effect of defocusing on laser-coupling into gold...

Effect of Defocusing on Laser-Coupling into Gold ConesI. Bush1, L. Gartside1, J. Pasley1,2, J. Green2, M. Notley2, H. Lowe2, C. Spindloe2, T. Winstone2, F. Cameron2, R. Clarke2, T. Ma3, T. Yabuuchi3, M. Wei3, F. Beg3, R.B. Stephens4, A. MacPhee5, A. J. MacKinnon5, M. H. Key5, W. Nazarov6

1Department of Physics, University of York, Heslington, York, YO10 5DD, United Kingdom2Central Laser Facility, Rutherford Appleton Laboratory, Chilton, OX11 0QX, United Kingdom

3Department of Mechanical and Aerospace Engineering, University of California San Diego, La Jolla, California 92093-0411, USA4General Atomics, San Diego, California 92121-1122, USA

5Lawrence Livermore National Laboratory, Livermore, California 94550-9234, USA6University of St. Andrews, School of Chemistry, St. Andrews, KY16 9AJ, United Kingdom

Overview

Results from an experiment to investigate the change in coupling with various degrees ofdefocusing, into cone-wire targets are described. The experiment was performed using theVulcan PetaWatt Laser at the Rutherford Appleton Laboratory. Gold cone targets, with copperwires attached to the tip were used. Diagnostics included a quartz crystal imager, HOPG crystalspectrometers, a single hit CCD and an optical probe. Results from the experiment show thatenhanced laser coupling is observed when the laser is defocused relative to the cone tip.

Cone Guided Fast Ignition

Fast ignition uses a separate igniter laser to heat thecentre of a compressed deuterium-tritium pelletAllows for smaller driver laser energy comparedwith conventional Inertial Confinement FusionGold cone allows laser energy to be coupled intothe fuel, by producing hot electrons from cone-tip

A typical FI target

Vulcan PetaWatt and Prepulse in High Power Lasers

Laser Parameters used on Vulcan PetaWatt:Nd Glass CPA laserShort pulse, 1-2psTotal energy of 600 J

λ = 1053 nmFocal spot ∼8µmIntensity on target of ∼1021 W m−2

Real high power lasersystems have significantprepulse, Vulcan PetaWatthas an intensity contrast of∼10−7 and an energy contrastof ∼10−4. This causes apreplasma in the cone tipextending to ∼100 µm [Batonet al., Phys. Plasmas (2008)],which could affect the lasercoupling into the cone.

Prepulse Measurement on Vulcan PetaWatt

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Time (ns)-6 -5 -4 -3 -2 -1 0 1 2

Prepulse ~1 ns

Diode saturated(main pulse)

Experimental AimA recent LLNL Titan laser experiment[L. Van Woerkom et al., Phys. Plasmas(2008)] looked at energetic photonemission from Cu cone targets using aTLD diagnostic [R. Clarke et al., J.Radiol. Prot. (2006)]. Enhancedemission was observed when the laserwas defocused.

Experiment aimed to collect more data when thelaser is focused before and after cone-tip:

Gold Cone-Wire Targets

Copper wires attached to thecone-tip allowed energycoupling into cone to bedetermined [Kodama et al.,Nature (2004)]Cone-wire targetsmanufactured on-siteOver 50 cone-wire targetsshot in total

Diagnostics - Target Chamber Layout

F 06 D

ElectronSpectrometer

Short Pulse

Long Pulse

Single Hit CCD

X-ray Pinhole Cameras HOPG Crystal SpectrometersCu K Crystal

Target ChamberCentral

f/3 Off-Axis Parabolic Mirror

K Image Plate

Optical ProbeLine

α

α

Primary diagnostics were a Cu Kα Imager, a HOPG Crystal Spectrometer, both used with imageplates, a single hit CCD and an an optical probe for shadowgraphy and interferometry. Thesingle hit CCD will be used for calibration of total yields of Kα emission. Also used werepinhole cameras and an electron spectrometer.

Copper Kα Imager

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Distance Along Wire (μ m)

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The Cu Kα signal (8.05 keV) was imagedby a Bragg reflecting quartz crystal (SiO22131) on to an X-ray image plate8×magnificationWire Kα emission brightens againtowards the end of the wire due torefluxing electronsScale length can be used to determineelectron energy spectrum, higher energyelectrons travel further in wire

HOPG Crystal Spectrometer

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Emission Frequency (keV)7.933 7.967 8 8.033 8.067 8.1 8.133

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spectrometers were used, with signalrecorded on to X-ray image platesCu Kα1 (8.05 keV) and Cu Kα2(8.03 keV) lines were both resolvableVery good correlation between the twoHOPG crystal spectrometers

2ω Optical Probe

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Distance Along Wire (μ m)

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Original Wire Diameter

Wire Diameter at 400 ps

Probe - Before Shot

Probe - 400 ps

Probe - InterferometryExp

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Optical probe set to 400 ps after main pulse, and has channels for shadowgraphy andinterferometry. This data is being used to reconstruct initial temperature profiles by comparisonwith radiation hydrodynamic modelling results.

Copper Kα Signal Variation with Defocus Distance

Defocus distance shows wherelaser was focused relative to thecone tip, -ve values are focusedbefore cone tip and +ve valuespast cone tipShows that significant amountsof energy are coupled into coneeven in defocused cases,highest Kα yields are fordefocused shots

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Defocus Distance (μ m)-800 -400 0 400 800

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Graph a) shows the variation of the length over which the Kα signal falls from the peak at thecone tip to e−1 of the peak value. The decrease of the scale length in the defocused casesindicates a slightly colder electron spectrum.

In b) a comparison of the HOPG crystal spectrometer signal against the signal from the Cu Kαimager is shown. The signal from the imager is integrated from 50 µm to 300 µm along the wire.Good agreement can be seen between the two Cu Kα diagnostics.

Conclusion

Defocusing of the laser into the cone-wire targets, by up to 800 µm, appears to enhance theenergy coupling to energetic electrons in the wires.

The effect of defocusing may be understood by considering:

Defocusing leaves cone intact during prepulse, no preplasma to interfere with laserLow density preplasma formed could allow for laser self-focusing

Modelling is currently underway to examine the relative importance of the effects. Care must betaken in extrapolating these results to a full scale FI system, as the energy in the prepulse islikely to be much higher.

Ian Bush, Department of Physics, University of York, Heslington, York, UK, YO10 5DD iab500@york.ac.uk