Laser Inertial Confinement Fusion Research Articles

Way of detecting entire beam path aberrations of laser systems based on a phase-retrieval method

A method is presented to detect entire beam path aberrations of many laser systems, especially the SG-III inertial confinement fusion (ICF) laser system, which is under construction in China. Instead of a Hartman-Shack (HS) sensor, a CCD will be introduced into the target chamber of the ICF laser system. The HS sensor and the CCD could record the near-field wavefront distributions and the intensity signals of their corresponding focal spots, respectively. An amendatory phase-retrieval method is introduced to reconstruct the aberrations of the entire beam path from near-field wavefront distributions and their corresponding focal spots intensity. Principle experimental results indicate that the aberrations of the entire beam path of a laser system can be precisely detected based on this method.

Research Progress on Laser Inertial Confinement Fusion at RCLF in China

In past years, the Research Center of Laser Fusion (RCLF) at the China Academy of Engineering Physics has developed many facilities and technologies in the field of ultrahigh-intensity lasers, physical diagnoses, and target fabrications for ICF experiments. This paper briefly reports some of the latest advances achieved and future plans at RCLF.

Lasers for inertial confinement fusion driven by high explosives

Proposed laser fusion power plant concepts suffer from the huge size and expense of the lasers needed for compression and ignition. In a 1969 study (classified in 1970 and declassified in 2007), the idea to use chemical high explosives for the pumping of megajoule lasers was explored. Apart from being less expensive by orders of magnitude, such lasers are expected to be much more compact, and with their large energy, output could simultaneously drive several thermonuclear micro-explosion chambers. Because of its topical importance, I accepted the journal's invitation to publish a previously classified work, but with new unpublished ideas, with the previously classified paper put into the appendix.

Relativistic laser-matter interaction: from attosecond pulse generation to fast ignition

The field of laser–matter interaction has branched out in two main directions. The first, motivated by laser inertial confinement fusion, warm-dense-matter, fast ignition and astrophysics in laboratory, and the second driven by ultra-high intensity, exotic physics, high-energy particle, photon beam generation and time-resolved attosecond (zeptosecond) science. The degree of maturity from both experimental and theoretical stand-points is such that a large European infrastructure for each branch is contemplated as part of the European Roadmap. The first one, HiPER-PETAL will be dedicated to fast ignition with the aim of obtaining a thermonuclear gain of 100, whereas the second, Extreme Light Infrastructure (ELI) could go beyond the relativistic regime to foray into the ultra-relativistic domain >1024 W cm−2. In this paper we highlight the intriguing perspectives that these two projects will offer.

Theory and experiment analysis of factors affecting THG efficiency for the TIL prototype laser facility

Based on the study and manufacture of the technical integration experiment line (TIL) prototype laser facility, and using the method that theoretical simulations guide experiments and experimental results check programs, the factors affecting third harmonic generation (THG) efficiency for a large-aperture, high-intensity inertial confinement fusion (ICF) laser facility have been analyzed and studied in detail. Furthermore, in the year 2004–2005, the output ability of a THG system for a TIL driver has been discussed adequately in a first beam eligibility experiment. As shown by experimental results, when the average drive irradiance of fundamental light, I 1 ω , is 2.7 GW/cm 2, the measured value of THG efficiency is 50–55% and the theoretical value is 60%, which indicates that the experimental result agrees well with the theoretical value.

Effect of a prepulse on ablation-pressure smoothing in laser heating of thin foils

The results of theoretical and experimental studies of plasma heating by nonuniform laser radiation and the symmetrizing effect of a laser prepulse on the uniformity of heating of thin foils, which simulated thin-shell laser inertial-confinement fusion (ICF) targets, are presented. The high efficiency of symmetrization by a prepulse was demonstrated both experimentally and theoretically.

Modeling of frequency doubling and tripling with measured crystal spatial refractive-index nonuniformities

Efficient frequency doubling and tripling are critical to the successful operation of inertial confinement fusion laser systems such as the National Ignition Facility currently being constructed at the Lawrence Livermore National Laboratory and the Omega laser at the Laboratory for Laser Energetics. High-frequency conversion efficiency is strongly dependent on attainment of the phase-matching condition. In an ideal converter crystal, one can obtain the phase-matching condition throughout by angle tuning or temperature tuning of the crystal as a whole. In real crystals, imperfections in the crystal structure prohibit the attainment of phase matching at all locations in the crystal. We have modeled frequency doubling and tripling with a quantitative measure of this departure from phase matching in real crystals. This measure is obtained from interferometry of KDP and KD*P crystals at two orthogonal light polarizations.

Ultrafast picket fence pulse trains to enhance frequency conversion of shaped inertial confinement fusion laser pulses

A high-frequency train of 5-100-ps pulses (picket fence) is proposed to improve significantly the third-harmonic frequency conversion of Nd:glass lasers that are used to generate high-contrast-shaped pulses for inertial confinement fusion (ICF) targets. High conversion efficiency of the low-power foot of a shaped ICF pulse is obtained by use of a low duty cycle, multi-gigahertz train of approximately 20-ps pulses with high peak power. Even with less than 10% duty cycle, continuous illumination is maintained on the target by a combination of temporal broadening schemes. The picket fence approach is analyzed, and the practical limits are identified as applied to the National Ignition Facility laser. It is found that the higher conversion efficiency allows approximately 40% more third-harmonic energy to be delivered to the target, potentially enabling the larger drive needed for high-yield ICF target designs. In addition, the frequency conversion efficiency of these short pulses saturates much more readily, which reduces the transfer of fluctuations at the fundamental and thus greatly improves the power stability of the third harmonic.

The relevance of HED lasers to astrophysics

The role of high energy density (HED) lasers in astrophysics is reviewed, emphasizing connections with laser physics experiments and inertial confinement fusion (ICF). Careful validation of astrophysical methods, by laboratory experiment, by critical comparison of numerical and analytical methods, and by observation are necessary for the development of simulation methods with reliable predictive capability. Recent results from hydrodynamic simulations of stellar evolution, HED laser experiments, and computer simulations will be discussed, and related to stellar evolution and supernovae.

Implosion de cibles laser magnétisées: influence de la limitation du flux thermique

The influence of thermal conduction on laser inertial confinement fusion is studied with a sophisticated CEA (Commissariat à l'Énergie Atomique) computer code. The beneficial effect of magnetic field on gain, shown by I.R. Lindemuth and R.C. Kirkpatrick, is qualitatively confirmed by our results.

The Role of Mixing in Astrophysics

The role of hydrodynamic mixing in astrophysics is reviewed, emphasizing its connections with laser physics experiments and inertial confinement fusion. Computer technology now allows us to use two-dimensional simulations, with complex microphysics, of stellar hydrodynamics and evolutionary sequences, and it holds the promise of simulations in three dimensions. Careful validation of astrophysical methods by laboratory experiment, by the critical comparison of numerical and analytical methods, and by observation are necessary for the development of simulation methods with reliable predictive capabilities. The recent and surprising results from isotopic patterns in presolar grains, from two-dimensional hydrodynamic simulations of stellar evolution, and from laser tests and computer simulations of Richtmeyer-Meshkov and Rayleigh-Taylor instabilities will be discussed in relation to stellar evolution and supernovae.

The spatial location of laser-driven, forward-propagating waves in a National-Ignition-Facility-relevant plasma

Ion acoustic and electron plasma waves, associated with backward-propagating stimulated Brillouin scattering and stimulated Raman scattering, have been diagnosed in a long-scale-length, nearly homogenous plasma with transverse flow. Thomson scattered light from a probe beam is employed to show that these waves are well localized in space and for a time much shorter than the laser pulse duration. These plasma conditions are relevant to hohlraum design for the National Ignition Facility inertial confinement fusion laser system. [R. Sawicki et al., Fusion Technol. 34, 1097 (1998)].

Beam Control and Laser Characterization for NIF

The demanding energy, power, pulse shape, focusability, pointing, and availability requirements placed on the 192 National Ignition Facility (NIF) beams lead to the need for an automatic operation capability that is well beyond that of previous inertial confinement fusion lasers.1 Alignment, diagnostic, and wavefront correction subsystems are integrated in an approach that, by permitting maximal sharing of instrumentation between subsystems, meets performance requirements at a reasonable cost.

Relativistic Multichannel Theory: Doubly Excited States of Atomic Helium

In the report, the recently developed relativistic multichannel theory (RMCT) is a full relativistic non-perturbative method and can be applied to the high-Z atomic ions which have given rise to some recent interests because of researches of inertial confinement fusion and x-ray laser. Based on the RMCT, the positions and widths of doubly excited sp2n+, sp2n-, and 2pnd1 P1 resonances are calculated, which are in agreement with the recent experimental measurements. It sets up a sound foundation on which the RMCT should be applicable to high Z atomic ions.

The OMEGA laser electronic timing system

A high-precision, computer-controlled electronic timing system has been built for the OMEGA inertial confinement fusion laser. The timing system consists of five modular subcomponents and is easily expandable to produce any number of delay channels. The timing from the system is synchronized by a distributed clock, which is derived from the radio-frequency-signal that drives the mode-locked laser master oscillator. The system produces IEEE RS-170 television synchronization signals and several subfrequencies for operating different parts of the laser. Performance of the system and cost will be discussed.

Recent Performance Results of the National Ignition Facility Beamlet Demonstration Project

The laser driver for the National Ignition Facility will be a departure from previous inertial confinement fusion laser architecture of a master oscillator single pass power amplifier (MOPA) design. The laser will use multi-segment Nd:Glass amplifiers in a multipass cavity arrangement, which can be assembled into compact and cost effective arrays to deliver the required multi- megajoule energy to target. A single beam physics prototype, the Beamlet, has been in operation for over two years and has demonstrated the feasibility of this architecture. We present a short review of Beamlet`s performance and limitations based on beam quality both at its fundamental and frequency converted wavelengths of 1.053 and 0.351 {mu}m.

Temporal evolution of stimulated Brillouin backscatter

A qualitative understanding of the time dependence of stimulated Brillouin scattering (SBS) provides estimates of the amount of temporal growth that occurs in current inertial confinement fusion and short laser pulse interaction experiments when the growth is limited by the length of the experiment or by motion of the ‘‘hot spots’’ of the laser intensity pattern induced by beam smoothing. In the weak coupling limit, the instantaneous growth rate depends upon the plasma initial conditions early in time, is proportional to t−1/2 later in time, and asymptotically approaches the absolute growth rate (in the absence of damping). When the instability is strongly coupled, the growth rate depends upon the plasma initial conditions early in time but is proportional to t−1/3 later in time. When the growth rate drops to a value comparable to that of the ion acoustic frequency, the instability becomes effectively weakly coupled. The effects of damping are also discussed.

Raman scattering in air: four-dimensional analysis

Inertial confinement fusion requires propagation of high-intensity, pulse-shaped IR and UV laser beams through long air paths. Such beams are subject to energy losses and decreased beam quality as a result by stimulated rotational Raman scattering (SRRS). In this paper we describe how quantum fluctuations, stimulated Raman amplification, diffraction propagation, and optical aberrations interact during the propagation of short, high-power laser pulses using a four-dimensional (4-D) model of the optical beams and the medium. The 4-D model has been incorporated into a general optical-propagation computer program that allows the entire optical system to be modeled and that is implemented on high-end personal computers, workstations, and supercomputers. The numerical model is used to illustrate important phenomena in the evolution of the optical beams. In addition, the OMEGA Upgrade laser system is used as a design case to illustrate the various considerations for inertial confinement fusion laser design.

Strengths and weaknesses of KrF lasers for inertial confinement fusion applications learned from the AURORA laser

The AURORA KrF laser at Los Alamos became operational in August 1989. AURORA is the first integrated system for demonstrating the capability of a KrF laser to perform target physics experiments for inertial confinement fusion (ICF) and is currently configured as a 5-kJ, 5-ns, 96-beam device. Both laser physics and ICF target physics experiments have been performed over the last year. Of the four major amplifiers in the AURORA laser system, one performed better than expected, one performed about as expected, and two performed below expectations. The causes of the variability in the amplifier performance are now well enough understood that this information can be used to improve the detailed design of the NIKE laser currently under construction at the Naval Research Laboratory. design of the NIKE laser currently under construction at the Naval Research Laboratory. High-dynamic-range pulse shapes have been propagated with minimal distortion through the AURORA amplifier chain, verifying theoretical predictions. Target physics experiments have been performed with intensities greater than 100 TW/cm2, pulse lengths ranging from 2–7 ns, and spot-size diameters from 500–1100 µm. The analysis of this first-generation kJ-class KrF laser target physics facility identified the strengths and weaknesses of KrF lasers for ICF applications. Detailed measurements of amplifier performance led to a better understanding of issues for KrF laser-fusion systems, and design studies for future KrF lasers for ICF applications incorporate improvements based in part on AURORA experience.

Large bandwidth frequency-converted Nd:glass laser at 527 nm with Delta nu / nu =2%

Large bandwidth in lasers for inertial confinement fusion is potentially capable of controlling laser-plasma instabilities. We have demonstrated the efficient generation of large bandwidth, \ensuremath{\Delta}\ensuremath{\nu}/\ensuremath{\nu}=2%, in a frequency-doubled Nd:glass laser in 527 nm, using stimulated rotational Raman scattering in atmospheric pressure nitrogen gas. Its use in a fusion laser would involve the use of a multilens array to produce multiple foci where the broadband conversion takes place. This technique is also effective for frequency tripled or quadrupled lasers at 351 or 263 nm.