Fast Ignition Experiments Research Articles

Controlled reproducible alignment of cone targets and mitigation of preplasma in high intensity laser interactions

The use of cone targets in high intensity laser-plasma experiments has been of recent interest because of their potential use in integrated fast ignition experiments. Simpler experiments provide a good avenue for understanding the underlying physics, however precise control of the alignment along with good pointing accuracy is of crucial importance. While on big laser facilities target alignment is done precisely with several microscopes, it is not always the case on smaller facilities. This can have a detrimental effect on the quality of the results. We have developed and characterized a method for accurate alignment of intense laser pulses into a cone target. This, along with optimal positioning of the focus compared to the tip, efficiently uses the shape of the target to microfocus the laser light and concentrates the hot electrons in the tip, and can mitigate preplasma issues.

Development of TOF neutron spectrometer for the measurement of degenerated plasma in fast ignition experiment

There are two essential requirements to achieve the Fast Ignition. One is high density compression of main fuel and the other one is additional heating of main fuel by ultra intense laser. Then the experimental determination of both the density of the main fuel before heating laser irradiation and ion temperature after heating laser irradiation is necessary. In the experiment of the Central Ignition, we developed both ion temperature measurement and density measurement, which is based on theoretical degenerated plasma model, by using Time-Of-Flight neutron spectrometer. The main problem in applying this measurement in Fast Ignition experiment is huge amount of high energy x ray, which is generated as a result of heating laser irradiation. Then we installed a total 15-cm thick lead shield as a measure against it. It was confirmed that the newly installed shield achieves x ray attenuation to 5.7±2.0×10−4 of previous level by Geant4 MC simulation. And the error for ion temperature measurement was confirmed to be smaller than 0.3 keV in an experiment whose neutron yield was 1.7±0.2×106.

Application of bubble detector in FIREX program

In a Fast Ignition Realization Experiment (FIREX), measurement of the primary and the secondary neutron yield is one of the key diagnostics. However, gamma-ray background generated by an ultra-short pulse laser irradiation makes it difficult to measure the neutron yield in the fast ignition experiment. We adopted a bubble neutron detector as a promising gamma-insensitive detector to be developed and started to examine the feasibility of the detector using commercially available dosimeters provided from BTI inc. [1].

Non-dimensional scaling of impact fast ignition experiments

Recent experiments at the Osaka University Institute for Laser Engineering (ILE) showed that ‘Impact Fast Ignition’ (IFI) could increase the neutron yield of inertial fusion targets by two orders of magnitude [1]. IFI utilizes the thermal and kinetic energy of a laser-accelerated disk to impact an imploded fusion target. ILE researchers estimate a disk velocity of 108 cm/sec is needed to ignite the fusion target [2]. To be able to study the IFI concept using lasers different from that at ILE, appropriate non-dimensionalization of the flow should be done. Analysis of the rocket equation gives parameters needed for producing similar IFI results with different lasers. This analysis shows that a variety of laboratory-scale commercial lasers could produce results useful to full-scale ILE experiments.

Fast Heating of Cylindrically Imploded Plasmas by Petawatt Laser Light

We produced cylindrically imploded plasmas, which have the same density-radius product of the imploded plasma rhoR with the compressed core in the fast ignition experiment and demonstrated efficient fast heating of cylindrically imploded plasmas with an ultraintense laser light. The coupling efficiency from the laser to the imploded column was 14%-21%, implying strong collimation of energetic electrons over a distance of 300 microm of the plasma. Particle-in-cell simulation shows confinement of the energetic electrons by self-generated magnetic and electrostatic fields excited along the imploded plasmas, and the efficient fast heating in the compressed region.

Fast ignition relevant study of the flux of high intensity laser-generated electrons via a hollow cone into a laser-imploded plasma

An integrated experiment relevant to fast ignition . A Cu-doped deuterated polymer spherical shell target with an inserted hollow Au cone is imploded by a six-beam 900-J, 1-ns laser. A 10-ps, 70-J laser pulse is focused into the cone at the time of peak compression. The flux of high-energy electrons through the imploded material is determined from the yield of CuKα fluorescence by comparison with a Monte Carlo model. The electrons are estimated to carry about 15% of the laser energy. Collisional and Ohmic heating are modeled, and Ohmic effects are shown to be relatively unimportant. An electron spectrometer shows significantly greater reduction of the transmitted electron flux than is calculated in the model. Enhanced scattering by instability-induced magnetic fields is suggested. An extension of this fluor-based technique to measurement of coupling efficiency to the ignition hot spot in future larger-scale fast ignition experiments is outlined.

Hydrodynamics studies of direct-drive cone-in-shell, fast-ignitor targets on OMEGA

Experiments have been performed on the OMEGA Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] to study the hydrodynamics of directly driven cone-in-shell, fast-ignitor targets. A 35° or 70° opening-angle gold cone was inserted into spherical plastic shells of ∼24-μm thickness and ∼870-μm diameter, which were imploded with up to 21kJ of 351-nm laser light. A backlighter was used on some experiments to compare the fuel assembly of targets with or without a high-pressure fill gas. The shock breakthrough to the inside of the cone, where the ultrafast laser propagates in integrated fast-ignitor experiments, was studied using a streaked optical pyrometer. No plasma was seen inside the cone before the assembled core reached peak compression.

Recent results and future prospects of laser fusion research at ILE, Osaka

Reviewed are the present status of the fast ignition researches. Since 1997, the fast ignition experiment and theory researches have been extensively continued at the Institute of Laser Engineering of Osaka University. In particular, the cone-shell target experiments and simulation research have been progressing. In order to demonstrate heating of imploded high density plasma to the ignition temperature, in the April of 2003, the construction of heating laser of 10 kJ/10 ps/1.06 μm (Laser for Fusion Experiment; LFEX), for FIREX-I (Fast Ignition Realization Experiment) has started. The fabrication of DT foam cryogenic cone target is also under development as a collaboration program between Osaka University and NIFS (National Institute for Fusion Science). The LFEX will be completed in 2008. After the completion of LFEX, the foam cryogenic cone shell target experiment will start in 2008. As a new approach toward a compact ignition, an impact fusion has been proposed, where the ablative acceleration to the order of 108 cm/s is the key issue. The ablation acceleration related to the impact fusion has been explored by experiments.

Laser Machining of RF Foam by Second Harmonics of Nd:YAG Laser

A way to make a hole in a resorcinol-formaldehyde (RF) foam shell target is described. A target used in fast ignition (FI) experiments consists of an RF foam shell and a guide cone. The fragile RF foam cannot accept conventional mechanical drilling or laser machining. We used the second harmonics of a Nd:YAG laser (532nm) to bore a hole for the cone by utilizing thermal decomposition of the RF foam.To optimize the cutting condition, the thermal characteristic and absorption spectrum of the RF foam were measured with a thermo-gravimetric meter and a spectrometer, respectively. Then, using these results, relationships between the laser power and cutting speed were optimized for RF foams with several densities.

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.

High-intensity laser interactions with mass-limited solid targets and implications for fast-ignition experiments on OMEGA EP

The modeling of petawatt laser-generated hot electrons in mass-limited solid-foil-target interactions at “relativistic” laser intensities is presented using copper targets and parameters motivated by recent experiments at the Rutherford Appleton Laboratory Petawatt and 100-TW facilities [Theobald et al., Phys. Plasmas 13, 043102 (2006)]. Electron refluxing allows a unique determination of the laser-electron conversion efficiency and a test with simulations. Good agreement between experiments and simulations is found for conversion efficiencies of 10%.

Surface Acceleration of Fast Electrons with Relativistic Self-Focusing in Preformed Plasma

We report an observation of surface acceleration of fast electrons in intense laser-plasma interactions. When a preformed plasma is presented in front of a solid target with a higher laser intensity, the emission direction of fast electrons is changed to the target surface direction from the laser and specular directions. This feature could be caused by the formation of a strong static magnetic field along the target surface which traps and holds fast electrons on the surface. In our experiment, the increase in the laser intensity due to relativistic self-focusing in plasma plays an important role for the formation. The strength of the magnetic field is calculated from the bent angle of the electrons, resulting in tens of percent of laser magnetic field, which agrees well with a two-dimensional particle-in-cell calculation. The strong surface current explains the high conversion efficiency on the cone-guided fast ignitor experiments.

Recent results and future prospects of laser fusion research at ILE, Osaka

We review the present status of fast ignition research. Since 1997, the fast ignition experiment and theory have been extensively continued at the Institute of Laser Engineering of Osaka University. In particular, the cone-shell target experiments and simulation research have made progress. In order to demonstrate heating of imploded high density plasma to the ignition temperature, in the April of 2003, the construction of heating laser of 10 kJ/10ps/1.06 m (Laser for Fusion Experiment; LFEX), for FIREX-I (Fast Ignition Realization Experiment) has started. The fabrication of DT foam cryogenic cone target is also under development as a collaboration program between Osaka University and NIFS (National Institute for Fusion Science). The LFEX will be completed in 2008. After the completion of LFEX, the foam cryogenic cones-shell target experiment will start in 2008. As a new approach toward compact ignition, an impact fusion scheme has been proposed, where the ablative acceleration to the order of 10 8 cm/sec is the key issue. The ablation acceleration related to impact fusion has been explored by experiments.

Simulation of heating-compressed fast-ignition cores by petawatt laser-generated electrons

In this work, unique particle-in-cell simulations to understand the relativistic electron beam thermalization and subsequent heating of highly compressed plasmas are reported. The simulations yield heated core parameters in good agreement with the GEKKO-PW experimental measurements, given reasonable assumptions of laser-to-electron coupling efficiency and the distribution function of laser-produced electrons. The classical range of the hot electrons exceeds the mass density-core diameter product pL by a factor of several. Anomalous stopping appears to be present and is created by the growth and saturation of an electromagnetic filamentation mode that generates a strong back-EMF impeding hot electrons on the injection side of the density maxima.This methodology is then applied to the design of experiments for the ZR machine coupled to the Z-Beamlet/PW laser. Sandia National Laboratories is also developing a combination of experimental and theoretical capabilities useful for the study of pulsed-power-driven fast ignition physics. In preparation for these fast ignition experiments, the theory group at Sandia is modeling various aspects of fast ignition physics. Numerical simulations of laser/plasma interaction, electron transport, and ion generation are being performed using the LSP code. LASNEX simulations of the compression of deuterium/tritium fuel in various reentrant cone geometries are being performed. Analytic and numerical modeling has been performed to determine the conditions required for fast ignition breakeven scaling. These results indicate that to achieve fusion energy output equal to the deposited energy in the core will require about 5% of the laser energy needed for ignition and might be an achievable goal with an upgraded Z-beamlet laser in short pulse mode.

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.

Integrated simulations for fast ignition with cone-guided targets

Including a density jump between a cone tip and imploded plasmas, we have performed preliminary FI 3 simulations for fast ignition with cone-guided targets, and found that the density gap with 2n was effective at heating the core. But this effect is not successful under the realistic parameters for recent fast ignition experiments. Even so, it was found that the core could be heated up to 0.44 keV, but we could not get core temperature observed in the experiments, 0.8 keV. From those results, it should be emphasizes that integrated simulations under realistic conditions are essential for the fast ignition research.

Full scale explicit PIC simulation of fast ignition experiment

A relativistic binary collision model is implemented in the particle-in-cell simulation code PICLS. Its applicability ranges from the non-relativistic to the ultra-relativistic regime. Using this model we have simulated a recent experiment on cone-guided fast-ignition experiment [Kodama et al., Nature 418, 933 (2002)] and found that there are three important heating process by fast electrons, fast ions and heat conduction from the cone.

Progress in direct-drive inertial confinement fusion research at the laboratory for laser energetics

Direct-drive inertial confinement fusion (ICF) is expected to demonstrate high gain on the National Ignition Facility (NIF) in the next decade and is a leading candidate for inertial fusion energy production. The demonstration of high areal densities in hydrodynamically scaled cryogenic DT or D2 implosions with neutron yields that are a significant fraction of the “clean” 1-D predictions will validate the ignition-equivalent direct-drive target performance on the OMEGA laser at the Laboratory for Laser Energetics (LLE). This paper highlights the recent experimental and theoretical progress leading toward achieving this validation in the next few years. The NIF will initially be configured for X-ray drive and with no beams placed at the target equator to provide a symmetric irradiation of a direct-drive capsule. LLE is developing the “polar-direct-drive” (PDD) approach that repoints beams toward the target equator. Initial 2-D simulations have shown ignition. A unique “Saturn-like” plastic ring around the equator refracts the laser light incident near the equator toward the target, improving the drive uniformity. LLE is currently constructing the multibeam, 2.6-kJ/beam, petawatt laser system OMEGA EP. Integrated fast-ignition experiments, combining the OMEGA EP and OMEGA Laser Systems, will begin in FY08.

Progress in direct-drive inertial confinement fusion research at the laboratory for laser energetics

Direct-drive inertial confinement fusion (ICF) is expected to demonstrate high gain on the National Ignition Facility (NIF) in the next decade and is a leading candidate for inertial fusion energy production. The demonstration of high areal densities in hydrodynamically scaled cryogenic DT or D2 implosions with neutron yields that are a significant fraction of the “clean” 1-D predictions will validate the ignition-equivalent direct-drive target performance on the OMEGA laser at the Laboratory for Laser Energetics (LLE). This paper highlights the recent experimental and theoretical progress leading toward achieving this validation in the next few years. The NIF will initially be configured for X-ray drive and with no beams placed at the target equator to provide a symmetric irradiation of a direct-drive capsule. LLE is developing the “polar-direct-drive” (PDD) approach that repoints beams toward the target equator. Initial 2-D simulations have shown ignition. A unique “Saturn-like” plastic ring around the equator refracts the laser light incident near the equator toward the target, improving the drive uniformity. LLE is currently constructing the multibeam, 2.6-kJ/beam, petawatt laser system OMEGA EP. Integrated fast-ignition experiments, combining the OMEGA EP and OMEGA Laser Systems, will begin in FY08.

High-Energy Petawatt Project at the University of Rochester's Laboratory for Laser Energetics

A high-energy petawatt laser, OMEGA EP, is currently under construction at the University of Rochester's Laboratory for Laser Energetics. Integrated into the existing OMEGA laser, it will support three major areas of research: (a) backlighting of high-energy-density plasmas, (b) integrated fast ignition experiments, and (c) high-intensity physics. The laser will provide two beams combined collinearly and coaxially with short pulses (~1 to 100 ps) and high energy (2.6 kJ at 10 ps). Cone-in-shell fuel-assembly experiments and simulations of short-pulse heated cryogenic targets are being performed in preparation for cryogenic integrated fast ignitor experiments on OMEGA EP.