Title:
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High gain ultra-short laser pulse raman amplification in plasma
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This thesis presents an experimental study of broad-band Raman chirped pulse
amplification in plasma. Amplification. of a short seed pulse occurs through
stimulated Raman backscattering in the presence ofa counter-propagating chirped
pump pulse in a preformed plasma waveguide channel.
300 and 200 Jlm diameter, 40 rom long, hydrogen-filled capillary discharge
waveguides have been investigated. The plasma channel has the dual purpose of
guiding the laser pulses over distances of many Rayleigh lengths and acting· as the
nonlinear medium for the Raman instability. Laser energy transmission through the
plasma channel, in single mode propagation, is observed ~o be better than 80 % for
the pulses with intensities up to 5.8 X 1014 W cm-2.
The dependence of the gain on pump intensity and plasma density is experimentally
studied and confirms theoretical predictions. It is shown that higher pump intensities
inside the capillary can be realised using unmatched guiding due to periodic
oscillations ofthe laser beam waist along the plasma channel.
Amplification of the seed pulse in the linear regime is obtained when the frequency
detuning between the seed and the pump matches th~ plasma frequency. Thus the
frequency difference between the pump and gain peaks enables to estimate the plasma
density, measured around 1018 cm-3.
The high gain regime has been investigated showing that peak gains of over 1200 %
can be routinely realised. A peak gain of almost 3000 % has 'been observed under
optimal conditions with the highest densities and pump energies.
Several diagnostic systems have been developed for simultaneous detection of the
spectral and energy gain together with the duration of the seed. Ultrashort pulse high
gl;lin amplification of 80 fs, 5 mJ pulse has been demonstrated, giving a peak value of
800% and energy gain of 40 % together with 40 % increase in pulse duration.
Amplification ofthe whole seed pulse spectra has been demonstrated using an optical
cut-off filter to provide a sufficient detul1ed seed pulse.
Future increase of the efficiency will occur in the nonlinear regime of the stimulated
Raman backscattering resulting in compression ofthe seed pulse.
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