Laser Fusion Experiments Research Articles

An enhanced gated MCP-PMT for neutron detection in laser fusion experiments

The microchannel plate photomultiplier tube (MCP-PMT) is a photodetector that utilizes microchannel plates for signal amplification, offering rapid pulse response time of sub-nanosecond with a compact design and low dark current. A series of enhanced gated MCP-PMTs have been developed by incorporating a gating electrode between the photocathode and the MCPs, which have been employed in neutron detection during the double-cone ignition laser fusion experiment at the SGII Upgrade laser facilities.

Tamping the movement of the laser absorption cutoff position using gold foam hohlraum

In indirect-driven laser fusion experiments, the movement of the laser absorption layer will distort the radiation uniformity on the capsule. The gold foam has advantages in symmetry control and lowering wall plasma blowoff when used in an inertial confinement fusion (ICF) hohlraum. This work investigates the motion of the laser absorption cutoff position using low-density foam gold walls. It is found that the motion of the laser absorption cutoff position can be significantly mitigated through optimal initial low density, tailored to a specific laser shape. For a short square laser pulse, the laser absorption cutoff position remains almost stationary at an initial density of approximately 0.6 g cm−3. For a long-shaped laser pulse, the minimal motion of the laser absorption cutoff position is observed at an initial density of about 0.1 g cm−3. This approach allows for the adjustment of the symmetry of the hohlraum radiation source. The insights gained from this study serve as a crucial reference for optimizing the hohlraum wall density.

Ultrafast two-dimensional x-ray imager with temporal fiducial pulses for laser-produced plasmas

It is challenging to make an ultrafast diagnosis of the temporal evolution of small and short-lived plasma in two dimensions. To overcome this difficulty, we have developed a well-timed diagnostic utilizing an x-ray streak camera equipped with a row of multi-pinhole arrays. By processing multiple sets of one-dimensional streaked image data acquired from various pinholes, we are capable of reconstructing high-resolution two-dimensional images with a temporal resolution of 38 ps and a spatial resolution of 18 μm. The temporal fiducial pulses accessed from external sources can advance the precise timing and accurately determine the arrival time of the laser. Moreover, it can correct the nonlinear sweeping speed of the streak camera. The effectiveness of this diagnostic has been successfully verified at the Shenguang-II laser facility, providing an indispensable tool for observing complex physical phenomena, such as the implosion process of laser-fusion experiments.

Radiation resistance of praseodymium-doped aluminum lithium fluorophosphate scintillator glasses for laser fusion experiments

We report the gamma (γ)-ray radiation resistance of praseodymium (Pr3+)-doped aluminum lithium fluorophosphate scintillator glasses. For its assessment as a scintillator material for laser fusion experiments, a 20Al(PO3)3-80LiF-PrF3 (Pr3+-doped APLF) glass was irradiated with γ-rays from a cobalt-60 (60Co) source resulting in an absorbed dose of 5.2 kGy. Although γ-ray-irradiation results in increased absorption due to phosphorus-oxygen hole centers (POHCs) and PO3 2− electron centers (PO3 ECs), these radiation-induced defects do not modify the glass emission as both non-irradiated and γ-ray-irradiated glasses exhibit similar emission spectra and decay times under optical and X-ray excitation. The emission peaks observed also correspond to the different interconfigurational 4f5d → 4f2 and intraconfigurational 4f2 transitions of Pr3+ ions which are neither oxidized nor reduced by irradiation. Our results show that Pr3+-doped APLF glass still maintains its characteristic fast decay time and that γ-ray irradiation does not affect the glass scintillation mechanisms.

Optimization of Backscatter and Symmetry for Laser Fusion Experiments Using Multiple Tunable Wavelengths

A new additional wavelength-tuning capability has been implemented on the National Ignition Facility (NIF) laser allowing for unprecedented control of crossed-beam energy transfer (CBET) between all groups of beams for better performance of indirect-drive inertial confinement fusion (ICF) ignition experiments. In particular, this advance allows for negation and reversal of flow-induced CBET and the rebalancing of the intensity of different groups of beams within an indirect-drive (ICF) hohlraum. Experiments conducted at the NIF using a 1.1 MJ laser pulse with peak power of 390 TW demonstrate this high level of control through measurements of the precise changes to stimulated Brillouin scattering, typically driven by flow-induced CBET at the end of the pulse. Additionally, this new capability is shown to be able to predictably control gold-wall plasma expansion in the target driven by early-time flow-induced CBET. Estimates of early-time CBET from the wall expansion are shown to be consistent with simulation expectations. This new additional capability to control symmetry and backscatter expands the design space of current experiments and provides extra margin for increased laser energy in near-future experiments.

Target alignment method of inertial confinement fusion facility based on position estimation

Target alignment technology is one of the most critical technologies in laser fusion experiments and is an important technology related to the success of laser fusion experiments. In this study, by combining the open-loop and closed-loop errors of the target alignment, the Kalman state observer is used to estimate the position of the target, which improves the observation precision of the target alignment. Then the optimized result is used to guide the alignment of the target. This method can greatly optimize the target alignment error and reduce uncertainty. With the improvement of the target alignment precision, it will greatly improve the reliability and repeatability of the experiments’ results, thereby improving the success rate of the experiments.

Low-Threshold Induced-Side-Scattering Instability at the Tokamak Edge Transport Barrier Predicted in O-Mode Electron Cyclotron Resonance Heating Experiments

The trapping of a lower hybrid wave in the tokamak edge transport barrier is predicted, reducing by 3 orders of magnitude the excitation threshold for the absolute parametric decay instability that leads to side scattering of the ordinary microwave pump in electron cyclotron resonance heating (ECRH) experiments. This process is similar to the stimulated Raman scattering instability in laser physics and can result in substantial anomalous scattering of the pump wave, like in laser fusion experiments. The corresponding broadening of the ECRH power deposition profile can reduce the ability of this method to control the neoclassical tearing modes both in present day machines, as ASDEX-Upgrade, where the theory can be checked, and in fusion reactors such as ITER and DEMO.

Principal factors in performance of indirect-drive laser fusion experiments

Progress in inertial confinement fusion depends on the accurate interpretation of experiments that are complex and difficult to explain with simulations. Results could depend on small changes in the laser pulse or target or physics that are not fully understood or characterized. In this paper, we discuss an x-ray-driven platform [Baker et al., Phys. Rev. Lett. 121, 135001 (2018)] with fewer sources of degradation and find the fusion yield can be described as a physically motivated function of laser energy, target scale, and implosion symmetry. This platform and analysis could enable a more experimental approach to the study and optimization of implosion physics.

Features and Mechanisms of the Generation of Neutrons and Other Particles in First Laser Fusion Experiments

The quantitative characteristics of the first successful experiments on the formation of a fusion plasma have been discussed. It has been shown that the generation of neutrons detected in these experiments is not directly due to fusion processes in a laser plasma with a comparatively low temperature. Alternative mechanisms of stimulation of a fusion reaction have been considered. It has been shown that the most probable mechanism of neutron generation is attributed to the processes of formation of correlated coherent states, which are generated by a shock wave in the undestroyed part of the target lattice or at the motion of slow ions emitted from the laser plasma in the target. It is reasonable to repeat these experiments, where the effective generation of not only neutrons but also other products of nuclear fusion should be expected.

Scaling of Laser Fusion Experiments for DD-Neutron Yield

Yields of neutrons produced in various laser fusion experiments conducted in recent decades are compared with each other. It has surprisingly been found that there is a possibility to make an overall elucidation of the variance in the number of neutrons produced in the various experiments. The common method is based on definition of the energy conversion efficiency as a ratio between the energy carried out by neutrons produced through the fusion reaction and the input energy given by laser energy, E, focused on a target. The neutron-yield – laser-energy diagram is the basic chart used to interpret the spread of experimental data in terms of the experimental efficiency of the laser-matter interaction. Experiments carried out using a single laser system show that laser energy dependence of the yield can be well characterized by a power law, Y = QE^a, where Q is the parameter reflecting possible dependence on the pulse duration, laser intensity, laser contrast ratio, focal geometry, target structure, etc. Sorting the values of the neutron yields obtained in various experiments shows that the power law Y=QE^1.65 is suitable to determine lines in the Y-E diagram each of them, where the value of Q is associated with quality of experimental conditions. This sorting shows the order-of-magnitude differences in yields found in various experiments, which can be characterized by the value of the parameter Q. Due to the easy feasibility and the large number of DD fusion experiments performed, the overall Y-E diagram gives a chance to identify suitable laser systems for effective DD fusion experiments. It can, therefore, be assumed that some of the conclusions of these experiments could also be applied to the experimental arrangement suitable for optimizing p11B fusion.

Polarization of x-ray radiation after electron-impact excitation of ions in plasmas

A new relativistic model for calculating (magnetic sublevel) electron-impact excitation process in plasmas is proposed in this paper, which is essential to predict the polarization property of x-ray radiation. Our model focuses on weakly coupled plasmas (assuming that the plasma is of low density and very hot), where we use a Debye-Hückel potential to represent the interactions among the charged particles, in order to calculate plasma shielding effects. As a practical example, we present results of accurate calculations of transition energy, cross section, and linear polarization of the photon emission following EIE of 1s2→1s2p3,1P1 lines of He-like Ar16+ ion over a broad range of energies and Debye lengths, i.e., 16 times ionized atom of Ar. Such highly-ionized species can exist in many environments, such as in laser-produced plasmas, x-ray laser, astrophysics, and inertial confinement fusion experiments. The energies and cross sections are found to be decreased with decreasing Debye length, these general characteristics also lead to a reduction in the linear polarization of the photon emission. The present approach compares well, with existing methods and experimental data. Apart from other applications, results obtained in this work should be useful in plasma diagnostic.

Three-dimensional target monitoring technique based on a morphology correlation for high-power laser applications.

Existing monitoring and target positioning systems used in high-power laser fusion experiments have a complex structure and size, making their operation slow and calibration difficult. A simple and accurate positioning technique is proposed here that is based on cross-correlation between the morphology of the target measured at various positions. The morphology of the target is obtained using a spectral interferometer at a video frequency, and its three-dimensional (3D) position is determined by the position of the maximum spatial correlation coefficient. This technique can determine the 3D position and orientation of the target at the sub-micrometer range using only a single compact optical head. The proposed technique is quite simple in its structure and very convenient in operation. The feasibility and performance of the proposed technique is demonstrated in experiments.

Tripled yield in direct-drive laser fusion through statistical modelling.

Focusing laser light onto a very small target can produce the conditions for laboratory-scale nuclear fusion of hydrogen isotopes. The lack of accurate predictive models, which are essential for the design of high-performance laser-fusion experiments, is a major obstacle to achieving thermonuclear ignition. Here we report a statistical approach that was used to design and quantitatively predict the results of implosions of solid deuterium-tritium targets carried out with the 30-kilojoule OMEGA laser system, leading to tripling of the fusion yield to its highest value so far for direct-drive laser fusion. When scaled to the laser energies of the National Ignition Facility (1.9 megajoules), these targets are predicted to produce a fusion energy output of about 500 kilojoules-several times larger than the fusion yields currently achieved at that facility. This approach could guide the exploration of the vast parameter space of thermonuclear ignition conditions and enhance our understanding of laser-fusion physics.

Controlled Generation of Double Emulsions for Laser Fusion Target Fabrication Using a Glass Capillary Microfluidic Device

We report the controllable generation of double emulsions for target fabrication using glass capillary microfluidic devices. Instead of a conventional triple-orifice droplet generator, user-friendly glass capillary devices are used to produce micrometer to millimeter-sized water-in-oil-in-water emulsions. The double emulsions have a relatively uniform size distribution with an average outer diameter of 1420 μm. The sizes of the emulsions can also be varied by changing the ratio of the inner, middle, and outer fluids. Increasing the flow rate ratio of the outer fluid to the other fluids [Qo/(Qm+Qi)] from 3 to 11, the outer radii of the emulsions decrease from 1120 to 950 μm. On the other hand, increasing the flow rate ratio of the middle fluid to the inner fluid (Qm/Qi) from 0.7 to 1.6, the aspect ratio of the emulsions increases from 4 to 8. Our experimental values are in good agreement with a simple theoretical model. These results suggest that our present method to control the generation of double emulsions can be used as an alternative approach to fabricate polystyrene targets for future laser fusion experiments.

Cu-oleate microspheres fabricated by emulsion method as novel targets for fast ignition laser fusion experiments

We report the successful fabrication of copper (II) oleate [Cu(C18H33O2)2, Cu-oleate] microspheres as novel targets for laser fusion. Cu-oleate microspheres were prepared using emulsion method with Cu-oleate dissolved in an organic solvent. The fabricated microspheres exhibit a uniform radius of around 103μm and a good sphericity of 0.97. The microspheres also have high atomic copper (Cu) content of ∼10wt%, and the Cu ions are homogeneously distributed along their radii. Although defects which are 1-μm deep can be observed on the microsphere surfaces, these defects do not cause any significant perturbation that affects the resulting areal density during implosion. With good morphology, Cu content, and target performance, the Cu-oleate microspheres fabricated by emulsion method can then be used as targets for fast ignition laser fusion experiments such as FIREX. The use of these targets enables the x-ray observation of the laser-induced fast electron energy transfer on the compressed core.

Single lens logarithmic confocal distance measurement array.

This article presents a method and results based on confocal microscopy for non-contact axial distance measurement from a lens to a partially reflective surface using an array of photodetectors. The ratio of the photodetector signals is independent of the surface reflectivity and light source variations over a range of distance. By using logarithmic photodetector signals the object distance is proportional to the difference between the signals. This technique potentially allows measurement of sound produced by single living cells, or the high velocity surface shape changes in the hohlraum in laser fusion experiments.

Tritium-doping enhancement of polystyrene by ultraviolet laser and hydrogen plasma irradiation for laser fusion experiments

We investigate the tritium-doping enhancement of polystyrene by ultraviolet (UV) laser and hydrogen plasma irradiation. Tritium-doped polystyrene films are fabricated by the Wilzbach method with UV laser and hydrogen plasma. The 266-nm laser-irradiated, 355-nm laser-irradiated, and hydrogen plasma-irradiated polystyrene films exhibit higher PSL intensities and specific radioactivities than the non-irradiated sample. Tritium doping by UV laser irradiation can be largely affected by the laser wavelength because of polystyrene’s absorption. In addition, UV laser irradiation is more localized and concentrated at the spot of laser irradiation, while hydrogen plasma irradiation results to a more uniform doping concentration even at low partial pressure and short irradiation time. Both UV laser and plasma irradiations can nevertheless be utilized to fabricate tritium-doped polystyrene targets for future laser fusion experiments. With a high doping rate and efficiency, a 1% tritium-doped polystyrene shell target having 7.6×1011Bqg−1 specific radioactivity can be obtained at a short period of time thereby decreasing tritium consumption and safety management costs.

Mechanical design of experimental apparatus for FIREX cryo-target cooling

Mechanical design of an experimental apparatus for FIREX cryo-target cooling is described. Gaseous helium (GHe) sealing system at a cryogenic environment is an important issue for laser fusion experiments. The dedicated loading system was designed for a metal gasket. We take U-TIGHTSEAL® (Usui Kokusai Sangyo Kaisha. Ltd.) with an indium plated copper jacket as an example. According to its specification, a linear load of 110 N/m along its circumference is the optimum compression; however a lower load would still maintain helium (He) leak below the required level. Its sealing performance was investigated systematically. Our system demanded 27 N/mm of the load to keep He leak tightness in a cryogenic environment. Once leak tightness was obtained, it could be reduced to 9.5 N/mm.

Hard x-ray transmission curved crystal spectrometers (10–100 keV) for laser fusion experiments at the ShenGuang-III laser facility

Two transmission curved crystal spectrometers are designed to measure the hard x-ray emission in the laser fusion experiment of Compton radiography of implosion target on ShenGuang-III laser facility in China. Cylindrically curved${\it\alpha}$-quartz (10–11) crystals with curvature radii of 150 and 300 mm are used to cover spectral ranges of 10–56 and 17–100 keV, respectively. The distance between the crystal and the x-ray source can be changed over a broad distance from 200 to 1500 mm. The optical design, including the integral reflectivity of the curved crystal, the sensitivity, and the spectral resolution of the spectrometers, is discussed. We also provide mechanic design details and experimental results using a Mo anode x-ray source. High-quality spectra were obtained. We confirmed that the spectral resolution can be improved by increasing the working distance, which is the distance between the recording medium and the Rowland circle.

Alpha Heating and Burning Plasmas in Inertial Confinement Fusion.

Estimating the level of alpha heating and determining the onset of the burning plasma regime is essential to finding the path towards thermonuclear ignition. In a burning plasma, the alpha heating exceeds the external input energy to the plasma. Using a simple model of the implosion, it is shown that a general relation can be derived, connecting the burning plasma regime to the yield enhancement due to alpha heating and to experimentally measurable parameters such as the Lawson ignition parameter. A general alpha-heating curve is found, independent of the target and suitable to assess the performance of all laser fusion experiments whether direct or indirect drive. The onset of the burning plasma regime inside the hot spot of current implosions on the National Ignition Facility requires a fusion yield of about 50kJ.