Acta Polytechnica Acta Polytechnica 53(2):246–248, 2013 © Czech Technical University in Prague, 2013 available online at http://ctn.cvut.cz/ap/ STIMULATED RAMAN BACKSCATTERING IN PLASMA — A PROMISING TOOL FOR THE GENERATION OF ULTRA-HIGH POWER LASER BEAMS Hana Turčičováa,∗, Jaroslav Huynha,b a Department of laser interactions, Institute of Physics of the Academy of Sciences of the Czech Republic, Prague, Czech Republic b Department of physical electronics, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Czech Republic ∗ corresponding author: turcic@fzu.cz Abstract. In the last fifteen years stimulated Raman backscattering (SRBS) in plasma has been intensively elaborated as a promising tool on the way towards high intense lasers. There are several advantages of this technique in comparison to the world-wide used the CPA-Chirped Pulse Amplification technique for a laser amplification. We present the principle of the SRBS technique, the best results so far obtained in theory and experiment, and a possible SRBS project at the PALS Research Centre in Prague. Keywords: stimulated Raman scattering, laser plasma, high intensity lasers. 1. Introduction Since the very beginning of the laser performing there has been a trend for the laser pulse shortening and power increasing. The first techniques manipulat- ing the pulse duration were Q-switching and mode- locking. Nanosecond duration has been achieved and further optimization of laser devices has led to en- ergy increase and output pulse intensity growing. In the middle of 80s, a revolutionary technique ap- peared [13], which profited from the results of [6] and that from a radar pulse coding and compres- sion [3]. The technique is known as CPA – Chirped Pulse Amplification which enabled the amplification of ultra-short pulses up to a femtosecond range. How- ever, a prerequisite for using this technique is a suffi- cient spectral broadness of the laser pulse being ampli- fied. The pulse is first spectrally swept in space, and thus prolonged in time (in a tretcher). Its intensity is now much lower and in the forthcoming amplify- ing medium the pulse will be amplified without any danger of an optical damage of the medium. Having gained energy, in the following, the pulse has to be optically compressed to its original duration (in an op- tical compressor). Both stretcher and compressor contain diffraction gratings and if the pulse is intense and large-sized after the amplification, the diffraction gratings in the compressor have to be similarly large- sized as they can sustain only limited pulse intensity. In the 90s the CPA technique became a part of another revolutionary method in the ultra-short-pulse amplifi- cation, OPCPA – Optical Parametric Chirped Pulse Amplification. This technique brought many benefits, among others substantially improved pulse contrast, which is a very important parameter in laser–target interactions. At present both techniques are used in many prestigious laser facilities in the world. How- ever, bulky compressors with demanding adjustment are an indispensable part of all of them. In the last fifteen years again a new potentially revolutionary technique has emerged from theoretical studies of wave instabilities in laser plasma, which presents a possibility of the production of intense and ultrashort laser pulses even without the large compres- sors. Raman backscattering has been known as a bad acting parametric instability in the laser energy de- position into the hohlraum target during the Iner- tial Confinement Fusion process [2]. In underdense plasma, however, stimulated Raman backscattering, i.e the scattering seeded by a counterpropagating beam, revealed some features [12] that made it a new promising tool for the generation of intense ultra- short laser beams. It was found that a short (< 1 ps) and weak laser pulse can be amplified by a counter- propagating long pump and remain short or even get shortened [8]. Since that time the topic has attracted attention of many laser-plasma physicists and theo- reticians. Here the laser plasma has ranked among the laser amplifying media, such as well-known solid- state materials (glass or crystals doped with Nd, sap- phire doped with Ti, etc.) or gas mixtures (He−Ne mixtures, CO2 or CO based mixtures, excited dimers etc.). Its eminent property is that it can withstand by several orders higher optical intensity than the lat- ter laser media can. The simulations have reported even hundreds of petawatts of the output power [15], whereas the practical realization up to now has not surpass 100 GW. In the paper we summarize the best results of simu- lations and experiments achieved in stimulated Raman backscattering (SRBS). In the following we propose a possible project on SRBS at the Research Center of PALS (Prague Asterix Laser System) in Prague, CZ. 246 http://ctn.cvut.cz/ap/ vol. 53 no. 2/2013 Stimulated Raman Backscattering in Plasma 2. SRBS in simulations and experiments There is a long track of theoretical works on stimulated Raman scattering since 70s and 80s in the last century. Even in those works a plasma is shown as a suitable medium for the amplification and compression of laser pulses. In 90s demanding simulation works enabled better insight into this process and other generated in- stabilities, see e.g. [9]. It was found that a short weak laser pulse, a seed, can be efficiently amplified and compressed in a plasma prior to the harmful instabili- ties, such as forward Raman scattering, filamentation and others, will grow. The computational methods in that time were based either on the classical three- wave-interaction model with the assumption of slowly- varying wave envelopes, where one of the waves was not electromagnetic but a plasma one, or on the PIC simulations, mostly one-dimensional, under certain simplifying conditions. A new regime of a Raman amplification was identified [12], so called SRA – su- perradiant amplification, when the seed amplitude grows linearly with time and simultaneously the seed is contracted. Reaching this regime needs an initial seed pulse of sufficiently high intensity, the pump wave then can get strongly depleted. A characteristic fea- ture of the superradiant amplification is that the par- ticipating electrons from the plasma wave backscatter the pump coherently. In [5] this regime was experimen- tally demonstrated and in 1D-PIC code numerically simulated. The plasma was produced in a H2 gas jet by the leading edge of a picosecond pump wave from a Ti:sapphire laser. The initial signal was of 80 fs long and after the amplification in the plasma its en- ergy increased from 70 µJ to ≥1 mJ and the duration was 56 fs. However, the output pulse was a train of sev- eral peaks. The experiment was explained as an initial SRBS process and when the seed intensity reached a certain value the process proceeded as the SRA amplification. At present the highest seed output achieved exper- imentally in the SRBS amplification is given in [10]. The plasma channel of 2 mm long was generated in a C2H6 gas jet, the pump and seed pulses were de- rived from a Ti:sapphire laser. Double-pass geometry was used when the seed beam passed twice the interac- tion area. The energy transfer pump–seed was 6.4 % which led to the amplification of the seed by a fac- tor of more than 20 000. The pulse compression was from 500 fs to about 50 fs. The seed intensity thus exceeded that of the pump by 2 orders. The output power of the seed was close to 100 GW level. It is evident that the seed output power data pro- vided by the simulations exceed those from the experi- ments by several orders. It was shown, see e.g. [17, 16], that mechanisms like detuning from the SRBS res- onance conditions due to an unintentional pump chirp or plasma channel inhomogeneities, plasma wave breaking at higher plasma temperature, or an inter- play between Brillouin and Raman scattering, effect negatively the SRBS efficiency. The achievement of the multipetawatt regime is presented in the sim- ulations of [15]. They performed large-scale multi- dimensional PIC simulations and came to a conclu- sion that multi-PW peak powers are within reach, however, only in a narrow plasma parameter win- dow. The plasma density should be kept in the range of (4.5 ÷ 18) × 1018 cm−3. The limiting factor is the growth of deleterious plasma instabilities. The au- thors recommend Raman amplification in plasma chan- nels of larger diameters so that the plasma density could be kept low enough. They report even 300 PW output if the plasma channel is of 1 cm (!) FWHM and pump beam (λ = 1 µm) of 1 PW in 25 ps. A too-many- orders difference thus persists between the seed output prognosis and the real up-to-now achieved value. 3. SRBS at PALS A crucial factor in the SRBS process is the plasma channel. It should be thoroughly adjusted so as to keep its good wave guiding properties along the interaction region over many Rayleigh zones [4]. The radial profile of the refractive index should have its maximum on the axis which prevents diverging tendencies of both waves. If the laser beams are strong enough, such index profiling can be self-formed by rela- tivistic or ponderomotive forces acting on the electrons. However, the beam peaking on the axis will push the charged particles away and their displacement might create new centers of diffraction. The pump and seed are usually focused into the plasma chan- nel which imposes spherical form of their wavefronts. Therefore the fronts of the waves should move with a lower speed than peripheral parts. Balancing all the counteracting processes in the channel is a de- manding task. In any way, a high quality laser beam, smooth in time and space, will be beneficial for the produc- tion of a plasma channel with defined plasma density profile and wave guiding properties. The Research Center of PALS has a single beam photodissocia- tion iodine laser [11] of such high qualities at a dis- posal. Its fundamental wavelength is 1315 nm and delivered energy ≤1 kJ@400 ps. Since 2000 the laser facility has been used by many international research groups namely due to the smooth top-hat beam pro- file in 1ω and also 3ω. A part of the beam could be used for the production of a gas-jet (e.g. C3H8) wave- guiding plasma channel. There is also experience with smoothing the laser plasma non-uniformities by additional gas jets [1]. The plasma channel can be produced in ∼ 0.25 × 3 mm2 size with a plasma density of ne ≈ (1 − 2) × 1019 cm−3, correspond- ing to a plasma frequency of ωpe ≈ 0.2 × 1015 Hz. The pump and seed beams can be produced by a high- power Ti:sapphire laser system [7] which is also op- erated at the PALS Research Center, λ = 810 nm, ∼1 J@40 fs. The laser beam will be split and adjusted into the pump (∼1 J, pulse duration τ = 10 ps) and 247 Hana Turčičová, Jaroslav Huynh Acta Polytechnica the seed (0.2 mJ, τ = 500 fs), both beams being fo- cused into a 0.25 mm spot. Thus similar experimental conditions to those of [4] would be produced and get-in-touch SRBS experiments could be performed. The seed amplification of almost two orders of magni- tude should be closely touched. Another SRBS experiment on the PALS laser sys- tem can be proposed. It aims at an output power upgrade on the third harmonics of the iodine laser. At present 400 J in 250 ps is reached if the PALS beam is frequency tripled, i.e. 438 nm, corresponding wave frequency being 4.3 × 1015 Hz. The output power at the maximum is therefore ∼ 1.6 TW. The third harmo- nics beam will be used as a source for both pump and seed waves, the remaining part of the fundamental beam after tripling will form a pre-pulse, i.e. generate the plasma channel, probably again in a gas jet with a low ionization potential. The channel should be of 1 ÷ 1.5 mm diameter and 15 mm length. Accord- ing to [14] the recommended ratio between the pump wave frequency ωpu and ωpe should be 20, giving the plasma frequency ωpe ∼ 0.2 × 1015 Hz and plasma density ne ∼ 1.5×1019 cm−3. The resonance condition provides the seed frequency, ωs = ωpu − ωpe, which means 460 nm wavelength. As the pump beam dura- tion is 250 ps, the seed beam one should be about 250 fs so as to keep up with the initial simulation param- eters of [14]. Such a seed beam can be provided by the Ti:sapphire laser mentioned above, if doubling its beam. The doubling will be performed prior com- pression to the fs duration, i.e. at the full duration of 250 ps [7]. The beam with about 400 nm than has to be Raman shifted to 460 nm. This can be done in a gas Raman shifter with a relevant Stokes shift (e.g. D2, CH4). After the Raman shifting, the beam will be compressed to the desired 250 fs and will func- tion as the seed beam. Following further the desired optimal plasma parameters given in [14], a pump in- tensity of about 6 × 1012 W cm−2 is required through- out the plasma channel. Performance parameters adjusted in this way would enable a conversion ef- ficiency of about 40 % and in its consequence an output power of 30 TW. Using the Raman amplifi- cation in the plasma, more than one order increase in the third harmonic frequency can be expected. 4. Conclusion The investigations of the stimulated Raman amplifi- cation in plasma performed in simulations are very optimistic regarding attainable output powers. The ex- perimental results so far achieved, however, stay be- hind by several orders. In the experiment, it is very difficult to keep the optimal conditions for an effi- cient energy transfer from the pump to the seed along the whole interaction path in plasma. The channel should be long enough (≤ 10 mm) and all along with good wave guiding properties. Technically it is not easy to maintain a uniform plasma density along such a long channel. However, it can be believed a single high-quality intense beam, smooth in time and space, the same as the PALS laser system in Prague, would be a good candidate for the production of a suitable plasma channel for the SRBS studies. References [1] D. Batani, R. Benocci, R. Dezulian, et al. Smoothing of laser energy deposition by gas jets. Eur Phys J Special Topics 175(1):65–70, 2009. [2] R. L. Berger, C. H. Still, E. A. Williams, A. B. Langdon. On the dominant and subdominant behavior of stimulated Raman and Brillouin scattering driven by nonuniform laser beams. Phys Plasmas 5(12):4337–4355, 1998. [3] E. Brookner. Phased-array radars. Scient American 252(2):94–103, 1985. [4] W. Cheng. Reaching the nonlinear regime of the Raman amplification of ultrashort laser pulses. Princeton university, U.S.A., 2007. Ph.D. thesis. [5] M. Dreher, E. Takahashi, J. Meyer-ter Vehn, K.-J. Witte. Observation of superradiant amplification of ultrashort laser pulses in a plasma. Phys Rev Lett 93(9):095001, 2004. [6] R. A. Fisher, W. K. Bischel. Pulse compression for more efficient operation of solid-state laser amplifier chains II. Appl Phys Lett 24(10):468–470, 1974. [7] J. Hrebicek, B. Rus, J. C. Lagron, et al. 25 TW Ti:sapphire laser chain at PALS. SPIE 8080, 2011. [8] V. M. Malkin, G. Shvets, N. J. Fisch. Fast compression of laser beams to highly overcritical powers. Phys Rev Lett 82(22):4448–4451, 1999. [9] V. M. Malkin, G. Shvets, N. J. Fisch. Ultra-powerful compact amplifiers for short laser pulses. Phys Plasmas 7(5):2232–2240, 2000. [10] J. Ren, S. Li, A. Morozov, et al. A compact double-pass Raman backscattering amplifier/compressor. Phys Plasmas 15(5):056702, 2008. [11] K. Rohlena, K. Jungwirth, B. Kralikova, et al. A survey of PALS activity and development, 2004. [12] G. Shvets, N. J. Fisch. Superradiant amplification of an ultrashort pulse in a plasma by a counterpropagating pump. Phys Rev Lett 81(22):4879–4882, 1998. [13] D. Strickland, G. Mourou. Compression of amplified chirped optical pulses. Optics Com 56(3):219–221, 1985. [14] R. M. G. M. Trines, F. Fiuza, R. Bingham, et al. Production of picosecond, kilojoule, and petawatt laser pulses via Raman amplification on nanosecond pulses. Phys Rev Lett 107(10):105002, 2011. [15] R. M. G. M. Trines, F. Fiuza, R. Bingham, et al. Simulations of efficient Raman amplification into the multipetawatt regime. Nature Physics 7(1):87–92, 2011. [16] D. et al. Turnbull. Simulataneous stimulated Raman, Brillouin, and electron-acoustic scattering reveals a potential saturation mechanism in Raman plasma amplifiers. Phys Plasmas 19(8):083109, 2012. [17] N. A. Yampolsky, N. J. Fisch. Limiting effects on laser compression by resonant backward Raman scattering in modern experiments. Phys Plasmas 18(5):056711, 2011. 248 Acta Polytechnica 53(2):246–248, 2013 1 Introduction 2 SRBS in simulations and experiments 3 SRBS at PALS 4 Conclusion References