Designing and Fabricating a Proton Beam Source Suitable for Fast Ignition Targets

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Designing and Fabricating a Proton Beam Source
Suitable for Fast Ignition Targets
PERSISTENT SURVEILLANCE FOR PIPELINE PROTECTION
AND THREAT INTERDICTION

• Richard B. Stephens
• General Atomics

9th International Fast Ignition
Workshop Cambridge, MA 3 November 2006
ICFT/P2006-054
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Contributors from a large collaboration

• M Mauldin, E Giraldez,C Shearer
• M Foord, A J MacKinnon, P Patel, R A
  Snavely, S C Wilks,
• K Akli, F Beg, S Chen, H-K Chung, D J Clark,
  K Fournier, R R Freeman, J S Green, C D
  Gregory, P-M Gu, G Gregori, H Habara, S P
  Hatchett, D Hey, K Highbarger, J M Hill, J
  A King, R Kodama, J A Koch, K L Lancaster,
  C D Murphy,, H Nakamura, M
  Nakatsutsumi, P A Norreys, N Patel, J Pasley ,
  H-S Park, C Stoeckl, M Storm, M Tabak, M Tampo, W
  Theobold, K Tanaka, R Town, M S Wei, L
  van Woerkom, R Weber, T Yabuuchi, B Zhang

• This work is from a US Fusion Energy Program
  Concept Exploration collaboration between LLNL,
  General Atomics, UC Davis, Ohio State and UCSD
• International collaborations at RAL have enabled
  the experiments
• Synergy with an LLNL Short Pulse ST Initiative
  has helped the work

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Proton ignition concept has evolved

• Initial concept avoided complexity
• External focusing surface
• Simple proton transport
• Velocity spread cause problems
• Energy must be delivered in short time
• Simple solutions
• Reduce energy spread (M. Hegelich, LANL)
• Reduce separation
• Introduce new problems
• Protection from the imploding shell

Roth et al., Phys. Rev. Lett. 86, 436 (2001)
Atzeni et al., Nucl Fusion 42, L1 (2002)
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Use a reentrant cone for protection

• Protects proton source from coronal plasma
• Limits accelerating surface
• Causes focusing edge effects
• Scatters proton beam

Laser
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Tested concept by making prototype

• Cone dimensions same as for electrons
• 30 full cone opening
• Focusing surface same as for hemi tests
• (existing focal length data)
• rc 170 mm
• dfocus 290 mm
• Limits accelerating area (125 mm dia)
• Target Cu foil - 32 mm thick (29 mg/cm2)
• Stops lt 4 MeV protons

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Proton source area depends on energy

• Accelerating electrons cool off as they travel to
  the edge

Patel et al., Phys. Rev. Lett. 91, 125004 (2003)
Hybrid PIC LSP simulation M. Foord - LLNL 100
fs, 50 mm FWHM Gaussian beam 45 J beam

• 200 mm dia includes most useful protons (flat
  foil data)

Our source will have limited energy output
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Low energy protons are most important to ignition
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• Protons must deliver energy in short time for
  ignition
• limits useful proton energy range

Proton Deposition
Proton Energy MeV
300
30
Power TW
200
20
10
100
Sim parameters Proton spectrum Tp 3 MeV,
dn/deµsqrt(e)e-e/Tp Total proton energy 26 kJ
Proton beam radius 10 mm Source distance 4
mm Target density 400 g/cc
45
65
85
105
125
t ps
Temporal et al., Phys of Plasma 9 3098 (2002)
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Protons are not easily scattered

• The cone tip can be far from the compressed core

• Scattering angle µ E-2
• 3 Mev Protons 5
• 15 Mev Protons 1
• Broadens spot 5-10 mm

5 mm Au
1-5
15
200 mm
End wall scattering is insignificant
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Prototype proton focusing cone was constructed
Construction is feasible
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Initial tests show moderate proton focusing and
heating
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Proton heating is reasonable for conditions

• Ratio of HOPG intensities gives slope temp 1-4
  MeV for protons
• Ka spots have 106 counts - to be compared to
  equivalent shots using full hemi
• Focal spot is rather large - 160 mm
• Could be consequence of side walls changing the
  proton focus.

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Measure focus changes by radiographing grids
Put grids in flat washers for simpler
construction

• Send proton beam through grid and detect with RCF
  stack
• Magnification determines focus position,
  fuzziness of grid shows focus size, number of
  grids show source area
• These experiments are in preparation

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Will use data to design integrated experiments
for Omega EP
Omega EP hydro simulations (S. Hatchett)
Conversion to protons, focusing/ heating?
40 µm
457 µm
PW laser
vacuum
CD2
more compact?
Hi-Z mix?
improve effy?
Blob rR 0.44 g cm-2 ltrgt 120 g cm-3 ltTgt 0.4
keV Total Energy in blob 0.6 kJ

• What is signature of heating, increased
  emission? Ka fluorescence? X-ray scattering?
  neutron production? Abs spectroscopy?

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Laser spot size influences proton focus

• The proton focal spot radius reduces as laser
  focal spot increases

• Trade-off between fully illuminating surface,
  and building edge effect

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Tight laser spot gives aberrated proton focus
X-ray phc image
Gekko PW data
Laser
Proton heating
PW laser
Cu Ka image
Cu Ka image
Cu Ka image
X-
RAL PW data

20mm heated spot