S. Atzeni

Inertial fusion fast ignitor: Igniting pulse parameter window vs the penetration depth of the heating particles and the density of the precompressed fuel

A key element of the fast ignitor scheme is the pulse of fast particles creating the igniting spark. In this paper, the dependence of the parameters (energy Ep, power Wp, intensity Ip) of such a pulse on the penetration depth R of the fast particles is studied by two-dimensional simulations of the evolution of a deuterium–tritium fuel, precompressed at density ρ, and heated by a beam of particles with assigned R. The ignition windows in the (Ep,Wp,Ip) space are found to depend very little on R over the interval 0.15⩽R⩽1.2 g/cm2. At ρ=300 g/cm3, the minimum ignition energy is about 14 kJ; an optimal set of parameters (with energy and power about the required minimum, and intensity relatively close to the minimum) is Ep≅17 kJ, Wp≅0.85×1015 W, and Ip≅6.5×1019 W/cm2 (achieved at R=0.6 g/cm2). The optimal energy scales with the density as Ep∝ρ−1.85. Scaling laws are also presented for the other pulse parameters and for the limiting energy gain.

Review of progress in Fast Ignition

Marshall Rosenbluth’s extensive contributions included seminal analysis of the physics of the laser-plasma interaction and review and advocacy of the inertial fusion program. Over the last decade he avidly followed the efforts of many scientists around the world who have studied Fast Ignition, an alternate form of inertial fusion. In this scheme, the fuel is first compressed by a conventional inertial confinement fusion driver and then ignited by a short (∼10ps) pulse, high-power laser. Due to technological advances, such short-pulse lasers can focus power equivalent to that produced by the hydrodynamic stagnation of conventional inertial fusion capsules. This review will discuss the ignition requirements and gain curves starting from simple models and then describe how these are modified, as more detailed physics understanding is included. The critical design issues revolve around two questions: How can the compressed fuel be efficiently assembled? And how can power from the driver be delivered efficient...

Numerical study of fast ignition of ablatively imploded deuterium-tritium fusion capsules by ultra-intense proton beams

Compression and ignition of deuterium–tritium fuel under conditions relevant to the scheme of fast ignition by laser generated proton beams [Roth et al., Phys. Rev. Lett. 86, 436 (2001)] are studied by numerical simulation. Compression of a fuel containing spherical capsule driven by a pulse of thermal radiation is studied by a one-dimensional radiation hydrodynamics code. Irradiation of the compressed fuel by an intense proton beam, generated by a target at distance d from the capsule center, and subsequent ignition and burn are simulated by a two-dimensional code. A robust capsule, absorbing 635 kJ of 210 eV (peak) thermal x rays, with fusion yield of almost 500 MJ, has been designed, which could allow for target gain of 200. On the other hand, for a reasonable proton spectrum the required proton beam energy Eig, exceeds 25 kJ (for d=4 mm), even neglecting beam losses in the hohlraum and assuming that the beam can be focused on a spot with radius of 10 μm. The effects of proton range lengthening due to ...

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

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, t...

A first analysis of fast ignition of precompressed ICF fuel by laser-accelerated protons

The main parameters of the beam required to ignite a precompressed DT fuel, as foreseen by the recently proposed scheme of fast ignition by laser-accelerated protons (Roth et al 2001 Phys. Rev. Lett. 86 436), are studied by 2-D numerical simulations and a simple model. For simplicity, instantaneous proton generation at distance d from the compressed fuel and exponential proton energy spectrum, dn/de ∝ exp(−e/Tp), are assumed. An analytical expression and parametric numerical results are then given for the dependence of the minimum required beam energy on d, Tp and on the fuel density ρ. For the parameters of Roth et al (d ≈ 4 mm; ρ ≈ 400 g/cm 3 ) the minimum total proton energy for ignition is about 40 kJ.

2-D Lagrangian studies of symmetry and stability of laser fusion targets

Abstract The use of 2-D Lagrangian codes for studying the symmetry and the stability of laser fusion targets is critically discussed. Physical models and their finite difference implementation are illustrated, with particular reference to the three-temperature code, DUED. Applications to “model problems” and to full-scale laser target experiments illustrate the potential of the approach.

Inhibition of fast electron energy deposition due to preplasma filling of cone-attached targets

We present experimental and numerical results on the propagation and energy deposition of laser-generated fast electrons into conical targets. The first part reports on experimental measurements performed in various configurations in order to assess the predicted benefit of conical targets over standard planar ones. For the conditions investigated here, the fast electron-induced heating is found to be much weaker in cone-guided targets irradiated at a laser wavelength of 1.057μm, whereas frequency doubling of the laser pulse permits us to bridge the disparity between conical and planar targets. This result underscores the prejudicial role of the prepulse-generated plasma, whose confinement is enhanced in conical geometry. The second part is mostly devoted to the particle-in-cell modeling of the laser-cone interaction. In qualitative agreement with the experimental data, the calculations show that the presence of a large preplasma leads to a significant decrease in the fast electron density and energy flux...

Fast ignitor target studies for the HiPER project

Target studies for the proposed High Power Laser Energy Research (HiPER) facility [M. Dunne, Nature Phys. 2, 2 (2006)] are outlined and discussed. HiPER will deliver a 3ω (wavelength λ=0.35μm), multibeam, multi-ns pulse of about 250kJ and a 2ω or 3ω pulse of 70–100kJ in about 15ps. Its goal is the demonstration of laser driven inertial fusion via fast ignition. The baseline target concept is a direct-drive single shell capsule, ignited by hot electrons generated by a conically guided ultraintense laser beam. The paper first discusses ignition and compression requirements, and presents gain curves, based on an integrated model including ablative drive, compression, ignition and burn, and taking the coupling efficiency ηig of the igniting beam as a parameter. It turns out that ignition and moderate gain (up to 100) can be achieved, provided that adiabat shaping is used in the compression, and the efficiency ηig exceeds 20%. Using a standard ponderomotive scaling for the hot electron temperature, a 2ω or 3ω ...

Stopping and scattering of relativistic electron beams in dense plasmas and requirements for fast ignition

The interaction of a relativistic electron with a dense plasma is studied in the context of inertial fusion fast ignition. Expressions for the electron stopping power and deflection are given and implemented in a three-dimensional (3D) Monte Carlo code. Electron range and penetration depth are computed as functions of the electron energy and plasma parameters; approximate expressions are also proposed. Conditions for fast ignition are studied by including the 3D Monte Carlo code in a 2D hydrodynamic code. The required beam energy is determined as a function of mean electron energy for monoenergetic and exponential energy distributions and a uniform initial deuterium?tritium plasma with a density of 300?g?cm?3. A simple model is shown to agree with the code.

Thermonuclear burn performance of volume-ignited and centrally ignited bare deuterium-tritium microspheres

A parametric study of the burn performance (measured by the fuel energy gain) of compressed deuterium-tritium fuel microspheres, with parameters of interest to inertial confinement fusion, has been performed by 1-D numerical radiation-hydrodynamics simulations. Both volume-ignited and centrally ignited (either initially isobaric or isochoric) configurations are considered. An overview is given of the relevant ignition conditions. For the first time a scaling law is presented for the limiting gain of volume-ignited fuels. Scaling laws are also given for the limiting gain of centrally ignited assemblies, which have the same functional dependences as predicted by previous analytical models, but different numerical coefficients. The advantage of isochoric assemblies over isobaric ones is confirmed, but found to be smaller than previously reported. The potentials of volume-ignited and of centrally ignited, isobaric assemblies as functions of the energy, density and pressure are also critically compared.