Demonstration of Detuning and Wavebreaking Effects on Raman Amplification Effciency in Plasma Page: 4 of 30
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I. INTRODUCTION
Powerful short laser pulses have wide applications in physics, including wake field accel-
erators [1], fast ignition scheme of fusion [2], compact x-ray lasers [3], and compact terahertz
sources [4]. Currently, short powerful laser pulses are generated using the chirped pulse am-
plification (CPA) technique. The CPA scheme is limited by fluency and intensity constraints
on the solid state gratings. Currently, meter-scale gratings are used to produce PW-scale
laser pulses.
Plasma-based laser amplifiers have been proposed as a means to circumvent the limits
on the output power. One of the leading candidates for a next generation laser sources is
the resonant backward Raman amplifier (BRA) [5]. In this scheme two counter-propagating
laser pulses (which are called "pump" and "seed") interact with each other through the
plasma wave, which is excited by the pondermotive force of beat wave. During the three-
wave interaction the long pump transfers energy to the short seed pulse.
According to Manley-Rowe relations, the ratio of pump energy which can be transferred
to the plasma wave is the ratio of the plasma to laser frequencies. This ratio is about a
factor of 0.1 for typical parameters of BRA, w/cup - 1/10. That means that almost all the
energy of the pump (90% for w/cup = 1/10) can be consumed by the seed pulse, while only
10% of the pump energy is absorbed to generate the plasma wave. [5-7]. Until recently,
the experimentally achieved efficiencies were at most on the order of one percent [8-12].
However, recent experiments [13, 14] showed significantly enhanced amplification efficiency.
This efficiency, on the order of several percent, remains much smaller than the theoretical
maximum. Theoretical studies suggest a number of the mechanisms which may limit the
efficiency of the scheme. The possible limiting mechanisms include resonance detuning
from either a plasma density gradient or pump chirp [6], forward Raman and Brillouin
scattering [15], amplified pulse self-focusing and self-phase-modulation [6, 16], small-scale
density perturbations [17], the amplification of precursors [18], plasma wavebreaking [6, 19],
thermal effects [20-22], and the ionization of low levels of the plasma ions [23]. The possible
importance of the detuning and the wavebreaking in BRA experiments has been noted
in earlier experiments [9]. However, the experimental data alone until now has not been
sufficient to fully identify which effects, in fact, play the dominant role.
While the various factors mentioned above can all limit performance, the recent observa-
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Yampolsky, N. A.; Fisch, N. J.; Malkin, V. M.; Valeo, E. J.; Lindberg, R.; Wurtele, J. et al. Demonstration of Detuning and Wavebreaking Effects on Raman Amplification Effciency in Plasma, report, November 6, 2008; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc896136/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.