Fast Ignition Targets Research Articles

A simulation study of fast ignition with ultrahigh intensity lasers

The coupling efficiency between the ignition laser and the target core for the fast ignition concept is studied using two-dimensional particle-in-cell simulations. The details of the energy transport within the weakly collisional overdense plasma of a fast ignition target are examined by performing a series of particle-in-cell simulations, where the intensity incident on a 100 times critical plasma with 50 μm radius is varied between each simulation. The simulations show that the peak energy flux of the ignition electrons is significantly lowered as the electrons traverse the collisionless plasma from the critical density through a weakly collisional overdense plasma region. This allows higher intensity lasers to be used thereby improving the coupling efficiency. In addition, we find that a higher percentage of the ignition laser energy is delivered to the core of the simulation target at higher intensity. The coupling efficiency increases in time during the simulations which are run for 2.5 ps. For a laser intensity of 8×1020 W/cm2 the simulations indicate that more than 15% of the incident laser energy is eventually absorbed in a dense plasma core.

Integrated simulations of implosion, electron transport, and heating for direct-drive fast-ignition targets

A thorough understanding of future integrated fast-ignition experiments combining compression and heating of high-density thermonuclear fuel requires hybrid (fluid+particle) simulations of the implosion and ignition process. Different spatial and temporal scales need to be resolved to model the entire fast-ignition experiment. The two-dimensional (2D) axisymmetric hydrocode DRACO [P. B. Radha et al., Phys. Plasmas 12, 056307 (2005)] and the 2D/three-dimensional hybrid particle-in-cell code LSP [D. R. Welch et al., Nucl. Instrum. Methods Phys. Res. A 464, 134 (2001)] have been integrated to simulate the implosion and heating of direct-drive, fast-ignition fusion targets. DRACO includes the physics required to simulate compression, ignition, and burn of fast-ignition targets. LSP simulates the transport of hot electrons from the place where they are generated to the dense fuel core where their energy is absorbed. The results from integrated simulations of cone-in-shell CD targets designed for fast-ignition experiments on OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997); C. Stoeckl et al., Fusion Sci. Technol. 49, 367 (2006)] are presented. Target heating and neutron yields are computed. The results from LSP simulations of electron transport in solid-density plastic targets are also presented. They confirm an increase in the electron divergence angle with the laser intensity in the current experiments. The self-generated resistive magnetic field is found to collimate the hot-electron beam and increase the coupling efficiency of hot electrons with the target. Resistive filamentation of the hot-electron beam is also observed.

Fast-ignition target design and experimental-concept validation on OMEGA

A comprehensive scientific program is being pursued at LLE to explore the physics of fast ignition. The OMEGA EP Laser was completed in April 2008, adjacent to the 60 beam, 30 kJ OMEGA Laser Facility. OMEGA EP consists of four beamlines with a NIF-like architecture, each delivering up to 6.5 kJ of UV laser energy in long pulse (ns) mode into the OMEGA EP target chamber. Two of the beamlines can operate as high-energy petawatt lasers, with up to 2.6 kJ each with 10 ps pulse duration. These beams can either be injected into the OMEGA EP target chamber or combined collinearly into the existing OMEGA target chamber for integrated fast-ignitor experiments. Fuel-assembly experiments on OMEGA have achieved high fuel areal densities, and the effects of a cone on the fuel assembly are being studied. Experiments on short-pulse laser systems in collaboration with other institutions are being pursued to investigate the conversion efficiency from laser energy to fast electrons. A coherent transition radiation diagnostic to study the transport of the electrons in high-density material is being developed. Integrated experiments with room-temperature targets on OMEGA will be performed in 2008. Simulations of these integrated experiments show significant heating of up to 1 keV due to the hot electrons from the short-pulse laser.

Simulations of electron transport and ignition for direct-drive fast-ignition targets

The performance of high-gain, fast-ignition fusion targets is investigated using one-dimensional hydrodynamic simulations of implosion and two-dimensional (2D) hybrid fluid-particle simulations of hot-electron transport, ignition, and burn. The 2D/3D hybrid-particle-in-cell code LSP [D. R. Welch et al., Nucl. Instrum. Methods Phys. Res. A 464, 134 (2001)] and the 2D fluid code DRACO [P. B. Radha et al., Phys. Plasmas 12, 056307 (2005)] are integrated to simulate the hot-electron transport and heating for direct-drive fast-ignition targets. LSP simulates the transport of hot electrons from the place where they are generated to the dense fuel core where their energy is absorbed. DRACO includes the physics required to simulate compression, ignition, and burn of fast-ignition targets. The self-generated resistive magnetic field is found to collimate the hot-electron beam, increase the coupling efficiency of hot electrons with the target, and reduce the minimum energy required for ignition. Resistive filamentation of the hot-electron beam is also observed. The minimum energy required for ignition is found for hot electrons with realistic angular spread and Maxwellian energy-distribution function.

Ignition and burn characteristics of D-3He-fueled fast ignition targets

The possibility of igniting D3He plasma in the fast-ignition, inertial confinement fusion scheme is discussed. Use of a small amount of DT fuel as an igniter is indispensable in order to mitigate the requirement on driver energy. Simulations have been made for a DT/D3He fuel compressed to 4000 times the liquid density by incorporating a newly-developed neutron diffusion code into FIBMET, a 2D fusion ignition and burning code. The DT igniter is placed at the edge of the compressed fuel. The work shows that it is possible to obtain sufficient pellet gains (∼60) with realistic driver energy below 10 MJ. The essential role of DT fusion neutron is clarified. The possibility to reduce the amount of DT fuel is discussed.

Numerical study on optimization of cone target and ignition pulse shape for fast ignition

Electron energy characteristics generated by the irradiation of ultra-intense laser pulses onto solid targets are controlled by using cone targets. Two important parameters characterizing the laser-cone interaction are introduced, which are cone angle and the ratio of the laser spot size to cone tip. By changing these parameters, the electron energy characteristics are controlled. They are optimized for fast ignition with the aid of 2D Particle-in-Cell (PIC) simulations.

Stopping power and range of energetic electrons in dense plasmas of fast-ignition fusion targets

Current theoretical predictions of the stopping range of fast electrons in the dense cores of fast-ignition fusion targets differ by about a factor of 2. Inconsistencies in the previous derivations are discussed and correct values of the stopping power, scattering coefficients, and ranges of fast electrons are obtained. Such more-accurate results can be used in theoretical studies of the ignition conditions and particles-in-cell and Monte Carlo simulations of the collisional electron transport in the plasma of fast-ignition targets. Convenient approximate relations for the stopping range and angular moments of the hot-electron distribution function are also obtained.

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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.

Gain curves and hydrodynamic simulations of ignition and burn for direct-drive fast-ignition fusion targets

Hydrodynamic simulations of realistic high-gain fast-ignition targets are performed, including one-dimensional simulations of the implosion and two-dimensional simulations of ignition by a collimated electron beam and burn propagation. These simulations are used to generate gain curves for fast-ignition direct-drive inertial confinement fusion. The minimum energy required for ignition is computed for fast-electron beams with a monoenergetic or Maxwellian distribution, generated by a constant or Gaussian laser pulse. It is found that realistic fast-ignition targets can be ignited by monoenergetic collimated electron beams with a radius of 20μm, duration of 10ps, and energy of 15kJ. Simulations using ponderomotive temperature scaling for fast electrons and Gaussian laser pulses predict a minimum laser energy for ignition of 235kJ (105kJ) for the energy conversion efficiency from the laser to fast electrons 0.3 (0.5) and the wavelength of 1.054μm. Such large energies are required because ultra-intense lasers are predicted to generate very energetic (multi-MeV) electrons with stopping distance exceeding the target size. The fast-electron energy, the stopping distance and the minimum energy required for ignition can be reduced using frequency-doubled laser pulses. Simulations of idealized cone targets are also performed in order to determine a lower bound of the gain deterioration due to the cone.

Targets for direct-drive fast ignition at total laser energy of 200–400kJ

Basic issues for the design of moderate-gain fast ignition targets at total laser energy of 200–400kJ (with less than 100kJ for the igniting beams) are discussed by means of a simple integrated gain model. Gain curves are generated and their sensitivity to several parameters is analyzed. A family of scaled target is designed, based on 1D hydrodynamic simulations of the implosion stage and 2D model simulations of ignition and burn. It is found that ignition and propagating burn can be achieved by targets compressed by 100–150kJ, properly shaped laser pulses (with wavelength λc=0.35μm), and ignited by 80–100kJ pulses. This requires adiabat shaped implosions to limit Rayleigh-Taylor instability, at the same time keeping the fuel entropy at a very low level. In addition, the igniting beam should be coupled to the fuel with an efficiency of about 25%, and the hot-electron average penetration depth should be at most 1.2–1.5g∕cm2. According to the present understanding of ultraintense laser-matter interaction, this limits the wavelength of the ignition beam to λig≤0.5μm. With the same assumptions, energy gain G=100 can be achieved by targets driven by a 250kJ compression laser pulse and an 80–100kJ ignition pulse.

Fabrication of Targets for Proton Focus Cone Fast Ignition Experiments

The fast ignition concept is a proposed method to reach fusion by two separate processes. The task of the first process is the compression of fuel and the second is the ignition of the compressed fuel by a rapid and directed energy deposition. One delivery method of this energy can be in the form of focused proton beams and this type of fast ignition target will be discussed. The target designs consisted of gold and plastic cones with a curved proton-generating surface (aluminum) within the cone and very close to the tip. The challenges of the given target specifications led to a new cone design consisting of a cone base and cone tip made in two pieces with the proton generating surface sandwiched between. The fabrication of these targets consisted of several steps and processes that included making PAMS shell mandrels, sputter coating deposition, electroplating, precision machining, chemical etching, and target assembly.

The formation of high-density core plasma in non-spherical implosion using high-resolution two-dimensional integrated implosion code

Fast ignition is an epoch-making scheme for inertial fusion energy. In this scheme, the formation of high-density core plasma is one of the key issues which must be solved, because the implosion dynamics of the fast ignition target, which is non-spherical with a conical target, is quite different from that of the conventional central ignition target. Some previous works showed the possibility of formation of a high-density core. However, the effects of hydrodynamic instability were not discussed in those works. In this paper, we simulate the whole implosion dynamics of a perturbed non-spherical target, and the effect of radiation transport instability is estimated using the two-dimensional integrated implosion (radiation hydrodynamics) code PINOCO. In the result, we have found that the hydrodynamic instabily has less of an effect on the formation of high-density core plasma in the cone-guided implosion, in comparision with the spherical implosion.

Analysis of core plasma heating in fast ignition target by two-dimensional Fokker-Planck-Hydro code

To investigate the core plasma heating in fast ignition targets, we recently developed a two-dimensional relativistic Fokker-Planck code for fast electron transport in dense plasmas. In the present study, we have coupled RFP-2D with a 2D hydrodynamics code for bulk plasma and analyzed the core plasma heating by assuming a condition similar to that in the GXII-PW laser experiments at ILE. It was found that when the source beam electrons enter the dense core region with low (< 1 MeV) average energy and a Super-Gaussian spatial distribution, the core plasma can be heated above 0.8 keV at maximum, which is almost the same temperature as measured in the experiments.

Computational study of implosion physics and target design for the fast ignition experiment FIREX-I

The fast ignition is an epoch-making scheme for inertial fusion energy. In this scheme, the formation of high-density DT fuel is one of the key issues which must be solved, because the implosion dynamics of the fast ignition target which is non-spherical with conical target is quite different from that of the conventional central ignition target. Some previous works showed the possibility of formation of high density core. But the effects of hydrodynamic instability and DT cryogenic target were not discussed in those works. In this paper, we simulate the whole implosion dynamics of a perturbed non-spherical DT cryogenic target, and evaluate the possibility of formation of high-density DT fuel using 2-D integrated implosion code PINOCO.

Conceptual design of laser fusion reactor KOYO-fast – Concepts of reactor system and laser driver

We have carried out the design studies of KOYO-Fast laser fusion power plant, using fast ignition cone targets, DPSSL lasers, and LiPb liquid wall chambers. Using fast ignition targets, we could design a middle sized 300 MWe reactor module, with 200 MJ fusion pulse energy and 4 Hz rep-rates, and 1200MWe modular power plants with 4 reactor modules and a 16 Hz laser driver. The liquid wall chambers with free surface cascade flows are proposed for cooling surface quickly enough to a 4 Hz pulse operation. We examined the potential ofYb-YAG ceramic lasers operated at 150 ∼ 225 K for both implosion and heating laser systems required for a 16-Hz repetition and 8 % total efficiency.

Core heating analysis of fast ignition targets by integrated simulations

A coupled PIC and Fokker-Planck hierarchical simulation model for analysis of fast ignition core heating was checked by comparing the heating profile with collisional PIC simulations. The integrated simulations using FI 3 code were carried out for the recent fast heating experiments and showed that the dense core (ρ > 10 g/cc) is heated up to 0.45 keV by the fast electrons via the Coulomb interactions (not anomalous processes).

Micromachining of Fast Ignition Targets

Fast ignition is a concept that is being actively investigated in the HED community. The fast ignition targets described here are highly precise targets composed of a small glow discharge polymer (GDP) shell (∼860 μm diameter) mounted on a gold hyperboloid tipped cone. The process of creating these targets is composed of several steps. The first step consists of machining a copper cone that is then plated with a layer of gold approximately 120 μm thick. Next, a hole is machined in a hollow GDP shell that will later be mounted on the gold gone. After the hole of this shell has been measured, the coated cone is machined to shape and to include a shelf so that the shell will sit at the desired location in relation to the tip of the cone. Finally, the copper mandrel is etched away from the gold and the target is assembled with the shell glued into place. At every step of this process, parts must be made and kept within tight specifications to meet the target requirements, not the least of which is that after assembly the shell center must be a specified distance from the gold cone tip with a tolerance of less than 10 μm.

Energy deposition of MeV electrons in compressed targets of fast-ignition inertial confinement fusion

Energy deposition of MeV electrons in dense plasmas, important for fast ignition in inertial confinement fusion, is modeled analytically. It is shown that classical stopping and scattering dominate electron transport and energy deposition when the electrons reach the dense plasmas in the cores of compressed targets, while “anomalous” stopping associated with self-generated fields and micro-instabilities (suggested by previous simulations) might initially play an important role in the lower-density plasmas outside the dense core. For MeV electrons in precompressed deuterium-tritium fast-ignition targets, the initial penetration results in approximately uniform energy deposition but the latter stages of penetration involve mutual couplings of energy loss, straggling, and blooming that lead to enhanced, nonuniform energy deposition. This model can be used for quantitatively assessing ignition requirements for fast ignition.

Power Plant and Fusion Chamber Considerations for Fast Ignition

Using a simple inertial fusion energy (IFE) power plant economic model, it is demonstrated that there are several potential advantages of an IFE power plant based upon fast ignition targets compared with one based upon central ignition targets. The fast ignition version can have a lower cost of electricity (COE) at the same output power, and a smaller fast ignition plant can have the same COE as a larger central ignition plant. This paper also considers the chamber issues raised by using fast ignition targets. Some direct-drive chamber concepts must be larger for cone-focus fast ignition targets because of the increase in the X-ray output. On the other hand, the use of fast ignition hohlraum targets may allow the use of thick-liquid-wall chambers, bringing the benefits of a smaller chamber and containment building, smaller amounts of hazardous waste, and a faster and cheaper development path. However, many technology issues need resolution before these benefits can become a reality.

Fabrication, Injection, and Tracking of Fast Ignition Targets: Status and Future Prospects

The current status of the fabrication, injection, and tracking of fast ignition targets is summarized including on cryogenic technologies for direct-drive, laser fusion targets with and without a reentrant guide cone for additional heating lasers. The fabrication of low-density foam and the drilling of fragile foam shells are current issues in the fabrication of fast ignition targets with reentrant cones. Fuel loading to the targets is a challenging issue in the mass-production process of the targets for a fusion power plant. Two fuel-loading methods are proposed for the fast ignition target with the cone. Preliminary experiments of injection and tracking of real-size targets have started showing promising results.