Cone Target Research Articles

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.

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.

Reentrant cone angle dependence of the energetic electron slope temperature in high-intensity laser-plasma interactions

Energy spectra of fast electrons, generated when high-intensity laser pulses irradiated hollow conical targets, have been measured experimentally. It is shown here that the slope temperature of the fast electrons is strongly dependent on the opening angle of the cone, and has a maximum value at 25°. The data confirms optical guiding of the laser pulse, by comparison of the measured electron temperature with ray-tracing calculations that include absorption in plasmas. The enhanced energy flow and intensity induced by optical guiding of the laser pulse inside the cone as a function of the opening angle as well as the f-number of the focusing optics is discussed.

High Energy Electron Generation by Laser-Cone Interaction

Electron acceleration processes taking place in the interaction of ultra-intense laser pulses with cone targets are studied by using two-dimensional Particle-in-Cell (PIC) simulations to understand the characteristics of electrons generated from cone targets. It is explained that there are two dominant acceleration processes which are distinctive in the laser-cone interaction. One is the acceleration and transport along the side wall of the cone target, where electrons are guided along the side wall surface towards the cone tip by surface magnetic and electric fields. The second is the ponderomotive acceleration at the cone tip by the laser field which is intensified by cone focusing. The understanding of these acceleration processes helps to design cone targets to control the electron energy characteristics.

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.

In-situ Attenuation Corrections for Radiation Force Measurements of High Frequency Ultrasound With a Conical Target.

Radiation force balance (RFB) measurements of time-averaged, spatially-integrated ultrasound power transmitted into a reflectionless water load are based on measurements of the power received by the RFB target. When conical targets are used to intercept the output of collimated, circularly symmetric ultrasound sources operating at frequencies above a few megahertz, the correction for in-situ attenuation is significant, and differs significantly from predictions for idealized circumstances. Empirical attenuation correction factors for a 45° (half-angle) absorptive conical RFB target have been determined for 24 frequencies covering the 5 MHz to 30 MHz range. They agree well with previously unpublished attenuation calibration factors determined in 1994 for a similar target.

WE-E-330D-01: The Production of Ultrafast Bright K-Alpha X-Rays From Laser Produced Plasmas for Medical Imaging

Purpose: To show the potential of improving image quality with a cleaner, brighter, quasi‐monochromatic X‐ray micro‐source via laser produced plasmas(LPP).Method and Materials: First generation targets consisting of ten micron thick gold formed into free‐standing pyramids have been built. PIC (Particle‐In‐Cell) simulations have been performed in order to validate this target geometry. Preliminary experiments with a Ti‐Sapphire CPA laser have been achieved with these targets. Second generation parabolic cone targets with an optimal angle for electron transport have also been built. This new nano‐fabricated target could optimize X‐ray source characteristics. Results:PIC (Particle‐In‐Cell) simulations show that conical targets optically guide laser light resulting in a higher density of hot electrons at the apex These simulations show a possible ten times augmentation in hot electron density and a three times increase in electron temperature with a conical verses flat target. This increase in collimated suprathermal electrons boosts total photon yield as well as possibly enhancing line emission verses the bremsstrahlung continuum. Preliminary experiments demonstrate a three‐fold higher X‐ray yield and a two‐fold reduction in focal spot with the pyramidal verses the flat target. Furthermore, the geometry of the conical targets not only reduces focal spot size to a few microns and pulse duration to a couple picoseconds, but allows the particles to escape the target perpendicular to the surface resulting in a particle‐free, ultra‐short X‐ray micro‐beam. Conclusion: Comparing LPP X‐ray source parameters to that of a standard X‐ray tube shows substantial improvements in focal spot size, photon flux, spectral range and emission duration. Focusing on target design can provide a cleaner, brighter, quasi‐monochromatic X‐ray source that could improve image quality in any medicaldiagnostic regime. Such advancements show promising applications in mammography and angiography. Conflict of Interest: Research sponsored by DOE/NNSA under University of Nevada Reno grant #DE‐FC52‐01NV14050.

Hot electron generation by laser-cone interaction

The hot electron generation by irradiation of intense short laser pulses onto cone targets is studied by using 2D PIC simulation. It is shown that the high energy electrons from the cone target are characterized with three effective temprature, while two-temperature for plane target. The electrons composing most energetic component are generated at the cone tip where laser field is intensified by cone-guiding, and electrons for middle component are generated at cone wing which is not observed for plane target. The acceleration processes at cone wing, cone tip, and rear surface are explained.

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.

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.

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.

Calculation of shock compression of porous media in conical solid-state targets with an outlet hole

The Godunov-type method that was earlier developed for the hydrodynamic equations of compressible media on regular grids is extended to the case of shock wave flows in elastoplastic porous media. The results of numerical simulations of a hypothetical experimental plant for checking the effect of an increased intensity of converging shock waves in porous media with increasing degree of porosity are presented. This effect was earlier discovered by numerical simulations.

Power Plant Concepts and Chamber Issues for Fast Ignition Direct-Drive Targets

We have analyzed the design windows for laser fusion power plants based on direct-drive fast ignition concepts and have examined the issues of chamber technologies and the feasibility of a small laser fusion experimental reactor suitable for developing their power plants. Target gain curves are assessed for power plants having 90- to 200-MJ fusion yields with 600-kJ to 1-MJ lasers, and for an experimental reactor [the laser fusion experimental reactor (LFER)], having a 10-MJ fusion yield with a 200-kJ laser, i.e., 100 kJ for implosion and 100 kJ for heating. The fast ignition LFER can produce its fusion output approximately one order of magnitude smaller than that of the central ignition design, so that we can use a rather small solid-wall chamber for the first stage of the LFER operation. We can also expect to decrease laser cost drastically, although for the heating laser we must develop a long-life final optics system. Using fast ignition direct-drive targets, we could design a smaller ~300-MW(electric) reactor, with 200-MJ fusion pulse energy and 4-Hz repetition rates. The smaller pulse energies mitigate pulse loads on the chamber walls and the final optics; then, we can flexibly design large 1200-MW(electric) modular plants by using multiple reactor modules. We identified the issues of liquid-wall and solid-wall chambers and proposed basic reactor concepts for a power plant (KOYO-Fast) and an experimental reactor using fast ignition direct-drive cone targets.

Generation and transport of fast electrons inside cone targets irradiated by intense laser pulses

Fast electrons are effectively generated from solid targets of cone-geometry by irradiating intense laser pulses, which is applied to fast ignition scheme. For realizing optimal core heating by those electrons, understanding the characteristics of electrons emitted from cone targets is crucial. In this paper, in order to understand the generation and transport processes of hot electrons inside the cone target, two-dimensional (2D) particle-in-cell (PIC) simulations were carried out. It is shown that hot electrons form current layers which are guided by self-generated surface magnetic field, which results in effective energy transfer from laser pulse to hot electrons. When the hot electrons propagate through the steep density gradient at the cone tip, electrostatic field is induced via Weibel instability. As a result, hot electrons are confined inside and emitted gradually from the target, as an electron beam of long duration. Energy spectrum and temporal profile of hot electrons are also evaluated at the rear side of the target, where the profile of rear side plasma is taken from the fluid code and the result is sent to Fokker-Planck code.

Heating Mechanisms in Short-Pulse Laser-Driven Cone Targets

The fast ignitor is a modern approach to laser fusion that uses a short-pulse laser to initiate thermonuclear burn. In its simplest form the laser launches relativistic electrons that carry its energy to a precompressed fusion target. Cones have been used to give the light access to the dense target core through the low-density ablative cloud surrounding it. Here the ANTHEM implicit hybrid simulation model shows that the peak ion temperatures measured in recent cone target experiments arose chiefly from return current joule heating, mildly supplemented by relativistic electron drag. Magnetic fields augment this heating only slightly, but capture hot electrons near the cone surface and force the hot electron stream into filaments.

Influence of under-dense plasma on laser conical target interaction

The laser light propagation inside the conical target has been studied by three-dimensional particle-in-cell simulations. It is found that the existence of under-dense plasma significantly affects the light propagation and the generation of relativistic electrons. The laser light can be focused at the tip of the cone and the intensity increases up to tens of times in the focal spot. With the under-dense plasma the laser light focused intensity is decreased. The existence of under-dense plasmas favors the relativistic electron generation in the laser conical target interaction in enhancing the number and energy of relativistic electrons.

反応性電子サイクロトロン共鳴（ＥＣＲ）スパッタ法によるフェライト薄膜の低温高速作製

A reactive sputtering apparatus utilizing electron cyclotron resonance (ECR) was developed for the deposition of thick oxide or nitride films usable in electric devices. A high deposition rate of 44 nm/min. was achieved using a conic alloy target and the deposited soft ferrite thin films were successfully crystallized without heat treatment during film deposition. Magnetic and physical properties for the film were analyzed in terms of saturation magnetization, coercivity, uniformity of film thickness, and inner stress. Obtained properties were consistent with soft magnetic films. The result confirms the oxygen partial pressure ratio to deposition rate strongly affected the magnetic properties. Well crystallized, ultra thin 3-nm-thick Ni-Zn ferrite (100) films, were also successfully deposited on MgO (100) underlayers. These results imply the ECR sputtering method is one of the most effective deposition methods for highly crystallized polycrystalline thick and thin films. Crystal orientation was improved by low target voltage sputtering as well as a relatively low deposition rate. Further improvement is considered to be possible by deposition at lower base pressures.

量産対応型ＥＣＲスパッタ装置によるフェライト薄膜の低温高速製造技術の開発

Reactive sputtering apparatus utilizing electron cyclotron resonance (ECR) microwave plasma was developed for deposition of ferrite thin-films. High deposition rate of 44 nm/min. was achieved using a conic alloy target, and the deposited soft ferrite thin films successfully crystallized without heat treatment during film deposition and post annealing. Magnetic and physical properties for the films were analyzed in terms of saturation magnetization, coercivity, uniformity of film thickness, and inner stress. These properties were proved to be good enough as soft magnetic films. The result described that the oxygen partial pressure ratio to deposition rate strongly affected the magnetic properties. Well crystallized ultra thin (3-nm-thick) ferrite films were also successfully deposited on MgO(100) underlayer. These results imply that ECR sputtering method is one of the most effective deposition methods for highly crystallized polycrystalline thick and thin films. Low target voltage sputtering as low as −100 V successfully improved crystal orientation of the ferrite films by suppressing the affect of recoiled argon ion bombardment to the substrate.

Innovative ignition scheme for ICF—impact fast ignition

A new ignition scheme is proposed, in which the compressed DT main fuel is ignited by impact collision of another fraction of separately imploded DT fuel, which is accelerated in the hollow conical target to super high velocities of about (1–2) × 108 cm s−1. Its kinetic energy is directly converted into thermal energy corresponding to temperatures >5 keV on the collision with the main fuel, and this self-heated portion plays the role of ignitor. The ignitor shell is irradiated typically by nanosecond pulses at intensities well beyond 1015 W cm−2 at such a short laser wavelength as 0.25 µm to exert ablation pressures of 150–300 Mbar. A preliminary two-dimensional hydrodynamic simulation demonstrates substantial heating of 3–5 keV on the impact. Simple physics, potential for high gain designs and low cost—these are the crucial advantages of the present scheme.

Towards realization of hyper-velocities for impact fast ignition

A new ignition scheme, impact fast ignition (IFI), is studied, in which the compressed DT main fuel is to be ignited by impact with another fraction of separately imploded DT fuel, which is accelerated in the hollow conical target. The first and distinct milestone in the IFI scenario is the demonstration of such a hyper-velocity, of the order of 108 cm s−1. Two-dimensional hydrodynamic simulation results obtained in full geometry using plastic instead of DT fuel are presented, in which some key physical parameters for the impact shell dynamics, such as an implosion velocity of 108 cm s−1, a compressed density of 300–400 g cm−3 and a converted temperature greater than 5 keV, are demonstrated. A preliminary experimental result with a planar target is presented to show the highest velocity, 6 × 107 cm s−1, ever achieved.