Multiplebeam fast ignition with KrF laser

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Multiple-beam fast ignition with KrF laser

• István B Földes1, Sándor Szatmári2
• 1KFKI-Research Institute for Particle and Nuclear
  Physics,
• Department of Plasma Physics
• Association EURATOM, H-1525 Budapest, P.O.B. 49.
  Hungary
• 2University of Szeged, Department of Experimental
  Physics2
• H-6720 Szeged, Dóm tér 9. Hungary
• (IAEA F1.30.11 CRP, Contract HUN 13759,
• Hungarian OTKA Foundation K-60531)

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Content

• Laser fusion with KrF lasers.
• 2. Requirements for fast ignition with KrF
  lasers How to reach
• the required intensities with KrF amplifiers?
• 3. The multiple-beam fast ignition Multiple
  beams of 1ps
• duration as alternatives for 20ps FI.

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Advantages of KrF lasersas a fusion driver

• Advantages of short (248 nm) wavelength
• - Less disturbing nonlinearities due to the
  I?2 scaling.
• - Higher critical density, deeper penetration
  into the
• plasma.
• - No frequency conversion needed for short
  wavelength
• in case of KrF lasers The UV
  wavelength may reduce the electric
• energy requirements in case of a fusion
  test facility.
• - A gas discharge laser can provide better
  beam quality,
• higher symmetry in target illumination.
  
• Direct drive compression is possible!

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KrF lasers real alternatives
Properties of KrF lasers - fast relaxation
time (6 ns) - long pumping time (e-beam 100
ns, discharge 15 ns) - to use efficiently
the full energy of the discharge angular
beam multiplexing is necessary - high
reprate possible - e-beam amplifier
lifetime considerably increased -
Electra program 400 J, 5 Hz, 100000 shots
obtained development of preionizer
(hibachi) - IFE 3?108 shots (2 years) needed
with 7 efficiency (Sethian)
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NRL design of a KrF DD laser fusion test facility

• 500 kJ system (2006)
• Number of moduls 32
• Modul aperture 1m2
• Amplified energy of a modul 30 kJ
• Number of angularly multiplexed
• beams 40
• Pump duration 225 ns
• High repetition rate

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Problems with present fast ignitor schemes
Au cones for the direct coupling of radiation
proved to be successful but it is hard to scale
them for a reactor size. The target is very
complicated, and it is difficult to inject them
with 10 Hz rate keeping the correct alignment.
Also the evaporating material will cover the
chamber walls and even the optics. Without a
cone however infrared lasers do not penetrate
deep enough, the velocity of the accelerated
electrons will be too high at the required
intensities, therefore they do not stop inside
the target. Short wavelength may be advantageous
for fast ignition, too!! Hydrodynamical
simulations (R. Betti) The optimal implosion
for fast ignition has low velocity, low adiabat
implosion with a large total mass. Optimal case
lt?gt?300-500g/cc, ? homogeneous. Fusion gain may
increase with a factor of 2, i.e. for a 1MJ laser
50 to 130
for 0.5 MJ
laser it will be 100 (50-200
kJ PW laser)
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Fast ignition and wavelength
The energy of fast electrons can be scaled with
the ponderomotive force, and the penetration
depth is energy-dependent
If Egtgt1MeV, the electron energy will be
significantly larger than the optimum for fast
ignition, the efficiency will be low. Shorter
laser wavelength reduces the average energy of
electrons, the stopping range and thus the
minimum ignition energy. According to the
scaling laws, for the 248nm KrF wavelength
1.8 ?1020W/cm2 intensity is
needed for 1 MeV electrons.
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The KrF fast ignitor
The idea of Zvorykin Using the same KrF
amplifier for the driver laser of ns duration and
for the short pulse PW ignitor laser. KrF
amplifiers When an amplified pulse depletes the
pump energy 2ns is needed for recovering
population inversion. Thus the duration of
electron-beam pumping (gt100ns) allows its
multiple using. This is the basis of multiplexed
amplification. Another property for short
laser pulses there exists a maximum output energy
of 6mJ/cm2 (saturated regime). Independent of
pulse duration (?ltlt2ns). Zvorykin-scheme For 2MJ
KrF system 100 kJ can be obtained by 25-pass
amplification (10-20ps), which takes only 50ns
from the 250ns of pumping time. The beams are
then angularly demultiplexed and focussed onto
the target. Due to the saturation properties of
the KrF amplifiers 6mJ/cm2 can be obtained
either with 20ps or with shorter (1ps) pulses.
Transform limited pulse 100fs.
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Fast ignition at the high repetition rate test
facility
The full aperture here is only 20m2 ? only 1.2kJ
energy/pass. 40 pass 48 kJ, 80 ns duration 80
pass 96 kJ, 160 ns duration. A significant part
of the total pumping time ? either longer pump
or reduced
compressor pulse energy Flexible system
!? Table shows focal diameters needed for 1MeV
electron-energy in case of two pulse durations
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Properties of KrF lasers, 20ps vs 1ps pulses

• 20ps pulses The 20?m focal radii needed for 1
  MeV electron-energy may be obtained. Required
  accuracy A 48kJ system corresponds to 80ns,
  which is 10-4 on 24 m,
• because the accuracy of demultiplexing must
  be some ps, which will be available.
• Problem
• Bandwidth of the KrF laser
  transform-limited pulse 100fs, therefore the
• bandwidth for 20ps is small.
  Amplifier efficiency maybe low, coherence effects
  
• must be suppressed, beam smoothing
  techniques are needed.
• Short pulse amplifiers work in saturation regime.
• The output laser energy is independent on pulse
  duration from 100fs to several 10 ps.
• It is possible to obtain the 48 kJ energy in a
  1ps pulse.
• The power is high, it is enough to combine the
  beams into a spot of 90?m radius.
• But Angular demultiplexing does not work well,
  the pulse duration corresponds to 300?m. The
  wavefronts must be matched as well.
• Interferometric multiplexing is needed, e.g. with
  the method of polarization multiplexing.

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Interferometric multiplexing
The 2 beams have the same path in different
directions, therefore the wavefronts are matched
automatically. The problem is that it is
applicable only for a few beams. For 2 beams 1.7
times energy multiplication obtained. Other
problem It is very difficult to combine the high
number of beams with an accuracy of 300?m on the
target.
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The alternative multiple beam fast ignition
Laser energy at the output of a 1m2 output of an
amplifier after interferometric multiplexing of
2 beams of 1ps pulse duration 120J. Focusing
r8?m ? I2?1020 W/cm2 . This is
sufficient for 1MeV electrons.
400 separately focused beams fulfill both the
energy- and intensity requirements!
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Requirements for multiple beams fast ignition
400 beams focussed separately fulfills both the
energy- and the intensity-requirements. - A
diffraction-limited beam 8?m spot size can be
obtained by f/32 focusing, which covers
10 of the total solid angle. - It is
sufficient to focus the separate beams onto the
target with 10ps accuracy. By changing the
delay between the beams even the pulse-form can
be varied. - The beams can be focused onto
different parts of the target if the corona
implosion model of fast ignition should
work. - In the case of separate spots ?R for the
ignition may be lt 0.1-0.3 but for the high
density isochoric case it may be reached even in
this case. - Alternatively they can be focused
to 1-2 spots with a larger total radius, then
each beam accelerates electrons separately and
they after diverging will ignite
together. - The critical density is still 3
orders of magnitude lower than that of the
fuel. 3-dimensional simulations for
self-focusing, electron- and burn-propagation is
needed.
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Conclusion

• A new fast ignitor scheme using multiple KrF
  laser beams from the driver amplifers is
  considered.
• 2. It is shown that for KrF lasers the short,
  1ps pulses have several advantages as compared
  with the longer pulses.
• 3. Experimental tests are needed for validating
  the interferometric multiplexing for large sized
  beams and for finding the paths for multiple
  amplification.
• 4. The feasability of the proposal and the
  optimal geometry including the number of focal
  spots should be determined by 3D simulations.
• Földes, Szatmári Laser and Particle Beams,
  accepted.