3) was developed and integrated into the hohlraum interior. A zinc oxide coating was applied throughout the poly(HIPE) foam using atomic layer deposition to achieve 149 ± 14 mg/cm3 bulk density. Preliminary data collected from actual shots at the National Ignition Facility suggest the inclusion of the poly(HIPE) liner reduced preheat threefold and stimulated Brillouin scattering (SBS) fivefold relative to an existing reference shot on a gold hohlraum (wavelength shift also contributed to SBS reduction).

StarDriver: Recent results on beam smoothing and LPI mitigation

StarDriver was recently proposed as a highly flexible laser driver for inertial confinement fusion and high energy density physics. It envisions a laser drive consisting of very many beams at an aperture and energy where the optical technology is well-developed, used in concert to create a large scale laser driver system. In this paper we describe a StarDriver-class laser with 5120 physical beamlets disposed about the target chamber in 80 evenly spaced ports, each port containing 64 beamlets, each beamlet having about ∼1.5THz of 2D SSD bandwidth and suitable phase plates, an aperture of ∼65mm, an energy of 80J, and frequency-converted to ∼351nm.many beamlets at an aperture where optical technology is well-developed, and each beamlet has energy ∼100J in a several times diffraction limited beam. The ensemble of beamlets has frequency bandwidth 2%-10%, thereby providing significant control of both hydrodynamic and laser-plasma instabilities The drive at the target is ∼400kJ, has a well-behaved low L-mode spectrum, and smooths very rapidly, reaching an asymptotic smoothness of <1% in less than Ins.

Damage Mechanisms Avoided or Managed for NIF Large Optics

After every other failure mode has been considered, in the end, the high-performance limit of all lasers is set by optical damage. The demands of inertial confinement fusion (ICF) pushed lasers designed as ICF drivers into this limit from their very earliest days. The first ICF lasers were small, and their pulses were short. Their goal was to provide as much power to the target as possible. Typically, they faced damage due to high intensity on their optics. As requests for higher laser energy, longer pulse lengths, and better symmetry appeared, new kinds of damage also emerged, some of them anticipated and others unexpected. This paper will discuss the various types of damage to large optics that had to be considered, avoided to the extent possible, or otherwise managed as the National Ignition Facility (NIF) laser was designed, fabricated, and brought into operation. It has been possible for NIF to meet its requirements because of the experience gained in previous ICF systems and because NIF designers have continued to be able to avoid or manage new damage situations as they have appeared.

A StarDriver-Class Laser Achieving 1 % Beam Uniformity in 1 ns

StarDriver was recently proposed as a highly flexible laser driver for inertial confinement fusion and high energy density physics. It envisions a laser drive consisting of many beamlets at an aperture where optical technology is well-developed, and each beamlet has energy ~100 J in a several times diffraction limited beam. Each beamlet has ~1.5THz of 2D SSD smoothing, but the ensemble of lasers has frequency bandwidth 2–10 %, thereby providing significant control of both hydrodynamic and laser-plasma instabilities. In this paper we illustrate the attractive features of the StarDriver concept with detailed calculations of beam smoothing for control of hydrodynamic instabilities in a direct drive ICF target, using a full 3D simulation of the laser drive. We describe here a StarDriver-class laser with 5120 physical beamlets disposed about the target chamber in 80 evenly spaced ports, each port containing 64 beamlets, each beamlet having about ~1.5THz of 2D SSD bandwidth and suitable phase plates, an aperture of ~65 mm, an energy of 80 J, and frequency-converted to ~351 nm. The drive at the target is ~400 kJ, has a well-behaved low L-mode spectrum, and smoothes very rapidly, reaching an asymptotic smoothness of <1 % in less than 1 ns. 2 MJ of drive at the same smoothing performance may be obtained by increasing the number of beamlets. An attractive feature of StarDriver is that the tolerances on the individual beamlet parameters are quite relaxed.

StarDriver: Recent Results on Beam Smoothing and 2ωpe Mitigation

StarDriver was recently proposed as a highly flexible laser driver for inertial confinement fusion and high energy density physics. It envisions a laser drive consisting of very many beams at an aperture and energy where the optical technology is well-developed, used in concert to create a large scale laser driver system. In this paper we describe a StarDriver–class laser with 5120 physical beamlets disposed about the target chamber in 80 evenly spaced ports, each port containing 64 beamlets, each beamlet having about ~1.5 THz of 2D SSD bandwidth and suitable phase plates, an aperture of ~65 mm, an energy of 80 J, and frequency-converted to ~351 nm. StarDriver has many beamlets at an aperture where optical technology is well-developed, and each beamlet has energy ~100 J in a several times diffraction limited beam. The ensemble of beamlets has frequency bandwidth 2%-10%, thereby providing significant control of both hydrodynamic and laser-plasma instabilities The drive at the target is ~400 kJ, has a well-behaved low L-mode spectrum, and smooth’s very rapidly, reaching an asymptotic smoothness of <1% in less than 1 ns. We also review recent results showing that the 2ωpe instability can be significantly reduced by 20 THz bandwidth.

A unified free-form representation applied to the shape optimization of the hohlraum with octahedral 6 laser entrance holes

The hohlraum is very crucial for indirect laser driven Inertial Confinement Fusion. Usually, its shape is designed as sphere, cylinder, or rugby with some kind of fixed functions, such as ellipse or parabola. Recently, a spherical hohlraum with octahedral 6 laser entrance holes (LEHs) has been presented with high flux symmetry [Lan et al., Phys. Plasmas 21, 010704 (2014); 21, 052704 (2014)]. However, there is only one shape parameter, i.e., the hohlraum to capsule radius ratio, being optimized. In this paper, we build the hohlraum with octahedral 6LEHs with a unified free-form representation, in which, by varying additional shape parameters: (1) available hohlraum shapes can be uniformly and accurately represented, (2) it can be used to understand why the spherical hohlraum has higher flux symmetry, (3) it allows us to obtain a feasible shape design field satisfying flux symmetry constraints, and (4) a synthetically optimized hohlraum can be obtained with a tradeoff of flux symmetry and other hohlraum performance. Finally, the hohlraum with octahedral 6LEHs is modeled, analyzed, and then optimized based on the unified free-form representation. The results show that a feasible shape design field with flux asymmetry no more than 1% can be obtained, and over the feasible design field, the spherical hohlraum is validated to have the highest flux symmetry, and a synthetically optimal hohlraum can be found with closing flux symmetry but larger volume between laser spots and centrally located capsule.

New two-dimensional space-resolving flux detection technique for measurement of hohlraum inner radiation in Shenguang-III prototype.

The space-resolving measurement of X-ray flux from a specific area (laser spot, re-emitting wall, or capsule) inside the hohlraum is an ongoing and critical problem in indirectly driven inertial-confinement fusion experiments. In this work, we developed a new two-dimensional space-resolving flux detection technique to measure the X-ray flux from specific areas inside the hohlraum by using the time- and space-resolving flux detector (SRFD). In two typical hohlraum experiments conducted at the Shenguang-III prototype laser facility, the X-ray flux and radiation temperature from an area 0.2 mm in diameter inside the hohlraum were measured through the laser entrance hole (LEH). The different flux intensities and radiation temperatures detected using the SRFD from the inner area of the LEH were compared with the result measured using the flat-response X-ray detector from the entire LEH. This comparison was also analyzed theoretically. The inner area detected using the SRFD was found to be the re-emitting wall area alone. This important improvement in space-resolving X-ray flux measurement will enhance the current X-ray flux space characterization techniques, thereby furthering the quantitative understanding of X-ray flux space behavior in the hohlraum.