Exploding Pusher Target Research Articles

A combined MeV-neutron and x-ray source for the National Ignition Facility.

In support of future radiation-effects testing, a combined environment source has been developed for the National Ignition Facility (NIF), utilizing both NIF's long-pulse beams, and the Advanced Radiographic Capability (ARC) short pulse lasers. First, ARC was used to illuminate a gold foil at high-intensity, generating a significant x-ray signal >1MeV. This was followed by NIF 10 ns later to implode an exploding pusher target filled with fusionable gas for neutron generation. The neutron and x-ray bursts were incident onto a retrievable, close-standoff diagnostic snout. With separate control over both neutron and x-ray emission, the platform allows for tailored photon and neutron fluences and timing on a recoverable test sample. The platform exceeded its initial fluence goals, demonstrating a neutron fluence of 2.3 ×1013 n/cm2 and an x-ray dose of 7 krad.

The neutron imaging system for inertial confinement fusion at the 100 kilo-Joule laser facility

The neutron imaging system based on the penumbral aperture for the inertial confinement fusion research at the 100 kilo-Joule laser facility has been developed. The aperture has been designed with a geometric model and fabricated by machining one half double-tapered cylinder into each of two tungsten slabs. The aperture is mounted into a box which helps to precisely fix two slabs to form a biconic-shaped aperture. The neutron image recording system has been constructed with a capillary-array neutron image detector, and tested with the static accelerator neutron source. The spatial resolution of neutron image detector can be within 1.1 mm. The preliminary results of neutron imaging system in the exploding pusher target experiment at the 100 kilo-Joule laser facility have been presented.

Fusion fuel ion temperature diagnostic for directly driven implosions

An ultrafast quenched plastic scintillation detector was used to measure the fusion fuel ion temperature at low implosion (DT) neutron yield (5×108–3×109) in the initial experiment performed at the Shenguang II laser facility. The typical temperatures of exploding pusher targets for direct drive were around 4 keV and the uncertainties were ±15–23%. The detection efficiency of the detector to DT neutrons was calibrated at a K-400 accelerator. The time response function of the detection system was calibrated by implosion neutrons from a DT-filled capsule, which can be regarded as a δ function pulsed neutron source due to its much narrower pulse width than that of the measured neutron time-of-flight spectrum.

DT FUEL AREAL DENSITY DIAGNOSTIC IN DIRECT-DRIVEN IMPLOSIONS

The diagnostic of DT fuel plasma areal density in the direct-driven implosion experiments on SG-Ⅱ laser facility has been performed by measuring the number of recoil D and T produced by 14MeV neutron′s scattering. The particles of recoil D and T were detected by CR39 solid-state nuclear track detector. The experimental results indicate that the uniformity of laser illuminating affects the compression to a certain extent. The areal density of exploding pusher target is about one order lower than that of ablative-driven target. This means that the compression of ablative-driven target is better than exploding pusher target. There is a strong relationship between areal density and suprathermal electron heating.

The investigation of high intensity laser driven micro neutron sources for fusion materials research at high fluence

The application of fast pulse, high intensity lasers to drive low cost DT point neutron sources for fusion materials testing at high flux/fluence is investigated. At present, high power bench-top lasers with intensities of 1018W/cm2 are routinely employed and systems capable of ⩾ 1021 W/cm2 are becoming available. These potentially offer sufficient energy density for efficient neutron production in DT targets with dimensions of around 100 μm. Two different target concepts are analysed - a hot ion, beam-target system and an exploding pusher target system - and neutron emission rates are evaluated as a function of laser and target conditions. Compared with conventional beam-target neutron sources with steadystate liquid cooling, the driver energy here is removed by sacrificial vaporization of a small target spot. The resulting small source volumes offer the potential for a low cost, high flux source of 14 MeV neutrons at close coupled, micro (⩽ 1 mm) test specimens. In particular, it is shown that a laser driven target with ∼100 J/pulse at 100 Hz (i.e. ∼10 kW average power) and laser irradiances in the range Iλ2∼1017-1019 W μm2/cm2 could produce primary, uncollided neutron fluxes at the test specimen in the 1014-1015 n cm-2 s-2 range. While focusing on the laser-plasma interaction physics and resulting neutron production, the materials science required to validate computational damage models utilizing ⩾ 100 dpa irradiation of such specimens is also examined; this may provide a multiscale predictive capability for the behaviour of engineering scale components in fusion reactor applications.

Neutron penumbral aperture images of inertial confinement fusion targets achieve 60 μm resolution (abstract)

Images formed by 14 MeV neutrons emitted from the core of inertial confinement fusion targets demonstrate 60 μm resolution for the penumbral aperture imaging system used in our 1988 experiments. Hohlraums containing deuterium–tritium filled capsules were irradiated at the Nova Laser Facility and produced images with detector limited resolution. Neutron yields ranged from 5×1010 to 2×1011. The results imply that convolution of the aperture point spread function with the neutron emitting region of the target core is less than 40 μm. Images produced by the indirect-drive hohlraum targets are compared with previously reported images of high-yield (1×1013) exploding-pusher targets that show 150 μm (full width half maximum) diameters for the emitting region.

Analysis of a proton driven, exploding pusher target on PBFA II (abstract)

During the past year we have conducted a series of PBFA II target experiments. The design of the experiments was threefold: to characterize the response of targets to ion beams, develop our experimental diagnostic capability, and benchmark our theoretical ability to design and predict the response of the experimental diagnostics to ion-beam-heated, ICF targets. We present here an analysis of spherical exploding pusher experiments. The targets were 6-mm-diam, 100-μm-thick, Cl-doped CH spheres, filled with 1.2 atm of deuterium doped with 7 Torr of H2S. The diagnostics included XRDs, x-ray pinhole, framing, and streak cameras, and a spatially resolved, x-ray crystal spectrometer. Our analysis includes comparisons between experimental results and theoretical predictions of diagnostics responses and sulfur line emission from the compressed target using one- and two-dimensional radiation/hydrodynamic code runs. This work supported by the U. S. Department of Energy under Contract No. DE-AC04-76-DP00789.

XUV radiation from gold exploding pusher targets irradiated with protons

In 1982, proton beam experiments were carried out on the PROTO I accelerator with up to 0.65 TW/cm 2 incident on gold exploding pusher targets. The total proton energy incident on the target was as high as 2.8 kJ, with up to 1.32 kJ deposited in 2.5-μm-thick gold cones, giving a specific power deposition as high as 27 TW/g for 1.5 MeV incident proton energy. XUV radiation (10 eV -2.5 keV) from the gold conical exploding pusher targets was measured with x-ray diodes (XRDs) and PIN solid state diodes. High resolution, time integrated XUV radiation measurements (60–400 eV) were also obtained with a 1-meter grazing incidence spectrograph. The radiation from these targets consisted of three major components: (1) XUV radiation from a high density but low temperature plasma radiation at the inner surface of the conical exploding pusher target, (2) high energy XUV radiation from a low density, high temperature plasma produced by the stagnation of the pusher material on axis, and (3) 2.1–2.2 keV gold Mα, β inner-shell line radiation from the inside surface of the target. The equivalent brightness temperatures at the inner surface of the conical targets were between 18 and 32 eV, assuming a 0.07 cm 2 emission area.

Diagnosis of laser produced plasmas using fusion reaction products

Experiments have been performed at the Central Laser Facility, Rutherford Appleton Lab., UK. using novel techniques in which laser produced plasmas have been diagnosed by measurements of the charged thermonuclear reaction products. Two types of experiment are being reported here. Firstly, thermonuclear alpha particles from an exploding pusher target have been used to determine the growth of the Rayleigh–Taylor instability in a separate laser driven planar foil. The resulting alpha particle range loss distributions provide a direct measurement of the foil thickness distribution and hence the instability. The R–T instability has been observed in a number of foils with range losses varying between almost zero to over lOμm. Secondly, a thermonuclear particle backlighting technique has been used in the measurement of the stopping power of hot plasma for different materials. The ratio of plasma stopping power to that of the cold material is measured and compared to a value obtained from theoretical modelling. The solid state nuclear track detector CR–39 has been used as a diagnostic for these experiments. A sophisticated image analysis system has been developed to enhance and improve data recovery.

Optimum Design of Exploding Pusher Target to Produce Maximum Neutrons

Exploding pusher target experiments have been conducted with the 1.052 µm GEKKO MII two beam-glass laser system to design an optimum target, which couples to the incident laser light most effectively to produce the maximum number of neutrons. Since hot electrons preheat the shell completely in spite of the strongly nonuniform irradiation, a simple model can design the optimum target, in which the shell/fuel interface is accelerated to 0.5 to 0.7 times the initial radius within a laser pulse. A 2-dimensional computer simulation supports this target design. The scaling of the neutron yield N with the laser power P is N∝P2.4±0.4.

Scaling laws for laser driven exploding pusher targets

An analytic model based on shock waves is used to describe the maximum density rho F, maximum temperature TF and minimum radius RF obtained in the laser driven implosion of a gas filled spherical shell target of shell thickness Delta T, shell radius R and initial gas density rho 1. The scaling behaviour rho F approximately rho 125/, rho FRf approximately rho 135/R and TF approximately ( alpha E0/M) rho 1-25/, is obtained, where E0 is the laser energy, M is the shell mass, and alpha is the hydrodynamically useful fraction of the laser energy. Instantaneous uniform heating of the shell is assumed in order to describe implosions of the 'exploding pusher' type.

Microfabrication and microcharacterization techniques for laser fusion targets

A review is presented of some recent accomplishments in the fabrication and characterization of laser fusion targets. These targets are used in a research program at the University of Rochester’s Laboratory for Laser Energetics whose goal is to demonstrate the scientific feasibility of laser fusion. Exploding pusher targets, consisting of deuterium–tritium filled glass microballoons, are discussed. Techniques are described for selecting high quality shells, filling them with D–T fuel by permeation, and characterizing their fuel content by nondestructive beta counting. More advanced targets have been employed in experiments designed to produce higher compressions using the six beam laser system ZETA. These consist of argon filled glass shells coated with plastic. A novel method is described for filling the shells with argon, which involves laser drilling a submicron hole in the balloon wall and sealing the gas with a meltable plug. Three methods are discussed for applying smooth and uniform plastic coatings to these targets. The thermal pyrolysis of di-paraxylene, plasma polymerization of organic monomers (e.g. p-xylene) and colloidal coatings of polyacrylates have all yielded excellent hydrocarbon coatings on glass microballons. Targets for upcoming experiments on the 24 beam laser system OMEGA involve multishell configurations. A new microfabrication molding and replication technique is being implemented to produce spherical and identical hemispherical shells used in the assembly of OMEGA targets. Details of this technique are provided, along with some preliminary examples of plastic and copper hemishells. A versatile physical vapor deposition system is also described which has the capability of coating levitated microballoons with metals and other materials amenable to sputtering. Finally, a unique method of examining opaque targets using contact microradiography is discussed. The x-ray source is generated by the focussed beam of a scanning electron microscope. Readout of the developed radiographic image is also performed in the SEM, using backscattered electrons from the silver grains in the emulsion. Edge resolution of 0.25 μm has been demonstrated in this radiography system. The characterization of localized defects and geometrical defects such as decentrations are possible with superior resolution in this system.

Neon spectral line broadening as a diagnostic for compressed laser-fusion targets

Experiments have been conducted on the JANUS laser system to measure fuel density in imploded laser-fusion targets filled with deuterium, tritium, and neon. An x-ray crystal spectrometer was used to record the line emission of the 1s-2p line of the Ne+9 ion. Simultaneous measurement of the linewidth and the neutron yield indicated that symmetrically irradiated exploding pusher targets reached fuel densities of approximately 1 g/cm3.

Diagnostics for the laser fusion program - Plasma physics on the scale of microns and picoseconds

Diagnostic techniques are reviewed in the context of insights they provide to physical processes dominating the implosion of laser fusion targets. Specific examples chosen from recent experiments demonstrate that adequate diagnostic techniques, resolved to microns and picoseconds, have been developed for the demanding requirements of laser exploding pusher targets. Future diagnostic needs are related to the special problems forseen with ablatively driven high density implosions.