Fast Ignition Concept Research Articles

High performance compact magnetic spectrometers for energetic ion and electron measurement in ultraintense short pulse laser solid interactions

An ultraintense short pulse lasers incident on solid targets can generate relativistic electrons that then accelerate energetic protons and ions. These fast electrons and ions can effectively heat the solid target, beyond the region of direct laser interaction, and are important to realizing the fast ignition concept. To study these energetic ions and electrons produced from the laser-target interactions, we have developed a range of spectrometers that can cover a large energy range (from less than 0.1 MeV to above 100 MeV). They are physically compact, high performance, and low cost. We will present the basic design of these spectrometers and the test results from recent laser experiments.

Cumulative phenomena due to the collision of laser-driven projectiles related to fast ignition and astrophysical applications

The results of contemporary investigations of cumulative phenomena due to the collision of projectiles laser-accelerated up to the velocities significantly higher than the sound velocity in solids are reviewed. The discussion is focused on the applications directed on the development of a concept of fast ignition by projectile impact and the laboratory modeling of astrophysical phenomena. Low-entropy acceleration of a projectile in a dense state up to ‘thermonuclear’ velocity of the order of 1000 km/s is grounded. The parameters of extreme matter state induced by laser-accelerated projectiles collision, namely, energy flows, pressure and temperature, spontaneous magnetic field and thermonuclear neutron yield are discussed. Astrophysical phenomena due to collision of cosmic objects such as a creation of a crater by meteorite's impact, as well as the emerging of a shock wave to a star surface, and formation of jets, ionization and emission waves are considered.

High-current fast electron beam propagation in a gas

Propagation of high-current relativistic electron beam in a gas has been studied experimentally in [1] in connection with the fast ignition concept of the inertial confinement fusion. In this paper, our numerical code based on the Particle-in-Cell method is utilized to simulate the electron beam propagation under the conditions similar to those of the experiment [1]. Our results demonstrate generation of a strong electrostatic field at the surface of the target from which the beam enters into the gas. The strength of this field depends in particular on the density of the gas and the field may reflect a significant part of the beam back into the target and decelerate the propagating beam. Ionization of the gas is provided by the electric field created by a dense bunch of runaway electrons and the ionization front may propagate with the velocity close to the velocity of light until the runaway bunch is depleted.

Application of High Energy Protons in Inertial Confinement Fusion by Using the Fast Ignitor Concept for DT Fusion

A short-laser-pulse driven ion flux is examined as a fast ignitor candidate for inertial confinement fusion. The main mechanism for ion acceleration is charge separation in a plasma due to high-energy electrons driven by the laser inside the target. Another very new branch of fast ignition research is the investigation of the use of laser generated proton beams. In the present paper aims to provide insights into the feasibility of the fast ignition concept with high energy beams of protons generated in laser–plasma interactions. The optimum parameters of an ion beam and laser pulse that are suitable for an ignition spark in a hot precompressed DT fuel are estimated as a rough guide. Also, in this paper we estimate the radius of Deuterium–Tritium (DT) fuel pellet that is equal to the protons range in DT plasma.

GARPUN-MTW: A hybrid Ti:Sapphire/KrF laser facility for simultaneous amplification of subpicosecond/nanosecond pulses relevant to fast-ignition ICF concept

The first stage of the petawatt excimer laser project started at the P.N. Lebedev Physical Institute, implements a development of multiterawatt hybrid GARPUN-MTW laser facility for generation of ultra-high intensity subpicosecond ultraviolet (UV) laser pulses. Under this project, a multi-stage e-beam-pumped 100-J, 100-ns GARPUN KrF laser was upgraded with a femtosecond Ti:Sapphire front-end, to produce combined subpicosecond/nanosecond laser pulses with variable time delay. Attractive possibility to amplify simultaneously short and long pulses in the same large-scale KrF amplifiers is analyzed with regard to the fast-ignition, inertial confinement fusion problem. Detailed description of hybrid laser system is presented with synchronized KrF and Ti:Sapphire master oscillators. Based on gain and absorption measurements at GARPUN amplifier and numerical simulations with a quasi-stationary code, we are predicting that 1.6 J can be obtained in a short pulse at hybrid GARPUN-MTW Ti:Sapphire/KrF laser facility, combined with several tens of joules in nanosecond pulse. Amplified spontaneous emission, which is responsible for the pre-pulse formation on a target, was also investigated: its acceptable level can be provided by properly choosing staged gain or loading the amplifiers by quasi-steady laser radiation. Fluorescence and transient absorption spectra of Ar/Kr/F2 mixtures conventionally used in KrF amplifiers were recorded to find out the possibility for femtosecond pulse amplification at the broadband Kr2F (42Γ → 1,2 2Γ) transition, which benefits in 100 times higher saturation energy density than for KrF (B → X) transition.

Review of Japanese fusion program and role of inertial fusion

The high compression of 600 times liquid density and the recent fast heating of a compressed core to 1-keV temperature have provided proof-of-principle of the fast ignition concept, and these results have significantly contributed to approve first phase of the Fast Ignition Realization EXperiment (FIREX) project. The goal of FIREX-I is to demonstrate fast heating of a fusion fuel up to the ignition temperature of 5–10 keV. Although the fuel size of FIREX-I is too small to ignite, sufficient heating will provide the scientific viability of ignition-and-burn by increasing the laser energy thereby the fuel size. Based on the result of FIREX-I, the decision of the start of FIREX-II to achieve ignition-and-burn can be made. The FIREX program is under the collaboration of the Institute of Laser Engineering and the National Institute for Fusion Science.

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.

Extension of atomic configuration sets of the Non-LTE model in the application to the Kα diagnostics of hot dense matter

High intensity short-pulse laser experiments are used for various applications from fast ignitor concept to proton beam generation. These systems produce substantial fluxes of energetic electrons, and the production of Kα spectrum by these energetic electrons has become a key diagnostics of interaction of short-pulse laser and solid-density matter. In general, spectral modeling is required to derive the thermal electron temperature from measured Kα spectrum and it is essential to use a Non-LTE (local thermodynamic equilibrium) model for this purpose. In this paper, we investigate the assumptions necessary for the application of the Non-LTE models and show that an extensive set of configurations is required to provide a valid Kα diagnosis of hot dense matter.

Runaway electron production during intense electron beam penetration in dense plasma

Relativistic electrons are efficiently generated when multiterawatt lasers focused to ultrahigh intensities ≳1019W∕cm2 illuminate the surface of dense plasma targets. A theoretical study finds that during typical picosecond pulse widths, significant amounts of Dreicer produced runaway electrons can build up due to the high axial electric field driving the neutralizing return current. An important consequence is that there will be a conversion of plasma current to runaway electron current, which is maximized at some optimum value of the beam-to-plasma density ratio Nb=nb∕ne, depending on the plasma collisionality. At collisionalities representative of solid target experiments, complete conversion to runaway electrons can only take place over a certain range of Nb values. At higher collisionalities and pulse widths, applicable to the fast ignition concept for inertial confinement fusion, it was found that conversion to runaways has a peak at ∼90% around Nb∼0.06. Significant lessening of target material heating by Joule current dissipation is also possible, since part of the beam energy loss is transferred through the electric field directly to the formation of energetic runaways. Implications for beam transport inhibition by the electric field are also discussed.

Role of current shear driven turbulence in anomalous stopping of energetic electrons

The transport and stopping of energetic electrons in a dense plasma medium is a crucial issue in the fast ignition (FI) concept of laser fusion. In this Letter nonlinear three-dimensional simulations of current shear driven electron-magnetohydrodynamics (EMHD) fluid instabilities have been presented. It is shown that these electromagnetic instabilities have a strong influence on the transport and stopping of inward directed energetic electrons. The implications of these results to the fast ignition concept of laser fusion are also discussed.

Present status of the FIREX programme for the demonstration of ignition and burn

The recent fast heating of a compressed core to 0.8–1 keV temperature as well as the previous high-density compression of 600 times liquid density have provided proof-of-principle of the fast ignition (FI) concept. These results have significantly contributed to official approval of the first phase of the FI realization experiment (FIREX) project. The goal of FIREX-I is to demonstrate fast heating of a fusion fuel up to the ignition temperature of 5–10 keV. Although the fuel of FIREX-I is too small to be actually ignited, sufficient heating will provide the scientific viability of ignition and burn by increasing the laser energy, thereby increasing the fuel size. The FIREX programme is being carried out in collaboration with the Institute of Laser Engineering, Osaka University and the National Institute for Fusion Science, including the development of cryogenic targets, holistic simulation systems and diagnostic equipment.

Development of a system code for an IFE reactor and investigation of a design regime for a dry wall chamber concept

A system code for designing laser fusion reactors has been developed, and the design possibility of the laser fusion commercial plant with a single dry wall chamber and a high repetition laser has been examined. In a fast ignition concept, it is confirmed that the target gain G ≥ 100 is achievable with the fusion pulse energy of 40 MJ. If we assume the laser repetition rate of 30 Hz and the pulse heat load limit of 2 J/cm 2, the plant which has moderate electric power ( ∼ 400 MW e) can be designed with a dry wall chamber of which radius R = 5.6 m. Although there are several technological issues needed to be solved, this design point can be characterized as a first-step reactor of the laser fusion study because of its relatively low construction cost.

Solid state laser design for inertial confinement fusion: Trends toward power production

Diode-pumped solid state lasers have the potential efficiency and repetition rate required for power production. One current candidate for laser materials is an Ytterbium doped ceramic. Yb doped sesquioxides are promising laser materials opening new fields during the research phase to demonstrate ignition and fusion gain (including the fast ignitor concept). We think that cryogenically cooled diode pumped Yb: ceramic laser is the best way to operating at high average power an that all of the operational features of the laser can be demonstrated at an aperture scale of only 10-15 cm.

Review of Japanese fusion program and role of inertial fusion

The high compression of 600 times liquid density and the recent fast heating of a compressed core to 1-kev temperature have provided proof-of-principle of the fast ignition concept, and these results have significantly contributed to approve first phase of the Fast Ignition Realization EXperiment (FIREX) project. The goal of FIREX-I is to demonstrate fast heating of a fusion fuel up to the ignition temperature of 5-10 keV. Although the fuel size of FIREX-1 is too small to ignite, sufficient heating will provide the scientific viability of ignition-and-burn by increasing the laser energy thereby the fuel size. Based on the result of FIREX-I, the decision of the start of FIREX-II to achieve ignition-and-burn can be made. The FIREX program is under the collaboration of the Institute of Laser Engineering and the National Institute for Fusion Science.

Proton Fast Ignition

Fast ignition by a laser-generated, ballistically focused proton plasma jet is a more recently proposed alternative to the original concept of fast ignition by a laser-generated beam of relativistic electrons. It has potential advantages in less-complex energy transport into dense plasma but has been investigated only at a preliminary level. Recent successful target heating experiments motivate further investigation of its feasibility. The concept and requirements, the characteristics of the proton plasma jets, the recent experimental work on focusing the jets and heating solid targets, and the overall physics constraints and unresolved questions are discussed.

Proof-of-Principle Experiments for Fast Ignition and the Fast Ignition Realization Experiment

This paper introduces the next generation of fast ignition research facilities now under construction and describes in detail the Japanese project Fast Ignition Realization Experiment (FIREX-I) and its proposed follow-up, FIREX-II. Both the facilities and their scientific objectives are presented. FIREX-I and the other two facilities described in subsequent papers - OMEGA EP at the University of Rochester and the Z-Petawatt at Sandia National Laboratories - will conduct proof-of-principle experiments for the fast ignitor concept. The facilities consist of two components: a long-pulse (τ > ns) driver capable of compressing and assembling the fusion fuel and a separate petawatt-class laser for heating. For the FIREX project, the present status of the construction of the 10-kJ-level, high-energy petawatt Laser for Fusion Experiment is reported, and the theoretical basis for high-density plasma heating with an ~10-kJ, 10-ps petawatt laser is discussed to show how this heating pulse is predicted to achieve the plasma parameters required for the fast ignition. The required petawatt spot size, the tolerable carbon fraction in the proposed D-T-loaded foam cryogenic target, appropriate heating laser pulse shape, and the required electron stopping range are explored. The theoretical analysis includes the use of Fokker-Planck simulation to describe the heating of the dense plasma by relativistic electrons created in the petawatt laser-plasma interactions. This modeling indicates that if 30% of the 10-kJ petawatt laser energy is coupled by relativistic electrons into D-T plasmas compressed to 100 to 200 g/cm3, the plasmas will be subsequently heated to 5 keV and fusion gains, defined as fusion energy produced divided by the total incident (compression and heating) laser energy, as high as 0.1 can result.

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.

Hydrodynamics of Conically Guided Fast Ignition Targets

The fast ignition concept requires the generation of a compact, dense, pure fuel mass accessible to an external ignition source. The current baseline fast ignition target is a shell fitted with a reentrant cone extending to near its center. Conventional direct or indirect drive collapses the shell near the tip of the cone, and then an ultraintense laser pulse focused to the inside cone tip generates high-energy electrons to ignite the dense fuel. Two-dimensional (2-D) calculations of this concept have sparsely explored the large design space available to optimize compaction of the fuel and maintain the integrity of the cone. Experiments have generally validated the modeling while revealing additional complexities. Away from the cone, the shell collapses much as does a conventional implosion, generating a hot, low-density, inner-core plasma that exhausts out toward the tip of the cone. The hot, low-density inner core can impede the compaction of the cold fuel, lowering the implosion/burn efficiency and the gain, and jetting toward the cone tip can affect the cone integrity. Thicker initial fuel layers, lower velocity implosions, and drive asymmetries can lead to decreased efficiency in converting implosion kinetic energy into compression. Fast ignition burn hydrodynamics can generate additional convergence and compression. We describe 2-D and one-dimensional approaches to optimizing designs for cone-guided fast ignition.

Relativistic self-focusing of a laser beam in an inhomogeneous plasma

This paper presents an analysis of relativistic self-focusing of intense laser radiation in an axially inhomogeneous plasma. The nonlinearity in the dielectric constant arises on account of the relativistic variation of mass for an arbitrary magnitude of intensity. An appropriate expression for the nonlinear dielectric constant has been used in the analysis of laser-beam propagation in the paraxial approximation for a circularly polarized wave. The variations of the beam width parameter with the propagation distance, the self-trapping condition and the critical power have been evaluated. The saturating nature of the nonlinearity yields two values of the critical power for beam self-focusing, . It is seen that the laser beam width tends to attain a constant value depending upon the plasma inhomogenity and the initial laser intensity. Numerical estimates are made for typical values of the laser–plasma interaction applicable for underdense and overdense plasmas. The most important experiments described by this model are related to the fast ignitor concept.

Implosion hydrodynamics of fast ignition targets

The fast ignition (FI) concept requires the generation of a compact, dense, pure fuel mass accessible to an external ignition source. The current base line FI target is a shell fitted with a reentrant cone extending to near its center. Conventional direct- or indirect-drive collapses the shell near the tip of the cone and then an ultraintense laser pulse focused to the inside cone tip generates high-energy electrons to ignite the dense fuel. A theoretical and experimental investigation was undertaken of the collapse of such targets, validating modeling, and exploring the trade-offs available, in such an asymmetric geometry, to optimize compaction of the fuel and maintain the integrity of the cone. The collapse is complex. Away from the cone, the shell collapses much as does a conventional implosion, generating a hot, low-density inner core. But because of the open side, hot plasma exhausts out toward the tip of the cone. This hot plasma is advantageous for implosion diagnostics; it can provide protons for angular dependent measurements of the shell wall, neutrons for temperature measurements, and self-emission for contamination measurements. But for FI it is a liability; the hot, low-density inner core impedes the compaction of the cold fuel, lowering the implosion/burn efficiency and the gain. Approaches to optimizing this shell design are discussed.