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C3 SBS Experiments

Julien Fuchs

Julien Fuchs is an experimental physicist in the field of high power laser-plasmas working on fusion plasmas, both with long and short pulses, particle acceleration, plasma optics and magnetized laboratory astrophysics. He obtained an engineering degree in optics in 1992, and worked in the industry before going into research for a Ph.D. he received from Université du Québec (Canada) in 1998. He joined CNRS (France) in 1998 and is now senior researcher at the LULI laboratory. He also taught at Université d’Orsay and Ecole polytechnique (France).

The C3 project [1], managed by J. Fuchs relates to the use of plasma optics for high-intensity beams. This includes extending the use of plasma mirrors to beam transport and focusing, as well as the use of plasma as an amplifying medium. It aims at overcoming the limitations of solid-state optics by using plasmas, where the damage thresholds are much higher by 3-4 orders of magnitude.

A plasma amplifier is based on the interaction and energy exchange between a long moderate-intensity pump pulse providing the energy, and a short less energetic seed pulse that is amplified. This energy amplification is made possible due to the coupling provided by the response of the plasma medium. Many other workgroups, including some affiliated to IZEST, use a plasma electron wave to provide the plasma response (Stimulated Raman Backscattering, SRS). In contrast, research at LULI (Ecole polytechnique, Palaiseau, France), ITP1, and ILPP (both Heinrich-Heine University, Düsseldorf, Germany), focuses on studying the Brillouin Backscattering mechanism in the regime of strong coupling (sc-SBS). Here, an ion acoustic wave is excited by the pump pulse and gives rise to seeded backscattering [2]

In the framework of this cooperation, the C3 project maintains the only workgroup worldwide studying sc-SBS in the amplification both in theory and experiment.

Plasma amplification by stimulated Brillouin backscattering in the strong coupling regime

Amplification by sc-SBS has been studied experimentally at LULI since the efficiency of this scheme has been first analyzed in theory by Andreev et al. [3]. Already in 2007, it could be experimentally demonstrated that this mechanism is suited to amplify a 400 fs few mJ seed pulse. It could be proven that the sc-SBS amplification partially compensated the losses of the seed in the plasma [4].

This report describes progress in experimental research on amplification of short laser pulses by sc-SBS obtained by LULI and ILPP in 2014/2015, aimed both at providing a better understanding of the process and at obtaining a higher amplification.

Self-similar regime

In an experiment in 2013, we have achieved absolute amplification for the first time. This is a major improvement over the proof-of principle experiment, since amplification completely overcame the losses in the plasma and even amplified the pulse by a factor of 4. This has been attributed to an optimized overlap of the pulses in the plasma. Additionally, further data analysis in 2014 and 2015 revealed that the self-similar regime of sc-SBS, necessary for obtaining a high gain, indeed has been reached. This could be concluded from the scaling behavior with respect to plasma density and interaction zone length, and from broadening of the spectrum [4]

Role of the chirp

Following independent theoretical analyses of ITP1 [5], we tried to investigate the role of the chirp experimentally.

A beam time on SBS of 0.7 ps, 4 mJ seed pulses has been carried out at the ELFIE laser system (January 12 to February 13, 2015). The chirp of the pump beam was systematically changed to investigate its impact on the amplification. The experiment allowed to reproduce the amplification (in terms of total pulse energy) obtained by the workgroup in a 2013 experiment. However, the pulse duration after interaction was significantly longer.

Role of the seed intensity

Amplification of high energy seed pulses had been studied previously by our group [6].

Following this, an experiment has been carried out at ELFIE (July 13 to September 4) on sc-SBS of higher energy seed pulses (0.7 ps, 35 mJ ...3.5 J). It showed some amplification for intense seed pulses. Data are still subject to analysis

SBS amplification of ultrashort pulses: Cooperation ILPP-LULI

A third experiment on SBS of ultrashort pulses (15 mJ, 30 fs ... 200 fs) has been done from Sept. 21 to Oct. 23 2015 at the ARCTURUS laser facility (ILPP, HHU Düsseldorf). It was carried out to provide some insight into the SBS process at lower seed durations than at ELFIE (i. e. < 700 fs), especially between 30 and 300 fs. Both simulations [7] and analytical theory (the dispersion relation for the strong coupling regime) predict amplification to be possible for pulses longer than 80 fs only.

In the cooperation between LULI and ILPP, this experiment was the first to take advantage of the new high-energy multi-beam capability of the ARCTURUS laser. After an upgrade in 2013/2014, the laser provides three beams (3 J, 3 J, 170 mJ before compression, all can be compressed down to 35 fs), one of only few lasers combining several beamlines for ultrashort pulses. After a preparation and test phase, needed to optimize laser parameters, we could do an experiment in a fully counterpropagating setup, using a 3 ps, 700 mJ pump and a 30 ... 300 fs, 13 mJ seed. Similarly to the 2013 LULI experiments, both beams were focused using long focal length optics to obtain pump intensities on the order of 1016 W/cm2 and seed intensities of 1013 ... 1014 W/cm2. Preliminary results hint at amplification happening for longer pulses rather than for shorter ones, in agreement with the theoretical predictions.

Data from the 2015 beam times are still subject to analysis to the date of this report.

References

[1] Mourou, G. A ,  Fisch, N. J. Malkin, V. M. ; Toroker, Z.; Khazanov, Khazanov, E. A. ; Sergeev ;Tajima, T, Le Garrec, B.•Exawatt-Zettawatt pulse generation and applications, OPTICS COMMUNICATIONS  Volume: 285, 5,  720-724  (2010)

[2] Frank, J. Fuchs, L. Lancia, G. Lehmann, J.-R. Marquès, G. Mourou, C. Riconda, K.H. Spatschek, T. Toncian , The use of damage-less optics for high laser intensities EPJ ST223, 1153 (2014)

[3] Andreev et al., Phys. Plasmas 13, 053110 (2006)
[4] L. Lancia et al. PRL, 104, 0250019, 20013
[5] G. Lehmann and H. Spatschek, Control Brillouin Short- Pulse Amplification by Chirping the Pump Pulse, Phys. Plasma 22. 043105 (2015)
[6] C. Riconda, 1 S. Weber, L. Lancia, J.-R. Marques, G. A. Mourou, and J. Fuchs, Spectral characteristics of ultra-short laser pulses in plasma amplifiers, Phys. Plasma 20, 083115 (2013)
[7]S. Weber, C. Riconda, L. Lancia, J.-R. Marquès, G. A. Mourou, and J. Fuchs, Amplification of Ultrashort Laser Pulses by Brillouin Backscattering in PlasmasPhys. Rev. Lett. 111, 055004, (2013)