High intensity laser coupling to a cone geometry for fast ignition
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??. Laser. High Intensity Laser coupling to a cone geometry for fast Ignition. R.B. Stephens General Atomics. 10 th International Workshop on Fast Ignition of Fusion Targets Hersonissos, Crete, Greece 12-18 June 2008.

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High Intensity Laser coupling to a cone geometry for fast Ignition

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High intensity laser coupling to a cone geometry for fast ignition

??

Laser

High Intensity Laser coupling to a cone geometry for fast Ignition

R.B. Stephens

General Atomics

10th International Workshop on Fast Ignition of Fusion Targets

Hersonissos, Crete, Greece

12-18 June 2008

This work was performed under the auspices of the U.S. Department of Energy under contracts No.DE-FG02-05ER54834 and Contract DE-AC52-07NA27344.

IFT\P2008-068


Collaborators

Collaborators

K.U. Akli

L.V. Van Woerkom, R.R. Freeman, E. Chowdhury, D.W. Schumacher, D.T. Offermann, A. Link, V.M. Ovchinnikov

F.N. Beg,T. Ma, S. Chawla, T. Bartal, M.S. Wei, J. King,J. Pasley

A.J. MacKinnon, A.G. Macphee, M. H. Key, H. Chen, R. Town, M. Foord, S. P. Hatchett, A.J. Kemp, A. B. Langdon, B. F. Lasinski, P. K. Patel, M. Tabak, T.H.Phillips

D. Hey

C. Chen, M. Porkolab

Y. Tsui

H. Habara, R. Kodama, H. Nakamura, K. Tanaka, T. Tanimoto

R. C. Clarke, J. Green, K. Lancaster, P. Norreys, D. Neely


A reentrant cone defines the laser plasma interface

A reentrant cone defines the Laser Plasma Interface

  • Ignition parameters

    • Laser: I ~ 1020 W/cm2,  ~ 10-20 psec, E ~ 100 kJ

    • Cone: tip, fuel size ~ 40 m

  • Must generate electrons that will stop at the core

    • Range < 1.5 g/cm2

    • Forward directed

  • How does the cone affect electron parameters?

    • Energy spectrum

    • Conversion efficiency

    • Directionality

Atzeni et al., POP 05702 (2007)


Cone could have several effects

Cu K imager

Spectralon

  • Tested with flat foils

  • Tested with thin-walled Cu cones

Cu K imager

Cone could have several effects

Good

  • Shape to focus light to tip

  • Walls to guide electrons to tip

    Bad

  • Confinement enhance effects of preplasma

75°


Reflectivity moderate at glancing incidence

S pol - 10 mAl/25mCu/1mmAl

P pol - 10 mAl/25mCu/1mmAl

60

S pol - 25mCu

50

40

30

0

40

80

120

160

200

240

20

10

0

Reflectivity moderate at glancing incidence

Specular Reflectivity, %

 = 75°

  • Reflectivity apparently decreases with increasing power

  • Difference between S- & P- polarizations within errors

 = 28°

Laser Peak Power, TW


Reflected light is partially scattered

3000

super-Gaussian fit tp reflected beam

2500

2000

Signal, arb. units

1500

1000

f/3 input beam

500

0

0

50

100

150

200

250

Distance, mm

Incident

Reflected

Incident

Reflected

Reflected light is partially scattered

  • Input beam is f/3, reflected beam f/2

  • Mask shadow disappears in reflected beam

  • Light focusing with cone wall has limited benefit


E generation at glancing incidence is weak and localized

Eprepulse~20 mJ

Eprepulse~400 mJ

20070830 s04 ka1

20070830 s03 ka1

20070827 s01 ka2

Near Normal Incidence

Glancing Incidence

e- Generation at glancing incidence is weak and localized

  • K production substantially reduced so e- generation is very weak

  • Fluorescence shows no electrons forward from beam

  • Not changed by pre-plasma

  • e- generation on cone wall not useful

Color scale shifted 7x


Any significant electron flow would be visible

4.0

3.5

3.0

2.5

2.0

1.5

Relative Inner shell ionization cross-section

1.0

0.5

3

2

1

4

10

10

10

10

Kinetic Energy (keV)

Any significant electron flow would be visible

H. Habara et al., Phys. Rev. Lett 97, 095004 (2006)

  • A few electrons been observed leaving target

    • Forward directed surface electrons detected with electron spectrometer.

    • Depends on target size - preplasma expansion can increase numbers.

  • Most electrons are trapped on the target

    • Losing 2x1011 electrons charges 1/2 mm sphere to MV

    • If all MeV electrons, loss of 30 mJ ( < 0.1% absorbed energy)

    • Current flowing up support stalk insignificant ~106 electrons

  • All trapped electrons can be seen

    • Target only 25 m thick (~ K abs length)

    • Scattering cross section ~ independent of energy

  • No significant e flows along the surface

T.Yabuuchi et al., Plasm & Fus Res 1,1 (2006)


E generation at glancing incidence is weak and localized1

Eprepulse~20 mJ

Eprepulse~400 mJ

20070830 s04 ka1

20070830 s03 ka1

20070827 s01 ka2

Near Normal Incidence

Glancing Incidence

e- Generation at glancing incidence is weak and localized

  • K production substantially reduced so e- generation is very weak

  • No significant electrons forward from beam

  • Not changed by pre-plasma

  • e- generation on cone wall not useful

Color scale shifted 7x

=> And best strategy is to focus light on cone tip


Imaged k fluorescence of cu cones shows electrons spreading unlike foils

20070823s03

700 m

But electrons spread a few hundred m up the cone

Tight focus at tip

Imaged K fluorescence of Cu cones shows electrons spreading unlike foils

Laser focus is smaller than flat cone tip -- should expect electrons concentrated at the tip

L. Van Woerkom et al., Phys. of Plasmas 15, 056304 (2008).


3x higher k emission from cones compared to foils

Cones

3X

Slabs 28° incidence

1011

Slabs 75° incidence

Yield (Ph/J/sr)

1010

109

1017

1018

1019

1020

1021

Peak intensity (W/cm2)

3X higher K emission from cones compared to foils

Where are they generated and which are useful?


E distribution in the cone not sensitive to focus

e- distribution in the cone not sensitive to focus

20070504s06

20070504s05

20070504s04

900 mm behind

450 mm inside

450 mm behind


Electron locations independent of focus

Cu K1

Electron locations independent of focus

  • Probably caused by plasma filling during prepulse

    • ~10-20 mJ during ~2 ns

  • Cones act like a ~300 m deep bucket for energy coupling

    • Electrons are free to travel within that space

  • Adding more prepulse energy deepens the bucket

    • 1 Joule of prepulse nearly fills it.


Electrons do couple to the tip

1 mm

Cu K - 8 keV

XUV - 256 eV

Electrons do couple to the tip

  • Previous shots at Vulcan showed similar plasma filling effects

    • Indication of heating (XUV) partway up the cone independent of focus location

  • And still strong coupling to electrons in wire

    • 15% for 40 m dia wire


Plans for integrated e generation tests this summer

Plans for integrated e- generation tests this Summer

  • Previous experiments did not have sufficient information

    • Prepulse and focus now measured on each shot at Titan

    • Prepulse reduced to ~ 3 mJ

  • Cone wire - e coupling into wire

    • Sensitivity of coupling to prepulse energy

  • Buried cones - electron generation

    • Thick walls simulate blow-off plasma

    • Image buried layer fluorescence to see electrons directions

  • Bookend - (2D cones)

    • Study confined plasma


Maximize efficiency by good focus and limited prepulse

Maximize efficiency by good focus and limited prepulse

  • Modest reflection at glancing incidence, minimal electron generation

  • Low reflection at normal incidence, maximum electron generation

  • No significant electron flow along surface

  • Maximize light intensity at tip with no reflection

  • Reflections just to add wings of focus in

  • Compound Parabolic Collector has the right properties

    • Single bounce gets all light to the tip

    • Concentrates light proportional to f/number

  • Possible improvement limited

    • Reflectivity ~ 50% - only use reflections to gather in wings of focal spot

    • Effective cone length limited by forward scattering - < 100 m

    • Near-critical plasma fill can refocus/filament light

  • Must limit prepulse?

    • 20 mJ filled cone 300 m deep

    • But current still coupled to wire

    • What’s the tolerable limit??


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