Inertial Confinement Fusion System Research Articles

Compressive Sensing for the Symmetry Distribution Analysis of Thermal Radiation on the Capsule inside a Cylindrical Hohlraum

The symmetric implosion of the pellet is very important in an inertial confinement fusion (ICF) system. The evaluation of symmetric distribution on the ICF pellet is time and memory consuming, especially when the elements of cylindrical hohlraums are iteratively subdivided. A novel compressive sensing (CS) based approach is presented in this paper to accurately valuate the distribution symmetry with much less computation. The core idea is that, the thermal radiation distribution is transformed in the spherical harmonic field, and the sparsity of spherical harmonics is accurately evaluated through CS approach. Finally, experiments are demonstrated to show higher efficiency of the proposed approach.

Stability Research of Laser Confocal Probe for Measuring the Micro-Shpere

Laser confocal probe, on an active focus adjusting principle, was introduced in this paper. This kind of non-contact laser probe has the abilities to do the detection on strong reflection of the surface of high precision metal parts and optical devices. So, a problem of shape inspection detection was resolved. The micro-sphere is the key element in the ICF (short for inertial confinement fusion) system. The success or failure of "Ignition" is directly related to the accuracy of the assembly location of micro-sphere in the micro-target. Because of the particularity of the micro-sphere, laser confocal probe is used to do the detection of the center error of micro-sphere. The required measurement precision is less than 3µm. The mathematical formula between the height defference or the displacement value of measured surface and the displacement valued of the tuning fork can be gotten by the time different principle. Good stability of frequency and the swing of tuning fork, which approved through the experiments, have little influence to the time different Δt and the final measurement results.

The influence of root mean square phase gradient of continuous phase plate on smoothing focal spot

In order to satisfy uniform irradiation requirement in inertial confinement fusion(ICF) system, the influence of surface characteristics of continuous phase plate on beam quality is studied in this paper. Based on wave optics and geometrical optics, theoretical analysis model is established, respectively. And the parameter, root mean square gradient, is used to unify both theoretical models. Meanwhile, the smoothing effects of root mean square phase gradient of continuous phase plate on laser beam with low-frequency aberrant wavefront are simulated. Numerical simulations show that the radius of focus spot will increase with the increase of root mean square phase gradient of continuous phase plate, and nonuniformity of beam top will decrease quickly, then reduce slowly and tend to be unchanged finally. So smoothing effect of continuous phase plate is obvious. In addition, energy availability factor has little change, then decreases gradually and reduces slowly. Spot size, nonuniformity of beam top and energy availability factor are all in a good range when the correlation length of continuous phase plate is not changed and root mean square gradient is 0.20.8 wave/mm.

Damage production and accumulation in SiC structures in inertial and magnetic fusion systems

Radiation damage parameters in SiC/SiC composite structures are determined in both magnetic (MFE) and inertial (IFE) confinement fusion systems. Variations in the geometry, neutron energy spectrum, and pulsed nature of neutron production result in significant differences in damage parameters between the two systems. With the same neutron wall loading, the displacement damage rate in the first wall in an IFE system is ∼10% lower than in an MFE system, while gas production and burnup rates are a factor of 2 lower. Self-cooled LiPb and Flibe blankets were analyzed. While using LiPb results in higher displacement damage, Flibe yields higher gas production and burnup rates. The effects of displacement damage and helium production on defect accumulation in SiC/SiC composites are also discussed.

Analysis of hydrodynamic instability growth in a 2D flow

Experiments with gas bubbles rising in water are presented. The experiments are found to be useful to explore effects related to the joint action and development of gravitational and shear instabilities in a 2D flow. This paper describes an experimental setup to look at the development of hydrodynamics instabilities on the dome of an air bubble rising in liquid. The processes observed during the development of such instabilities are extremely important as they provide better insight into the nonlinear stage of gravitational instability in a 2D case with a curvilinear unstable interface with initial perturbations. The practical relevance of such effects is associated with their occurrence in inertial confinement fusion systems. It is shown that initial short-wavelength perturbations on the dome of a rising bubble quickly decay. Such decay is not attributed to dissipative mechanisms (viscosity or surface tension). Instead, it is a manifestation of general hydrodynamic laws that result in a so-called sub-harmonic instability. The experimental technique used in the experiments could also be developed to check the hypothesis of scale factor in the problem of turbulent mixing. The possibility of a large-scale experiment is discussed.

EFFECTS OF DEUTERIUM–LITHIUM FUSION REACTION ON INTERNAL TRITIUM BREEDING

The optimal usage of designed fuel pellets is one of the very important parameters in inertial confinement fusion (ICF) systems. In this research, time-dependent dynamical equations for D/D fuel are written by considering impurity of 6 Li . Then dependency of gain on temperature, density and pellet radius is studied using Runge–Kutta method. The obtained results show that the energy gain will be maximized at the initial temperature 35 keV, density, 5000 g/cm3 and ratio impurity of 6 Li , 0.05.

Ion Equation of State using scaled binding energy model

Global Equation of State models are used to carry out hydrodynamic calculations of inertial confinement fusion systems. There are some such models like QEOS which are used quite routinely in ICF simulations. The empirical content of QEOS, which otherwise does have a strong theoretical basis, provides scope for further research aiming at making the EOS theoretically more sound by reducing the empirical nature of the EOS. To this end, we have made some progress by developing an equation of state which uses a more accurate form for the zero temperature isotherm and a method to calculate the Grüneisen parameter so as to yield an ion equation of state which is better than the Cowan's model used in QEOS. The parameters of the model are taken from experiments or electronic structure calculations. The EOS is verified against the shock Hugoniot data and the melting data of Pb and U, and the phase diagram and Hugoniot of Hf. The stability analysis of some metals like Cu and Mo had also been carried out. The agreement of the calculated results with available experimental data is reasonably good.

Stability analysis of linearly chirped Gaussian pulse stacking in laser plasma reaction

We analyze the stability of linearly chirped Gaussian pulse stacking (LCGPS) in the laser plasma reaction (LPR) in the inertial confinement fusion (ICF) system. The LPR can be treated as the process that the stacked pulse is first intensity filtered and then induces the plasma due to the thermalization time of the plasma. We also examine the stability of LCGPS over the change of the thermalization time of the plasma, the timing delay, and the intensity attenuation of the stacked pulse in the LPR, and compare the results with those of none chirped Gaussian pulse stacking (NCGPS). Our results show that LCGPS is more stable than NCGPS.

Deuterium/helium-3 fusion reactors with lithium seeding

Optimal usage and consumption of designed fuel pellets in the inertial confinement fusion system (ICF) is one of the very important parameters. In this research, nuclear fusion in D/3Hey/6Lix mixtures is considered using a time dependent model based on nuclear reactions, including ion–electron collisions, bremsstrahlung losses and inverse Compton effects and mechanical expansion [1]. The effect of the initial lithium concentration on ignition temperature and energy gain is analyzed. It was found that with x = 0.37 (x: lithium content parameter) and ρR = 23 g cm−2 (ρR: areal density) and Ti = 32 keV (Ti : ignition temperature) we get the maximum energy gain.

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 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.

Diagnostic components in harsh radiation environments: Possible overlap in R&D requirements of inertial confinement and magnetic fusion systems

The next generation of large scale fusion devices--ITER/LMJ/NIF--will require diagnostic components to operate in environments far more severe than those encountered in present facilities. This harsh environment is the result of high fluxes of neutrons, gamma rays, energetic ions, electromagnetic radiation, and in some cases, debris and shrapnel, at levels several orders of magnitude higher than those experienced in today's devices. The similarities and dissimilarities between environmental effects on diagnostic components for the inertial confinement and magnetic confinement fusion fields have been assessed. Areas in which considerable overlap have been identified are optical transmission materials and optical fibers in particular, neutron detection systems and electronics needs. Although both fields extensively use cables in the hostile environment, there is little overlap because the environments and requirements are very different.

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.

Equation of state using scaled binding energy model

Global equation of state models are currently used for hydrodynamic simulations of inertial confinement fusion systems. The quotidian equation of state (QEOS) is one of such models. In spite of a sound theoretical basis, QEOS uses two empirical corrections for obtaining agreement with measured shock wave data. While the first is used for all materials, the second is to account for structural phase transitions. Evaluation of these corrections requires a priori knowledge of experimental data. Two improvements to QEOS proposed in this article obviate the use of both corrections. First, a modified version of the universal scaled binding energy is used for the zero-temperature isotherm. Then, an expression is derived for the Grüneisen parameter γ(ρ), which also includes the noncentral features of interparticle potential. The Debye temperature and melting temperature, deduced from γ(ρ), show excellent agreement with experimental results or electronic structure calculations. Predictions of the improved EOS model also compare very well with measured Hugoniot for normal metals such as Al, Cu, Pd, Pt, etc., and Fe and Zr, which undergo shock induced phase transitions.

Measurement of transient near-infrared laser pulse wavefront with high precision by radial shearing interferometer

It is hard to measure the transient near-infrared pulse laser wavefront in inertial confinement fusion (ICF) system by conventional methods for its short pulse width, large energy, high power and large distortion. A circular radial shearing interferometer based on spatial phase modulation is proposed to measure the transient near-infrared laser pulse wavefront with high precision. Transient, highly precise measurement for near-infrared laser pulse wavefront, with pulse width of nanometer scale and central wavelength of 1064 nm, can be carried out with common path, no reference plane. The theory of wavefront reconstruction has been validated by the computer simulation, and an error less than 1/1000 λ is obtained. Comparing with the results of ZYGO interferometer, an error less than 1/15 λ for both peak valley and root mean square value, is gained with good repeatability. The system has already been used in the ICF system to test the pulsed laser wavefront, and can also be applied to other lasers of visible and infrared wavelengths.

Effects of spectral shifting in an inertial confinement fusion system

Abstract The main objective is to study the effects of spectral shifting in an inertial confinement system for kT/shot energy regime on the breeding performance for tritium and for high quality fissile fuel. A protective liquid droplet jet zone of 2 m thickness is used as coolant, energy carrier, and breeder. Flibe as the main constituent is mixed with increased mole-fractions of heavy metal salt (ThF4 or UF4) starting by 2 moles% up to 12 moles%. Spectrum softening within the inertial confinement system reduces the tritium production ratio (TBR) in the protective coolant to a lower level than unity. However, additional tritium production in the 6Li2DT zone of the system increases TBR to values above unity and allows a continuous operation of the power plant with a self-sustained fusion fuel supply. By modest fusion fuel burn efficiencies (40 to 60 %) and with a few mol.% of heavy metal salt in the coolant in form of ThF4 or % UF4, a satisfactory TBR of > 1.05 can be realized. In addition to that, excess fissile fuel of extremely high isotopic purity with a rate of ∼ 1000 kg/year of 233U or 239Pu can be produced. Radiation damage through atomic displacements and helium gas production after a plant operation period of 30 years is very low, namely dpa < 1 and He < 2 ppm, respectively.

Design and characterization of a neutralized-transport experiment for heavy-ion fusion

In heavy-ion inertial-confinement fusion systems, intense beams of ions must be transported from the exit of the final-focus magnet system through the fusion chamber to hit spots on the target with radii of about 2 mm. For the heavy-ion-fusion power-plant scenarios presently favored in the U.S., a substantial fraction of the ion-beam space charge must be neutralized during this final transport. The most effective neutralization technique found in numerical simulations is to pass each beam through a low-density plasma after the final focusing. To provide quantitative comparisons of these theoretical predictions with experiment, the Virtual National Laboratory for Heavy Ion Fusion has completed the construction and has begun experimentation with the neutralized-transport experiment. The experiment consists of three main sections, each with its own physics issues. The injector is designed to generate a very high-brightness, space-charge-dominated potassium beam, while still allowing variable perveance by a beam aperturing technique. The magnetic-focusing section, consisting of four pulsed quadrupoles, permits the study of magnet tuning, as well as the effects of phase-space dilution due to higher-order nonlinear fields. In the final section, the converging ion beam exiting the magnetic section is transported through a drift region with plasma sources for beam neutralization, and the final spot size is measured under various conditions of neutralization. In this paper, we discuss the design and characterization of the three sections in detail and present initial results from the experiment.

Comparison and physical interpretation of MCNP and TART neutron and γ Monte Carlo shielding calculations for a heavy-ion ICF system

Inertial confinement fusion (ICF) aims to induce implosions of D–T pellets to obtain a extremely dense and hot plasma with lasers or heavy-ion beams. For heavy-ion fusion (HIF), recent research has focused on “liquid-protected” designs that allow highly compact target chambers. In the design of a reactor such as HYLIFE-II [Fus. Techol. 25 (1984); HYLIFE-II Progress Report, UCID-21816, 4.82-100], the liquid used is a molten salt made of F 10, Li 6, Li 7, Be 9 (called flibe). Flibe allows the final-focus magnets to be closer to the target, which helps to reduce the focus spot size and in turn the size of the driver, with a large reduction of the cost of HIF electricity. Consequently the superconducting coils of the magnets closer to the D–T neutron source will potentially suffer higher damage though they can stand only a certain amount of energy deposited before quenching. This work has been primarily focusing on verifying that total energy deposited by fusion neutrons and induced γ rays remain under such limit values and the final purpose is the optimization of the shielding of the magnetic lens system from the points of view of the geometrical configuration and of the physical nature of the materials adopted. The system is analyzed in terms of six geometrical models going from simplified up to much more realistic representations of a system of 192 beam lines, each focused by six magnets. A 3-D transport calculation of the radiation penetrating through ducts, that takes into account the complexity of the system, requires Monte Carlo methods. The technical nature of the design problem and the methodology followed were presented in a previous paper [Nucl. Instr. and Meth. A 464 (2001) 410] by summarizing briefly the results for the deposited energy distribution on the six focal magnets of a beam line. Now a comparison of the performances of the two codes TART98 [TART98: A Coupled Neutron-Photon 3-D Combinational Geometry Monte Carlo Transport Code, Lawrence Livermore National Laboratory, UCRL-ID-126455, Rev. 1, November, 1997] and MCNP4B [MCNP – A General Monte Carlo N-Particle Transport Code, Version 4B, La-12625-m, March 1997, Los Alamos National Laboratory] for two different configurations of the system is discussed, separating the n and γ contributions, in the light of the physical interpretation of the results in terms of first flight and of scattered neutron fluxes, of primary γ and of secondary γ generated by inelastically scattered or radiatively captured neutrons. The final conclusions indicate some guidelines and suggest possible improvements for the future neutronic shielding design for a HIF facility.

Symmetric Inertial-Confinement-Fusion-Capsule Implosions in a Double-Z-Pinch-Driven Hohlraum

An inertial-confinement-fusion (ICF) concept using two 60-MA Z pinches to drive a cylindrical hohlraum to 220 eV has been recently proposed. The first capsule implosions relevant to this concept have been performed at the same physical scale with a lower 20-MA current, yielding a 70+/-5 eV capsule drive. The capsule shell shape implies a polar radiation symmetry, the first high-accuracy measurement of this type in a pulsed-power-driven ICF configuration, within a factor of 1.6-4 of that required for scaling to ignition. The convergence ratio of 14-21 is to date the highest in any pulsed-power ICF system.

Ablative Acceleration of Foils, Their Pulsations, and Interchange Instability

The problem of hydrodynamic stability is important for inertial confinement fusion (ICF) systems based upon high compression of fuel before its ignition. This problem for the case of complicated multilayer foils has been studied here by a new approach describing the development of Rayleigh-Taylor or interchange instability in compressible media with inhomogeneous distribution of “entropy”s = ρ/ρk, ∂ where K = (∂ In ρ/∂ In ρ)s is an adiabatic derivative taken in the local hydrostatic values of ρ and ρ. Inhomogeneous distribution of s simulates the dynamics of development of perturbations of multilayer flyer foils and shells. Besides instability, the same approach has been used for analysis of ID pulsations of a levitated foil. The problem of pulsations is real in the case of foils. Indeed, (1) an ablative acceleration is equivalent to an effective gravity field, which causes the appearance of an atmospheric-type distribution of thermodynamic functions, (2) the duration of ablative flight of foil is at least several times larger than the time that is necessary for an acoustic wave to travel from one side of the foil to another side, and (3) there is a strong initial impulse that initiates the motion of foil. This impulse together with (1, 2) is a reason for the powerful pulsations of foils. The period of pulsations is defined by the velocity of sound in the foil material, which is dependent on the derivatives of an equation of state (EOS). The check of the derivatives gives us finer information concerning the current state of matter and the EOS than the usual measurements of material velocity and pressure that are rougher measures. Therefore, an analysis of pulsations seems to be a promising tool for tracking the dynamics of flyer foil and for the definition of thermodynamic properties of matter.