Laser Fusion Experiments Research Articles

Growth of large KDP crystals for laser fusion experiments

The growth of huge KDP crystals which have a 40 cm × 40 cm cross section for a frequency converter for high power laser system for nuclear fusion experiments is described. The main factor limiting the growth rate is spontaneous nucleation. We could successfully grow a crystal by a three-vessel method (TVM) at the rate of 2.9 mm/day over half a year without nucleation by setting the temperature of the crystallizer lower than room temperature. The TVM proved to be superior to a conventional temperature reduction method. The bulk laser damage threshold was improved two to three times that of conventionally grown crystals by reducing organic impurities in the growth solution. These techniques are very important for the growth of large KDP crystals of good quality for high power lasers.

An investigation of the X-ray point-source brightness for a short-pulse laser plasma

The production of X-rays is examined for photon energies in the range ∼3–5 KeV Created when a laser of 0.53 μm wavelength, 100 psec pulse duration and pulse energy ∼15 J is focussed onto various targets of dimension ≲ 10 μm. The aim is to produce a point source (⩽15 μm) of duration ≲ 100 psec suitable, for example, as a backlighter for laser-fusion experiments. We found that tin (Sn) is the best emitter in the 3–5 keV range of the materials studied (Ca, Ti, Sn, Te, CsI) and that stripe targets are better emitters than fibre targets.

Laser-field effects on the interaction of charged particles with a degenerate electron gas.

We formulate a general description of the inelastic interaction between charged particles and a degenerate plasma in the presence of a strong laser field. The excitations in the plasma are described according to the random-phase-approximation formalism, in terms of the dielectric function of the medium, and including the effects of the laser field on the dynamical response. The energy exchange and the scattering rate of the particle in the plasma are modified by multiphoton processes. The formalism describes the excitation of plasmons and electron-hole pairs, with simultaneous emission or absorption of photons. We calculate the contribution of these processes to the energy-loss rate and to the mean free path of the particle in the range of solid-state densities. New effects due to the laser field are expected for intensities similar to those used in laser fusion experiments.

High-Z elements in laser fusion targets

Recent laser fusion experiments conducted by Los Alamos physicists have required the use of high-Z elements in atypical applications. Gold, a commonly used element, has been deposited in low-density smoke form onto plastic and glass spheres. Thorium and uranium have been requested as x-ray backlighter materials, and wires etched to <10 μm have been produced for this purpose. Uranium has also been vapor deposited onto spheres and 7–10 μm fibers with varying degrees of success.

A source of hot spots in frequency-tripled laser light

Laser light, frequency tripled to the ultraviolet (UV), shows numerous, strongly modulated hot spots in the target plane for the OMEGA laser used in laser-fusion experiments. However, at the fundamental frequency in the infrared (IR), the hot-spot modulation is relatively small. High-resolution measurements of the intensity and phase in the laser near field, combined with computer modeling of beam propagation to the target plane, show that the intensity fluctuations are dominated by the near-field phase errors. The appearance of hot spots in the UV and not in the IR are found to be the result of phase-error tripling caused by the frequency conversion.

Investigation of the mutual coherence of laser radiation in the Del'fin-1 facility

A method is proposed for investigation of the mutual coherence function Γ (x1, x2, τ) of a laser beam on the basis of an analysis of the Fourier transform of a speckle field formed as a result of interaction of laser radiation with a random Gaussian scatterer. The proposed method was used to study the coherence of high-power laser beams at various stages of pulse amplification in the Del'fin-1 facility designed for controlled laser fusion experiments. It was found that when the radiation from a master oscillator in this facility was practically coherent throughout the whole beam, subsequent amplification of a pulse reduced the half-width of the function Γ (x1, x2) to 0.1–0.2 of the beam diameter and this was due to nonlinear effects in the active medium at power densities ~ 1 GW/cm2. A study was also made of the dependence of the function Γ (x1, x2) on the spectral composition of laser radiation.

Detection system of the cryogenic target default for laser fusion experiment.

Cryogenically cooled deutrium (D2) targets have been used for laser fusion experiments. They requirevery delicate operations to control the amount of the liquid fuel (D2) in a plastic foam shell andto set the shell precisely at the focal position. If these targets should fall away before laser irradiation, intense laser beams may cause severe damages to some optical components. An automatic protectionsystem has been designed, fabricated, and tested to protect the optical components in the highpower laser system from any costly damage.

Tunable lasers. Optical nonlinear frequency converters.

Frequency conversion techniques using nonlinear optical materials are simple and useful to obtain lasers ranging from visible to ultra-violet region. Recently new nonlinear optical crystals have been or being developed for this purpose. Since KDP and LAP can be grown to big size, they are suitable for big laser systems of laser fusion experiments. New crystals KTP, BBO and LBP are promising for high power, high repetition lasers and optical parametric oscillators. For small power lasers such as diode lasers, potassium niobate, lithium niobate and KTP are used. Many new organic materials are being developed in place of these inorganic materials. Laser properties of these inorganic and organic crystals are described.

Laser Fusion Research in the USSR

The history of laser fusion experiments in the USSR is briefly reviewed. Stress is placed on recent key experimental and theoretical results. These results, taken together with data from the worldwide laser fusion community, support the feasibility of break-even and high-gain targets. Concepts for both pure fusion and hybrid reactors are outlined.

Gated x-ray pinhole camera for time-resolved 2-D images of laser-driven targets (abstract)

We describe the operation of a multiframe x-ray pinhole camera with time resolutions on the order of 200 ps. This diagnostic provides complementary information to that obtained with a conventional x-ray streak camera. We will describe the characterization of the instrument, obtained optically and electronically, and show x-ray data obtained from laser fusion experiments. This work was performed under the auspices of the U. S. DOE by the Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.

Radiation-induced conductivity: The high-speed detection of photons and particles (invited) (abstract)

High-speed photoconductors have been used to detect photons from the infrared to gamma rays. Their technology has been extended to new materials and to the detection of particles (radiation-induced conductivity). The new materials include thin films of GaAs and diamond and bulk type IIa insulating diamond which complement previously tested material, bulk InP and GaAs. Thin films offer high speeds and discrimination against penetrating radiation. Insulating diamond is insensitive to visible light (scattered laser radiation). Both properties can be extremely valuable in a laser–plasma environment. In addition, these detectors have been used to detect 14-MeV neutron pulses from laser fusion experiments on the Nova laser. The thermonuclear burn rate and its temporal relationship to the driving laser pulse have been determined with 100-ps time resolution. This work was performed under the auspices of the U.S. DOE by the Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.

"Coulomb logarithm" for inverse-bremsstrahlung laser absorption.

The ``Coulomb logarithm'' for inverse-bremsstrahlung laser absorption is examined for plasmas of different ionic charge, spanning the classical and quantum-mechanical limits. Previously, this term has not been calculated exactly for the conditions of interest in laser fusion experiments; it has only been estimated from physical considerations. For short-wavelength irradiation (e.g., 0.35 \ensuremath{\mu}m), uncertainties in the ``logarithmic'' factor can produce variations of 20\char21{}50 % in the laser absorption coefficient. A more exact treatment of this term is presented here. For low-Z plasmas, a modified Born approximation is used that reproduces previous results for long-range interactions that cannot be described by a single electron-ion collision, and it simultaneously treats the short-range electron-ion encounters. For high-Z plasmas, the Coulomb logarithm is calculated in terms of the classical, nonlinear electron trajectory in a self-consistent electrostatic potential; strong ion-ion correlations are treated by the nonlinear Debye-H\uckel model. There are no indeterminate quantities in the calculations.

Optimization of the parameters of high-aspect-ratio targets for laser thermonuclear fusion experiments with a laser energy of 1–2 kJ

A scheme was developed, based on theoretical models and numerical calculations, for optimizing gas-filled shell targets for laser fusion experiments with energies of 1–2 kJ. The results of numerical calculations are compared with data obtained in experiments on the Del'fin-1 facility and shown to be in satisfactory agreement.

Fabrication of polyvinyl alcohol coated polystyrene shells

Glass shells have been used traditionally as the deuterium–tritium fuel container for direct-drive laser fusion experiments because of their convenience and availability, but lower-Z fuel containers have superior implosion characteristics and diagnostic possibilities. Unfortunately, polymers such as polystyrene (PS) that produce shells easily have very high permeabilities so require cryogenics to retain fuel, and impermeable polymers such as polyvinyl alcohol (PVA) are difficult to make into high-quality shells. We have developed improved methods of making PS shells with diameters from 0.2 to 0.7 mm and coating them with a 3-μm layer of PVA to obtain the advantages of both. Both the PS shells and the PVA coating are made in drop towers using gas-stripped nozzles. Details of the procedures and product quality are discussed.

Data processing computer system for laser fusion experiment.

This article describes the development and present situation of the data processing system for laser fusion experiment using GEKKO XII glass laser. To achieve flexibility of the system and capability to process large amount of image data, we introduced and developed a system where several 16 bit microcomputers are connected to a 32 bit super minicomputer (FACOM S-3300). This report shows the present potential of the S-3300-microcomputer system, and presents the method of file transfer between these computers.

Possibility of controlling the coherence properties of high-power laser beams with a plasma filter

An investigation was made of the energy, angular, spectral, and coherence characteristics of high-power radiation from a neodymium laser, scattered by a plasma corona on the surface of an irradiated solid target. The high homogeneity of the beam formed from the scattered radiation was due to the loss of coherence on a reflecting surface which acquired a rippled structure. An analysis was made of the possibility of using plasma filters in laser fusion experiments to provide the required homogeneity of irradiation of spherical targets.

Suprathermal electron generation in cannonball targets

Suprathermal ( T s =20−300 keV) electron production in cannonball targets was investigated in Gekko laser fusion experiments. Very high temperatures ( T sh =200−350 keV) and energy conversion rates (η sh=10 -5−10 -4 J/J str) were observed for the cannonball targets as compared to directly driven targets ( T sh =20–200 keV, η sh =10 -8−10 -6 J/J str). Correlations of superthermal electron conversion with SRS and 3ω 0 2 light conclusively indicated that these electrons were mainly generated by parametric processes in the region below n c 4 .

Reduction of a time of flight experiment to tomography

Abstract We propose a novel generalization of time of flight analysis which yields unambiguous information about the velocity spectrum as a function of time of a point pulsed particle source. The records of several particle counters placed at various prescribed distances from the source are combined in a way derived from tomography theory to give a well defined set of moments of the source emission function. A variety of reconstructions can then be made, which we illustrate by computer examples. The resolution of the method is investigated. There are possible applications to plasma focus and laser fusion experiments.

Laser fusion experiments at the University of Rochester

The paper reviews the contributions of the Laboratory for Laser Energetics of the University of Rochester to inertial fusion research, including investigation of the direct-drive approach to laser fusion, the development of short-wavelength laser systems and the development of high-density diagnostic techniques.

Summary Abstract: Fabrication of Rayleigh–Taylor instability experiment targets

A new class of targets for laser fusion experiments was fabricated using plasma-deposition and etching technology. Plasma polymer coatings seeded with silicon or sulfur were deposited as 300..mu..m diameter microspots inside holes of equal diameter in a pure hydrocarbon polymer film. The target was designed to study large-scale plasma instabilities and measure the temperature and density histories of laser induced plasmas. The microspot target required three new development: freestanding stress-free CH films, technology to define and form holes in CH films, and development of seeded films deposited as 300..mu..m diameter discs, nested tightly in the precision holes. Hydrocarbon films were deposited by plasma polymerization or by solution casting (polystyrene in dichloromethane) onto potassium-chloide-coated glass slides. Holes were defined either by masking with a 300..mu..m diameter disc or by reactive ion etching through a washer mask. Sulfur or silicon seeded CH polymer microspots were deposited through a mask using plasma polymerization. Seeded polymer films were prepared with compositions as high as 12 atomic percent, with most diagnostic targets made with 2 a/o. Silicon seeded polymers, when deposited at 750 mtorr (100 Pa) pressure, wer transparent and colorless.