Measurements of AblatorIon Spectra for Preheat and Compression Studies

1
Measurements of Ablator-Ion Spectra for Preheat
and Compression Studies

• N. Sinenian, J.A. Frenje, C. Li, F.H. Séguin, R.
  PetrassoMassachusetts Institute of Technology
• J. A. Delettrez, C. StoecklLaboratory for Laser
  Energetics, University of Rochester, NY
• American Physical Society50th Annual Meeting of
  the Division of Plasma PhysicsFriday, November
  21, 2008

2
Abstract
Measurements of ablator-ion spectra produced at
the OMEGA laser facility are presented. The
mechanism responsible for the acceleration of
these ablator ions is the presence of hot
electrons generated by laser-plasma interactions.
The effect of hot electrons on implosion
performance was modeled using the 1D hydra code
LILAC. The results of those simulations are
compared to measured areal density of several
types of capsule implosions. Two magnetic
charged-particle spectrometers have been used for
the measurements of the ablator-ion spectra, but
this technique does not distinguish between fast
particles with low charge states and slow
particles with high charge states making the
measurement ambiguous. To break this degeneracy,
we propose to use a Thomson Parabola Spectrometer
that allows for accurate measurements of absolute
ablator-proton spectra and equally important,
energy spectra for various charge states of
higher-Z ablators such as Carbon.
3
Preheat of fuel in Inertial Confinement Fusion
(ICF) targets may affect compression of fuel and
hence confinement quality
2. Laser generates hot electrons faster, more
mobile electrons escape capsule and
accelerate ablated ions with them
1. Low-a laser drives are desired to compress
target along a low adiabat so as to achieve a
high compression ratio
4. Trapped electrons may preheat the fuel
during the compression stage, leading to a
reduced ?R and hence lower confinement
quality
3. Escaping electrons leave behind a potential
well in fuel which traps slower, but
nevertheless energetic electrons in the fuel
4
Laser-plasma interactions are responsible for the
generation of hot electrons a fraction of these
electrons become trapped in the target and may
significantly preheat the fuel

• Laser-plasma interactions generate a hot
  electron population (Thot 60-140keV) in
  addition to the usual thermal plasma
• Temperature of hot electrons is well
  correlated with laser intensity and wavelength (I
  ?2), and determines the amount of capsule
  charging for a given shot

5
Measurements of the fast ablator ion energy
spectra along with a simple target charging model
are used to infer the hot electron temperature
and quantify preheat effects
Sheet potential (F )Ref.3
Potential
Isotropic source of hot electrons with temp
(Thot )
Escaping electrons
Ee lt eF
Ee gt eF
3-D Maxwellian fhot(E)
Potential (F )
LILAC profile _at_ 1ns (35004)

• 1. J.S. Pearlman et al., Applied Phys. Lett. 31
  (1977) 414.
• D.M Villeneuve et al., Phys. Fluids 27 (1984)
  721.
• D.G. Hicks et al., Phys. Plasmas 12 (2000) 5106.

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Deuteron ablators from cryogenic targets and
proton ablators from CH-shell targets carry
approximately 125 J and 20 J of energy,
respectively, at laser intensities of 1015 W/cm2
7
Spectra of carbon and proton ablator ions were
measured with magnetic charged particle
spectrometers and for a DT(15)CH20 implosion at
intensity of 1015 W/cm2
Proton ablators
Carbon ablators
60 J (meas)
1 J (meas)
0.7 MeV
Assumed C(1)
The carbon ablator yield is typically 10 times
greater than the proton ablator yield. Carbon
ions carry significantly more energy than protons
8
Thot inferred from the proton ablator data is in
the range of 60-140keV and shows good agreement
with Thot inferred from the hard X-rays
A simple implosion model linking Thot to the
amount of preheat may be used to compare ?R
variations directly with the amount of energy
deposited in the fuel
9
Utilizing the Li-Petrasso electron stopping power
modelRef.1-2 electrons were transported through
LILAC density profiles to determine the energy
deposition in the fuel and shell low energy
electrons were found to deposit a fraction of
their energy into the shell
The duration of the maximum potential
F is 200 ps at the end of the laser
pulseRef.3 this is the window for preheat by hot
electrons. This timeframe seems consistent with
the timing of the hard X-ray productionRef.4
1. C.K. Li and R.D. Petrasso et al., Phys. Rev E
73 (2006) 016402. 2. C.D. Chen et al., Journal of
Applied Phys. accepted (2007). 3. D.G. Hicks
et al., Phys. Plasmas 12 (2000) 5106. 4. C.
Stoeckl et al., Phys. Rev. Letters 90 (2003)
235002.
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The simple model applied to experimental data
shows no degradation of ?R with increased
electron energy deposition in CH capsules LILAC
simulations with and without the presence of hot
electrons support these conclusions
?R Shell vs. Preheat
Low energy electrons were found to deposit some
energy into the shell during the 200ps window
whereas the fuel remained unaffected. LILAC
simulations with and without hot electrons showed
no degradation of shell ?R experimental data is
normalized to this 1-D value
No significant degradation of ?R is demonstrated
for 20 - 40J of preheat in the CH shell
11
Any detailed modeling of the effect of electron
preheat must include quantitative info on any
fast ions, including spectra and total energy
Total Ablator-Ion Energy
LILAC simulations indicate that up to 2 kJ goes
into the ions for Cryo D2 at 1015 W/cm2.
Observations are about and order of magnitude
smaller for deuterons and protons carbon
ablators carry significantly more energy
12
In CH-capsule implosions, Carbon ablator ions are
produced and can carry energies of order 200-800J
Carbon charge states cannot be resolved a
single spectrum is obtained for Carbon, and then
assumed to be entirely C1, C2, etc.The
maximum proton energy is 0.8MeV for this shot.
Thus, it is expected that C1 would have a
maximum energy of 0.8MeV, C2 a maximum of 2 x
0.8MeV 1.6MeV, C3 a maximum of 2.4MeV, and so
on. Using this reasoning, higher charge
states of carbon (i.e. C3 and higher) are not
possible the carbon ablators are thus dominated
by C1 and C2
13
Questions to be addressed

• Do the carbon ions and protons experience the
  same potential?
• Do the carbon ions recombine with electrons along
  their way to the spectrometer (through plasma and
  vacuum)?

14
Fast ablator ion energy spectra are measured with
charged particle spectrometers (CPS), leading to
uncertainties in the energies of the heavy ions
and hence the ablator-spectra inferred Thot
Fast ions from target
CPS Magnet (7.6kG)
30MeV
10MeV
1MeV
3MeV
Filtering is used to remove heavy ion species
from protons or deuterons, but charge states of
heavy ions are not accurately known
Cannot resolve degeneracies between charge states
and energies of incoming ions (e.g. high energy,
low charge state vs. low energy, higher charge
state)
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Resolution of carbon charge states and accurate
measurement of the total energy carried by
ablators may be obtained with the use of a
Thomson Parabola
Individual energy spectra may be obtained for
various charge species (for each charge to mass
ratio)
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Ablator protons and carbon ions can be resolved
D and T parabolas overlap with C6 and C4 these
ions may be separated based on track size
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Summary

• Charged particle spectrometers are used to
  measure ablator ion spectra the presence of fast
  ions indicates a significant hot electron
  population, Thot.
• Carbon ablator ions (CH) carry away anywhere from
  200-800J of the laser energy whereas protons (CH)
  carry 20J deuteron ablators (cryo) typically
  carry 100J
• Carbon ablator ion spectra suggest that higher
  carbon charge states (C3, C4, etc) are
  unlikely a more sophisticated analyzer such as a
  Thomson Parabola may be used to break the
  magnetic charged particle spectrometer degeneracy
  and obtain accurate measurements of high-Z
  ablator spectra.
• Thot may be inferred from the ablator ion
  spectra and is shown to be in the range of
  60-80keV, in good agreement with Thot inferred
  from hard X-rays
• ?R does not degrade with respect to inferred Thot
  in the range of 60-80keV, which corresponds to
  preheat in the amount of 15-40J as determined by
  the Li-Petrasso electron stopping power model.
• LILAC simulations both with and without hot
  electron generation and transport support these
  findings