Nonlinear Optics in Plasmas

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Nonlinear Optics in Plasmas
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What is relativistic self-guiding?
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What is relativistic self-guiding?
At
the combinational effect of relativistic
self-focusing and ponderomotive self-channeling
overcomes the natural diffraction, leading to the
self-guiding of the laser pulse.
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What is Raman forward scattering in a plasma?
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What is Raman forward scattering in a plasma?
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Setup for characterizing relativistic
self-guiding and Raman forward scattering
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Setup for characterizing relativistic
self-guiding and Raman forward scattering
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Side imaging of Thomson scattering for various
laser powers
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Transverse profiles of the self-guided laser beam
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Channel length vs. laser pulse duration and chirp
A longer self-guiding channel is observed for
longer pulse duration. Chirp has no effect.
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Breakup of the laser pulse due to a lower contrast
contrast at -1 ns lt5x105
Ne 1.3x1019 cm-3, E 310 mJ
Low contrast produces a longitudinal plasma
density gradient which results in bifurcation of
the laser beam.
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Raman spectra for various plasma densities
The frequency shift is equal to plasma frequency
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Raman spectra for various laser powers
The frequency shift is independent of laser power.
We can conclude that the frequency shift is due
to RFS.
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Dependence of RFS on the chirp of a laser pulse
Raman forward scattering is expected to be
assisted by a positively chirped pulse and
suppressed by a negatively chirped pulse.
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Raman intensity vs. laser pulse duration and chirp
The intensity of the Raman Stokes satellite is
observed to be stronger for positively chirped
pulse in comparison to negatively chirped pulse.
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Raman spectra vs. laser pulse duration and chirp
The narrowing and spread of the Stokes satellite
spectrum for positively and negatively chirped
pulses respectively confirms that Raman forward
scattering is enhanced by positive chirp and
inhibited by negative chirp.
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Setup for producing a collinearly-propagating
second-harmonic probe pulse
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Setup for time-resolving the phase modulation
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Blue shift of the probe pulse wavelength versus
delay
Blue shift of the probe occurs due to the rapid
change of plasma density at the ionization front.
when strong Raman satellite is observed
when no Raman satellite is observed.
(nm)
??
High contrast results in a significant ionization
at within 1 ps prior to the peak of the pulse.
Such an ionization front excites a plasma wave to
seed the growth of Raman forward scattering.
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Summary
Relativistic self-guiding can be enhanced by
increasing pulse duration.
Channel bifurcation of relativistic self-guiding
can be suppressed by raising the nanosecond-scale
temporal contrast.
Raman forward scattering is enhanced by positive
chirp and suppressed by negative chirp.
Raman forward scattering is suppressed when the
picosecond-scale temporal contrast falls below a
threshold of 104.
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Laser-Plasma-Based Electron Accelerator
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Acceleration of electrons by an electron plasma
wave
Raman forward scattering instability can drive an
electron plasma wave with a phase velocity close
to c. Electrons with initial energy higher than a
threshold can be trapped and accelerated by the
plasma wave.
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Use collective Thomson scattering to measure an
electron plasma wave
Scattering efficiency of the probe into the first
Stokes and anti-Stokes satellites is
Scanning the probe delay maps out the the
temporal evolution of the electron plasma wave.
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Thomson satellites vs. plasma density
pump pulse 260 mJ, 275 fs (?-) Dt 0
The frequency shift is equal to plasma frequency
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Thomson satellites vs. probe delay
pump pulse 260 mJ, 275 fs (?-) Ne 21019 cm-3
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Temporal evolution of the amplitude of the plasma
wave
pump pulse 260 mJ, 275 fs (?-) Ne 21019 cm-3
The decay rate of the electron plasma wave driven
by Raman forward scattering is measured to be
about 1 ps-1. Since the plasma wave lasts for
only 1 ps, the macro-bunch duration of the
accelerated electrons should be less than 1 ps.
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Acceleration of electrons by an electron plasma
wave
250 mJ, 55 fs 3x1019 cm-3
The same dependence of the electron number and
Raman satellite energy on laser pulse duration
and chirp proves that the electrons are
accelerated by the plasma wave driven by Raman
forward scattering instability.
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Total number of electrons accelerated vs. plasma
density and laser peak power
3.3x1019 cm-3
250 mJ
55 fs laser focus at the front edge of the gas jet
Above an appearance threshold the total number of
electrons accelerated increases exponentially
with plasma density and laser peak power, and
saturates at gt1x109.
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Divergence of the electron beam
Since the source size should be smaller than the
laser channel size (10 mm), the transverse
emittance is lower than 0.1 p-mm-mrad, better
than that of a state-of-the-art electron gun.
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Low-energy electron spectrum
250 mJ, 55 fs 3.3x1019 cm-3 focus at front edge
The dispersion of the image on the LANEX under
magnetic field identifies that it is indeed from
electrons. The electron energy spectrum can be
fitted into an exponetial decay with a
characteristic temperature of 1.8 MeV
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High-energy electron spectrometer
The existence of electrons of a certain energy
can be confirmed by checking if they indeed move
in the path expected.
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High-energy electron spectrum
250 mJ, 55 fs 3.3x1019 cm-3 focus at front edge
Electrons with kinetic energy up to 45 MeV was
observed. The energy spectrum can be fitted into
an exponetial decay with a characteristic
temperature of 8.5 MeV, a flat region, and a
high-energy cut-off.
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Laser channel and electron beam profile vs. gas
jet position
3.3x1019 cm-3
0 laser focus at the center of the gas jet
400 laser focus at the front edge of the gas jet
The electron beam with smaller divergence angle
is produced when the laser focus is positioned at
near the front edge of the gas jet. It is
correlated with the onset of relativistic
self-guiding.
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Laser channel and electron beam profile vs.
plasma density and laser peak power
55 fs laser focus at the front edge of the gas jet
Once the small-divergence electron beam appears,
its divergence angle shows little variation with
increasing plasma density and laser peak power.
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Laser channel and electron beam profile vs. laser
pulse duration and chirp
250 mJ, 55 fs 3.1x1019 cm-3
The large-divergence electron beam is stronger
for positive chirp, while the small-divergence
electron beam is stronger for negative chirp. The
onset of the small-divergence electron beam does
not correspond to a sudden decrease of the
large-divergence electron beam.
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Summary
The decay rate of the electron plasma wave driven
by Raman forward scattering instability is
measured to be about 1 ps-1 using probing
collective Thomson scattering.
The same dependence of the electron number of the
large-divergence electron beam and Raman
satellite energy on laser pulse duration and
chirp proves that these electrons are accelerated
by the plasma wave driven by Raman forward
scattering instability.
The generated electron beam has a total electron
number of gt109, a divergence angle of 1.8º, and a
maximum electron energy exceeding 40 MeV.
The appearance of an electron beam with small
divergence angle is correlated with the onset of
relativistic self-guiding, possibly resulting
from a direct laser acceleration mechanism.