Fast Ignition Targets Research Articles

Directly driven magnetized fast-ignition targets with steep density gradients for inertial fusion energy

The development of advanced targets capable of achieving ignition with improved energy gain at lower driver energies is one of four key technical challenges to be solved in order to realize economical inertial fusion energy. We report the minimum energy necessary for a small hemispherical mass of fast-ignited high-density deuterium–tritium fuel to explosively ignite a significantly larger hemispherical mass of assembled cold fuel with much lower mass density, both with and without a flux-compressed magnetic field connecting the two regions. With the magnetic field, the burn rate improves, and lower energy states become more effective. The imploded fuel reservoir available in the lower-density, larger-mass region of the steep density gradient determines whether the fusion yield is several hundred MJ or up to a few GJ. We report a case wherein the cold reservoir ignited and produced high gain with the assistance of only ∼700 kJ of hotspot yield, an amount that has already been demonstrated as feasible in laboratory experiments using indirect-drive targets.

Behavior of Gas Injected Fast Ignition Targets

Behavior of Gas Injected Fast Ignition Targets

Optimizing laser focal spot size using self-focusing in a cone-guided fast-ignition ICF target

This paper presents a scheme for strong self-focusing of a laser beam interacting with a cone-guided fast-ignition inertial confinement fusion target using cone pre-plasma filling as an optical medium for reducing the laser beam waist. The objective is to reduce the focal spot size at the interior of the tip of the re-entrant cone to that required for efficient coupling to the dense imploded fuel core. This is challenging to achieve in a large laser system using the standard optical components of a chirped-pulse-amplified (CPA) laser-beam chain where the spot sizes produced are often significantly larger than would be desirable for fast ignition. The approach described also makes use of the presence of pre-plasma in the cone. Such pre-plasma filling is difficult to avoid entirely when illuminating a cone with a high-energy CPA laser system due to the challenges of reducing laser pre-pulse to below the threshold for plasma production. For deriving the differential equation which governs the progress of the laser beam-width with propagation distance, paraxial theory in a WKB approximation has been used. A simulation is performed assuming strong self-focusing in accordance with the laser parameters and plasma density profile chosen.

Density diagnosis based on ps-duration-pulse X-ray backlighting for fast ignition compression

To provide significant parameters for fast ignition coupling efficiency and density diagnosis for higher compression, a ps-duration X-ray backlighter has been produced with ps-duration laser on Shenguang-Ⅱ updated facility. The radiation properties such as backlighting image resolution and the photons arriving at the image plate have been successfully measured. Based on the conformed conditions, density diagnosis of an indirectly-driven fast ignition target using ps-duration backlighting has been carried out. The obtained short-pulse backlighting radiography shows that the imploded shell shape agrees well with that of simulation and the areal density exceeds 50 mg/cm2. The short-pulse backlighting radiography also shows the hydrodynamic instability which might be caused by the asymmetric compression. Further investigations and attempts to improve implosion performance to a higher density are in progress.

Development of aluminum cone tip for fast ignition target

We have developed a novel method to fabricate an aluminum cone tip to attach the conventional gold cone for fast ignition. An aluminum cone tip is fabricated by laser thermal chemical vapor deposition. Trimethylaluminum (TMA) is used as a chemical precursor of aluminum. TMA is introduced into a vacuum chamber with argon gas and heated on a substrate by using a heater and a laser. Aluminum products are measured by using digital zoom microscope. It is found that the deposition rate is increased with the increase of the substrate temperature, argon gas flow, and the laser intensity. However, the increase of the laser intensity increases the temperature of the deposit, and also causes the melt of the deposit. Under suitable conditions, we succeeded in making desired aluminum conical shape with 32 μm height, whose size is difficult by conventional cutting technology. Therefore, this method will be important not only to the fusion experiments but also to many other industrial applications.

Study of fast ignition target design for ignition and burning experiments

For the fast ignition of laser fusion, a reliable target design is required for an ignition scale target. This paper shows the first optimized target design for the implosion phase of fast ignition, which is scalable to larger targets. The requirements from the heating process are taken account. In conclusion, a target can be highly compressed using multi-step laser pulse irradiation to a solid spherical target with an inserted gold cone. In an 8 kJ scale implosion, the maximum areal density of deuterium–tritium fuel reaches 0.39 g cm−2 according to 2D simulation results, which is 62% of the case without the cone. Based on the similarity rule, we estimate that the requirement of the implosion laser energy for ignition scale targets (ρRmax =1.0 g cm−2) is 135 kJ.

Ignition threshold in cylindrical fast ignition targets

In this paper, the ignition stage of heavy ion driven fast ignition is evaluated numerically by using a quasi-1D model. Recently, the minimum value of the fuel areal density in cylindrical fast ignition was reported by Ramis and Meyer-ter-Vehn [Laser Part. Beams 32, 41–47(2014)]. Here, we intend to examine the impact of the initial fuel temperature on the ignition threshold. It is expected that the initial temperature and fuel areal density provide an acceptable practical framework to manage a successful ignition scenario. Assuming a precompressed DT fuel with temperature between 0.1 keV and 1.0 keV and the areal density ⟨ρr⟩DT ≥ 0.45 g/cm2, the minimum required ignition energy has been derived. It is found that as the ignition energies decrease, the burn wave propagation into the fuel layer is suppressed and the ignition condition becomes more sensitive to the initial fuel temperature. Moreover, at fixed fuel areal density, the ignition threshold energy reveals a weak dependence on the fuel radius smaller than rDT ≤ 40 μm. In order to attain the high energy gain, G > 30, the minimum ignition energy of 250 kJ corresponding to beam intensity higher than 1.6 × 1019 W/cm2 is recommended. While, in the burn strategy, fb ≥ 0.33, it has been found that the threshold ignition energy scales exponentially with the fuel radius, which becomes vivid for the critical areal density of ⟨ρr⟩crt= 0.45 g/cm2. Finally, it has been shown that the contribution of the released fast neutrons provides the additional plasma heating, which generally improves the burn stage.

Improvement in the heating efficiency of fast ignition inertial confinement fusion through suppression of the preformed plasma

The study of fast electron spectrum optimization by suppression of preformed plasma in fast ignition targets is presented in this work. Integrated fast-electron spectra for electron energies below 3 MeV—the energy range responsible for core heating—are compared for different preformed plasma conditions. The pulse contrast (the ratio of peak-to-pedestal laser intensities) is compared for 108, 109 and 1011 conditions at constant laser energy (~500 J), pulse duration (2 ps), spot size (30% encircled energy on 50 µm diameter) and laser intensity (around 1 × 1019 W cm−2). The best electron spectrum optimization, consisting of maximized electron number for energies below 3 MeV was obtained with 14 µm thick cone targets. The energy coupling efficiency from heating laser to core plasma, assuming typical core plasma parameters, was estimated to be 2%, although 0.37% was obtained with previous conditions with poor pulse contrast and a 7 µm thick cone target.

A simple method to prevent hard X-ray-induced preheating effects inside the cone tip in indirect-drive fast ignition implosions

During fast-ignition implosions, preheating of inside the cone tip caused by hard X-rays can strongly affect the generation and transport of hot electrons in the cone. Although indirect-drive implosions have a higher implosion symmetry, they cause stronger preheating effects than direct-drive implosions. To control the preheating of the cone tip, we propose the use of indirect-drive fast-ignition targets with thicker tips. Experiments carried out at the ShenGuang-III prototype laser facility confirmed that thicker tips are effective for controlling preheating. Moreover, these results were consistent with those of 1D radiation hydrodynamic simulations.

Development of 4.5 keV monochromatic X-ray radiography using the high-energy, picosecond LFEX laser

Development of a monochromatic x-ray imaging system using a high-energy short- pulse laser LFEX and a spherical crystal is reported. Irradiation of the intense short-pulse laser produces a flash of 4.51 keV Ti K-alpha x-ray while the spherically bent quartz crystal provides a narrow spectral bandwidth and high spatial resolution. This high spatiotemporal imaging technique was applied for recording 2-D monochromatic x-ray images of laser-driven Fast Ignition targets. The results show a sufficiently high spatial resolution to characterize the implosion core, suggesting that the core information extracted from the radiograph images can be used to benchmark a 2-D radiation-hydrodynamic code for accurate hydrodynamic modelling and optimization of FI fuel assembly in the asymmetrical implosion.

Quantitative Kα line spectroscopy for energy transport in ultra-intense laser plasma interaction

Absolute Ka line spectroscopy is proposed for studying laser-plasma interactions taking place in the cone-guided fast ignition targets. X-ray spectra ranging from 20 to 100 keV were quantitatively measured with a Laue spectrometer. The absolute sensitivities of the Laue spectrometer system were calibrated using pre-characterized laser-produced x-ray sources and radioisotopes. The integrated reflectivity for the crystal is in good agreement with predictions by an open code for x-ray diffraction. The energy transfer efficiency from incident laser beams to hot electrons, as the energy transfer agency, is derived as a consequence of this work. The absolute yield of Au and Ta Ka lines were measured in the fast ignition experimental campaign performed at Institute of Laser Engineering, Osaka University. Applying the hot electron spectrum information from the electron spectrometer, an energy transfer efficiency of the incident LFEX [1], a kJ-class PW laser, to hot electrons was derived for a planar and cone-guided geometry.

Optimizing design of irradiation uniformity of direct drive cone-in-shell target for fast ignition

The irradiation uniformity of a cone-in-shell target directly driven by laser beams has been considered. First, a model is established to include the influence of the cone on laser beam propagation. Then, the irradiation uniformity on the target surface outside the cone during the initial imprinting phase is analyzed, and highly uniform irradiation on the target surface outside the cone is achieved by optimizing the intensity distribution within laser beams, as well as the polar direct drive displacement. As an illustrative example, direct drive irradiation uniformity of a typical cone-in-shell target is improved for Shenguang III laser facility, the illumination non-uniformity is reduced from 5.8% to 1.1%. Irradiation on the cone surface outside the target is also analyzed, and it is found that for the laser-target configuration considered in this work, a gold cone thicker than 50μm is needed to avoid shock breakout. Moreover, sensitivity to beam uncertainties (power imbalance and pointing error) is analyzed, indicating that this scheme can tolerate a certain amount of beam errors.

Physical studies of fast ignition in China

Fast ignition approach to inertial confinement fusion is one of the important goals today, in addition to central hot spot ignition in China. The SG-IIU and PW laser facilities are coupled to investigate the hot spot formation for fast ignition. The SG-III laser facility is almost completed and will be coupled with tens kJ PW lasers for the demonstration of fast ignition. In recent years, for physical studies of fast ignition, we have been focusing on the experimental study of implosion symmetry, M-band radiation preheating and mixing, advanced fast ignition target design, and so on. In addition, the modeling capabilities and code developments enhanced our ability to perform the hydro-simulation of the compression implosion, and the particle-in-cell (PIC) and hybrid-PIC simulation of the generation, transport and deposition of relativistic electron beams. Considerable progress has been achieved in understanding the critical issues of fast ignition.

Measurements of Preformed Plasma Generation and Its Suppression Inside a Cone in a Cone-in-Shell Target for Fast Ignition

Measurements of Preformed Plasma Generation and Its Suppression Inside a Cone in a Cone-in-Shell Target for Fast Ignition

快点火靶的新型概念和结构分析

快点火靶的新型概念和结构分析

Time-resolved compression of a capsule with a cone to high density for fast-ignition laser fusion.

The advent of high-intensity lasers enables us to recreate and study the behaviour of matter under the extreme densities and pressures that exist in many astrophysical objects. It may also enable us to develop a power source based on laser-driven nuclear fusion. Achieving such conditions usually requires a target that is highly uniform and spherically symmetric. Here we show that it is possible to generate high densities in a so-called fast-ignition target that consists of a thin shell whose spherical symmetry is interrupted by the inclusion of a metal cone. Using picosecond-time-resolved X-ray radiography, we show that we can achieve areal densities in excess of 300 mg cm(-2) with a nanosecond-duration compression pulse--the highest areal density ever reported for a cone-in-shell target. Such densities are high enough to stop MeV electrons, which is necessary for igniting the fuel with a subsequent picosecond pulse focused into the resulting plasma.

Hot electron spectra in hole-cone shell targets and a new proposal of the target for fast ignition in laser fusion

The traditional fast ignition scheme is that a compressed core created using an imploding laser is auxiliary heated and ignited by hot electrons (produced using a short pulse laser guided through the cone). However, sufficient heating cannot be achieved due to the highly hot-electron energy spectra, the energy loss in the cone, the large divergence of the hot electron and the long distance between the core and the generation point. Here, we propose a new target based on a hole-cone shell target experiment.

Preliminary target design for integrated direct-drive fast ignition experiments on Shenguang-II upgrade facility

The laser energy will be increased substantially when the Shenguang-II laser facility upgrade is completed and the petawatt picosecond laser beam will be equipped at the same time. For the fast ignition approach, direct-drive implosions have some advantages over indirect-drive ones, such as higher energy efficiency and lower mixing of cone material into fuel. Based on Shenguang-II upgraded laser facility, integrated direct-drive fast ignition experiments will be carried out and it will contribute to the further understanding of the relevant physics such as integrated coupling efficiency. The radiation hydrodynamic code Multi1D is used to design fast-ignition targets, and the optimized target parameters are achieved. The optimized target has a relatively thick wall (35 μm) and 420 μm-outer-radius CH shell, which are consistent with the scaling laws in target design. The deposition in the optimization target of the hot electrons generated by the picosecond petawatt pulse is also calculated according to the hot electrons scaling relation. The results show that the achieved areal density is high enough to stop the hot electrons.

Ultra-Precision Turning Technology of Gold Cone in Fast Ignition

Fast Ignition (FI) attracts much attention owing to its advantages. The fabrication of fast ignition targets is one of the key technologies in FI study. Based on the single point diamond turning (SPDT) technology, Diamond post-turning method is adopted in this paper for the fabrication of gold cone. It not only helps to reduce the end-effect of cone mandrel and consequently improve the coaxiality of internal and external cone surface, but also helps to improve the quality of cone surface and the wall thickness consistency. Besides, the processing parameter of diamond post-turning is experimentally studied in this paper for its effect on the cone surface roughness. According to results, the cone surface roughness is Ra 9.21nm, the wall thickness consistency is 3μm and the cone end surface roughness is Ra5.72nm。

Quantitative measurement of hard X-ray spectra from laser-driven fast ignition plasma

Absolute Kα line spectroscopy is proposed for studying laser–plasma interactions taking place in the Au cone-guided fast ignition targets. X-ray spectra ranging from 20 to 100 keV were quantitatively measured with a Laue spectrometer composed of a cylindrically curved crystal and a filter-absorption method for Bremsstrahlung continuum emission. The absolute sensitivities of the Laue spectrometer systems were calibrated using pre-characterized laser-produced X-ray sources and radioisotopes. The integrated reflectivity for the crystal is in good agreement with predictions by an X-ray diffraction code. The energy transfer efficiency from incident laser beams to hot electrons, as the energy transfer mechanism, is derived from this work. The absolute yield of Au and Ta Kα lines were measured in the fast ignition experimental campaign performed at Institute of Laser Engineering, Osaka University. Applying the hot electron spectrum information from electron spectrometer and scaling laws, the energy transfer efficiency from the incident LFEX, a kJ-class PW laser, to hot electrons was derived for the first time.