Shock Ignition : an alternative ignition scheme for Hiper

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Shock Ignition an alternative ignition scheme
for Hiper

• Marion Lafon
• Xavier Ribeyre
• Guy Schurtz
• Stefan Weber

6th Direct Drive Fast Ignition Workshop 11th-14th
May 2008 LISBOA
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Motivations

• Baseline Hiper design is
• Cone-in-a-shell target
• Electron driven fast ignition
• Major uncertainties remain unsolved
• Electron production and transport (cf this
  wkshop)
• High convergence cone guided implosions
• High rep operation
• Need for a PLAN B
• Simple, spherical, scalable targets
• Demonstrated control of main physical issues
• ? SHOCK IGNITION

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Outline

• How does it work
• Shock Ignition of Hiper Baseline capsule
• Chronometry
• 1D robustness
• Absorption
• 2D ignition
• Plasma physics issues
• Conclusion

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Basics of shock ignition

• Principle Central Ignition threshold is lower
  for non isobaric fuel assembly
• roughly, EniEiso/F3 DE(F) with FPaniso/Piso
  
• ?assist spontaneous hot spot ignition by
  driving a strong convergent shock before
  stagnation time
• Proposed by J. Perkins , R. Betti as an
  alternative to fast ignition
• SI experiments at Omega produced record areal
  densities and demonstrated dramatic yield
  enhancement
• Numerical simulations predict high gain at the
  200-300kJ level
• Shock Ignition is assisted central ignition, not
  fast ignition

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How does it work

• Classical medium velocity,
• low isentrope fuel assembly
• Drive the ignition shock
• by means of a final spike in
• laser intensity
• The ignition shock collides with the return shock
  and provides the necessary amount of energy to
  trigger ignition from a central hot spot
• RESULTS IN A NON-ISOBARIC FUEL ASSEMBLY

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Fuel conditions at maximum density
Shocked
No shock
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Rationale for shock ignition design

• Pressures must match at shock collision time in
  order to maximize entropy growth
• Collision radius determines the hot spot mass and
  pressure
• Shocks must collide near the inner boundary of
  fuel
• Too large mass at given energy means low
  temperature and no ignition at all
• Too small roR means ignition is not self
  sustained
• The classical hot spot ignition model must be
  modified to account for the pressure ratio
  FPsi/Piso (R.Betti al. PRL 98,155001-2007)

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Shock Igniting HiPER target
1044 µm
DT ice
DT gas
833 mm

• 180 kJ compression
• 70 kJ, 350 ps ignitor
• 20 MJ (TN) G80

M0.59 mg
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Different conditions are met when varying the
shock launching time
10.7 ns Too early ?inhibates shell compression
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Shock launching time
Too late 0 yield
Too early slow burn
On time 20 MJ
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Iso energy curves

• Run series of 1D calculations using radial rays
  and total absorption at critical
• Vary the launching time of the igniting shock and
  the laser power in spike

Slope of cliff consistent with shock velocity
I1/3
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Laser absorption efficiency is the first
identified issue
Delivering 80 TW to the target requires 150-200
TW from the laser

• Temperature at critical exceds 7 keV during the
  spike
• DT absorbs poorly
• Critical radius is half its initial value at
  spike launching time

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Absorption

• Motion of critical surface suggests the use of
  reduced focal spots (zooming ?)
• An alternative is to use dedicated beams with
  specific RPP
• (bipolar shock ignition)

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Using dedicated beam lines

• 48 compression beam lines 12 shock ignition
  beam lines at 32 from polar axis
• 2D ignition simulations (allDT , 200 TW, 3D rays )

Laser absorbed power
500 mm 250 mm
Pressure
7 ns
10.5 ns
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Using dedicated beam lines

• 48 compression beam lines
• 12 shock ignition beam lines at ? from polar
  axis
• 2 D ignition simulations (allDT , 150 TW )

Pressure during shock ignition propagation
T33.2
T0
T54.7
100 ps later
Pressure becomes isotropic In 100 ps Little
sensitive to ignitor incidence
Whathever ? G80
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S.A.s target with a wetted foam ablator
CH(DT)6 DT
1020 µm 950 mm

• Target
• 1D CHIC (MG rad, 3D rays)
• ?Vi250 Km/s,
• rR1.55, rmax570g/cc
• absorption efficiency
• Is strongly enhanced

833 mm
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Experimenting shock ignition on LMJ
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• Use 49 and 59 cones to
• compress fuel
• (cf B. Canaud al. JL Feugeas, this wkshp)
• 49 59 bracket 53.7
• exact cancellation of P2 by tuning
• the balance between cones
• ?0.3 rms non uniformity on target
• Max available energy is 1 MJ.
• ? 300 kJ is routinely achievable
• - safe, low velocity , 1 lt a lt2 implosions
• 20 beams at 32 may provide the
• 120 TW (max is 200) ignition pulse

59 49 32
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Shock production relies on thermal transport in
a quite unusual regime

• Heat conduction greatly helps smoothing out shock
  pressure
• Probably non local
• Magnetic fields likely to be self generated

End of ignition pulse
Start of ignition pulse
gradTe
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LPI issues
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Hot electrons (if any) are welcome
Betti al. FSC Meeting February 28,
2007 Chicago, IL
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a perturbated core still ignites( with a
narrower window)

• According to Rochester simulations 1

80
2D simulations Modes l4-100, NIF 2D-SSD Energy
400kJ Normal Incidence Thomas-Fermi EOS No
radiation
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1D Gain
Gain
40
20
0
FSC Meeting February 28, 2007 Chicago, IL
11.1
11.3
11.5
Ignitor shock launching time (ns)
1 how do they get higher gains in 2D ?
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Summary / Conclusions

• Shocks driven by a 80 (150) TW intensity peak
  ignite the .6 mg Hiper target proposed by Stefano
  Atzeni al. with target gains up to 80.
• Absorption is a critical issue, not drive
  symmetry
• Dedicated, more tightly focused ignition beams
  should be used
• Wetted foam ablators significantly increase
  absorption
• Several plasma physics issues may show up.
• I gt 2.5x1015 W/cm2, long plasmas
• Tegt8 keV, 2D transport is non local, B fields
  likely
• Survives laser imprint in Rochester simulations
• Shock Ignition is an attractive candidate for
  Hiper high rep operation.
• LMJ ( 3 cones ID geometry) is well suited for
  shock ignition experimentation
• Work in progress at CELIA

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