Increase of hot electron production

1
June 16 2008
10th Fast Ignition Workshop Crete Greece
Increase of hot electron production its
behavior under strong static field Kazuo A.
Tanaka Graduate School of Engineering, Osaka
University, 2-1 Yamada-Oka, Suita, Osaka 565-0871
Japan Institute of Laser Engineering, Osaka
University, 2-6 Yamada-Oka, Suita, Osaka 565-0871
Japan 4th talk in morning
ILE
GRE
2
Co-authors.

• Endo, K.1), 2), Habara, H.1), 2) , Honda S.1),
  2), Katayama M.1), 2), Kumar, R. G.3), Kodama,
  R.1), 2), Krushelnek, K.4), Lei, A. L.5),
  Matsuoka, T.4), Mima, K.1), Nakamura, T.1), 2),
  Nagai, K.1), Norimatsu, T.1), Sentoku, Y.6) ,
  Tanimoto, T.1), 2), Yabu-uchi, T.1), 2)
• 1) Graduate School of Engineering, Osaka
  University, Suita, Osaka 565-0871Japan
• 2 Institute of Laser Engineering), Osaka
  University, Suita, Osaka Japan
• 3) Tata Institute of Fundamental Research,
  Mumbai, India
• 4) University of Michigan, Center for Ultrafast
  Optical Science, MI USA
• 5) Shanghai Institute of Optics and Fine
  Mechanics, Shanghai, China
• 6) Department of Physics, University of Nevada,
  Nevada U.S.A

3
My presentation will include

• Foam or low density target can increase the
  coupling efficiency for hot electrons.
• Hot electrons are subject to strong E B fields
  when they leave a target.

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Introduction

• Gold cone was used to guide a fast heating laser
  pulse in order to heat a highly compressed plasma
  core up to 1 keV.
• 30 coupling efficiency was indicated in the
  experiment from the heating laser to the core.
  Based on this high efficiency 10 kJ PW laser is
  now under construction to test even higher fast
  heating temperature up to several keV at the
  Inst. Laser Engineering. This should be close to
  fast ignition.
• Important issues are to understand the physics of
  this high efficiency, to increase further the
  efficiency and to study the both electro-static
  potential formation at around the target
  affecting on the relativistic hot electron energy
  transport.

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Compression and heating can be separated in fast
ignition.
ILE Osaka
Compression by multiple laser beams
Heating by ultra-intense laser pulse
Ignition Burn
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Gold foam inside the cone

• To increase the heating efficiency of the core
  plasma, we propose a foam cone-in-shell target
  design.

Gold cone with inner tip covering with a foam
layer
Relativistic laser
Fuel shell
Multiple implosion beams
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Element experiment demonstration of the
improvements of the foam-in-shell target design
for fast ignition

• ILE target group are now fabricating the foam
  cone and foam cone-in-shell target.
• We used planar targets in the element
  experiments. Planar configuration does not change
  the physics behind the cone tip.
• -to measure e yield
• -to measure e temperature
• -to measure e beam divergence

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Element experiment 1monitoring the heating of
the target rear and measuring e energy spectra

• Target types 20um Mo with front surface coating
  with 2mm thick solid Au or 2mm thick 20 solid
  density Au foam
• -micro-structured targets (nanoparticles, foams,
  etc) experimentally demonstrate high laser
  absorption. Expected more hot electrons
  generated.
• -gold foam used (expected) to avoid severe
  suppression of hot e transport in high-Z thin
  foam.
• Planar targets used
  gold foam material

K. Nagai et al., Fusion Sci. Tech. 49, 686
(2006)
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Element experiment 1monitoring the heating of
the target rear and measuring e energy spectra

• Experimental setup
• -GXII PW laser 0.6ps/1.053um/100J on
  targets/70um focus/OPCPA 10-8 contrast
  ratio/f7.6/p-pol/26deg incidence
• -planar targets 2um Au20um Mo, and 2um Au
  foam20um Mo
• -front XPHC 18um size pinhole/40um Be filter/KeV
  x-ray range/M8.6
• -back XPHC 200um size pinhole/40um Be/KeV x-ray
  range
• -ESM along the laser axis, energy range
  1100MeV.

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Au foam coating enhances laser absorption and hot
electron generation

• Hot -e yield measurement via the back x-ray
  emission from the target rear due to the heating
  from hot e beams
• -weak front x-ray emission from the Au
  foam-coated target. This is due to the low
  density of the foam.
• -stronger back x-ray emission from the Au foam
  coated target. This is attributed to higher laser
  absorption and more hot electrons generated with
  the foam coated target. Back x-ray emission is
  caused by the hot e beam heating of the target
  rear.
• -target is thick so that the front x-ray emission
  may not be responsible for the enhancement of
  back x-ray emission with foam coated target.
  Moreover, if it happens, one would expect weak
  x-ray emission from the foam coated target rear,
  contrary to the experimental results.
• -narrow band-width x-ray image diagnostics
  needed to give the relative hot e yield through
  assuming Plankian emission from the target rear.
  
• -quantitative models and simulations needed

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Au foam coating does not change the hot
electron energy spectral characteristics

• Hot -e energy spectra are very similar for solid
  gold coated and gold foam coated targets, showing
  a temperature 1.5 MeV, a typical value for solid
  aluminum targets
• There is a question why there is no comparable
  increase in the amount of hot electrons observed
  with Au foam coated target?
• In vacuum electrons escaping from the target is
  fully limited by the static potential.
• T. Yabu-uchi et al., submitted to Phys. Rev. E.

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Carbon nano tubes are grown on Ta target by CVD.
CVD Electron beam deposition make Fe/Al layer on
surface. The surface is treated by CVD with He
/acetylene (gas). at 650 degrees.
Collaboration with Prof. M. Katayamas laboratory
Osaka University.
SEM photo of CNT grown normal to surface. 1010
CNT/cm2.
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Ultra-intense laser irradiates carbon nano tube
on Ta substrate.
Target
Target CNT (Length 60um?SubstrateTa 10umt) W
(35umt)
Incident Laser
g-ray detector
X-ray diode
Diagnostics X-ray diodemonitor x-ray
emission NaIScintilatormonitor g-ray emission.
Collaboraiton w/ CUOS Michigan
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Carbon nano tube is irradiated at 1019 W/cm2
indicating increase of hot electron generation.
Gamma ray Intensity (mV/TW/mm)
X-ray Intensity (mV/TW/mm)
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Electron spectrum and spatial distribution are
measured of CNT.
Target CNT (Length40um?SubstrateTa 10umt) Al
(10umt)
(???????)

• Diagnostcis
• ESM monitor electron spectrum
• Stack monitor electron proton spatial
  distribution.
• X-ray pinhole cameramonirot x-rays.
• Interferometer monitor prepulse.

Experiment_at_ GM2
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CNT electrons are strongly scattered by Ta.
Angle
Simulated scattered angle on Al Ta.
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Electron spectrum for CNT- Ta target
Electron/MeV/Sr
Energy (MeV)
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E B fields formation also strongly affect
electron behaviors.
Intense laser creates MV/mm E and tens of
MGauss B fields at the target rear.
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Measured electron number isalways less than
produced.
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Target rear plasma is created using another laser
beam to control the rear sheath potential.
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Electrons are generated more with rear plasma
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Factor 2-3 increase with rear plasma
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Retardation time is determined with rear plasma
capacity.
nplasma l Nmax mm/cm3 Total Number
Electr.
.
nhot c Ncmax mm/cm3s Hot Electr. Flux
Total No. Electr.
Tretard.
Hot Electr. Flux.
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PIC simulation set up
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1-D PIC shows retardation of potential growth for
rear plasma case.
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Analytical line fits well with PIC results.
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Net electron increase of factor 2-3 consistent
with Alfven limit.
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Summary

• Au foam target increased a plane target heating
  efficiency.A.L.Lei, K.A. Tanaka et al., Phys.
  Rev. Lett., 96, 255006(2006).
• Carbon nano tube appears to increase the
  laser-hot electron coupling.
• Electro-static potential formation is studied to
  understand hot electrons leaving from a
  target.T. Yabu-uchi, K.A. Tanaka et al., Phys.
  Plasmas 14, 040706 (2007)

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PIC shows clear difference of E field formation.
w/rear plasma
wo/rear plasma