Demonstration of laser pulse amplification by stimulated Brillouin scattering Download: 608次
1. Introduction
Exploration of the intensity frontier is an exciting challenge for physicists. Advances in laser technologies, particularly those associated with increased power and decreased pulse duration, are of great interest due to their application to many fields in science and engineering, for example in laser-driven inertial fusion energy, in laser- and beam-driven particle accelerators, and in next-generation light sources. Currently, most high power lasers rely on the chirped pulse amplification (CPA) technique, in which a laser pulse is stretched before going to an amplifying medium, then expanded to large area (1 m or more) and recompressed, in order to avoid optical damage that occurs at intensities close to . Present-day high power lasers typically reach around 1 PW peak powers. Next-generation laser systems have been designed to reach powers of 10 PW or more, by the employment of the optical parametric CPA (OPCPA) technique. However, the achievement of intensities beyond this level is still uncertain, mainly due to the requirement for precise wavefront delivery at the final focusing optic of multiple large area laser beams.
Pulse compression methods using plasmas have been promoted as a way of overcoming these obstacles. The enormous energy densities associated with focused high power lasers excite nonlinear wave amplification in a medium that is already ionized. The plasma can support intensities of up to , i.e., 5 orders of magnitude larger than solid-state systems, before disruption to the medium occurs[1]. Laser pulse amplification in plasma rests upon an energy transfer between a relatively long duration pump pulse and a shorter seed pulse through the generation of either an electron plasma wave, known as stimulated Raman scattering (SRS), or an ion-acoustic wave, known as stimulated Brillouin scattering (SBS). Experimental[2] and numerical[3, 4] results have already been demonstrated in the case corresponding to SRS excitation.
As SBS produces a frequency shift in the scattered wave spectra, it is necessary for the seed laser to be downshifted by an amount equal to the ion-acoustic frequency in order for coupling between the laser beams to be realized. When utilizing long duration beams, which naturally have a very narrow bandwidth, an adjustment to the seed laser is essential for coupling between the laser beams to ensure that the necessary frequency component for scattering is present in the seed. This creates an additional technical complexity to the achievement of Brillouin scattering in plasma. However, when the seed beam is sufficiently short, and its bandwidth is sufficiently wide, the necessary downshifted frequency to trigger Brillouin scattering of the pump pulse will already be available in the seed pulse, and no additional frequency modification will be needed.
In this paper, we report on experimental observations of Brillouin scattering using two beams incident from the same laser system, one long (15 ps) pump beam and one short (1 ps) seed beam counter-propagating with respect to one another through a volume of plasma, with no modifications made to the frequency of either pulse. These findings are corroborated by 1D numerical simulations using the particle-in-cell code OSIRIS[5], confirming that for sufficiently short pulses the necessary Brillouin downshifted frequency is presented in the laser bandwidth, therefore negating the requirement for a frequency downshift in the seed pulse to be performed before Brillouin scattering can be obtained. A pump-to-probe energy transfer of up to 2.5% has been obtained, which confirms earlier results by Lancia
In addition, our results extend the results by Lancia
2. Theory
SBS in plasmas can be characterized as the scattering of a high frequency transverse electromagnetic wave by a low frequency ion-acoustic wave into a second transverse electromagnetic wave. This corresponds to the decay of an incident photon in the laser beam, with frequency and wavenumber , into a phonon (ion-acoustic quantum) with frequency and wavenumber , and a scattered photon, with frequency and wavenumber , which travels in approximately the opposite direction to the incoming laser photon. Following directly from linear theory[7], the frequency and wavenumber matching conditions, often invoked when studying the Brillouin instability, are
In the case of Raman amplification, the minimum frequency shift that can occur is equal to the plasma frequency, meaning that the maximum density where Raman amplification techniques can be utilized is one quarter of the critical density. However, in the case of Brillouin amplification, the minimum frequency shift is equal to zero, therefore allowing Brillouin scattering to operate at densities up to the critical density[8]. In addition to this, more energy can be transferred into the scattered wave via Brillouin scattering than via Raman scattering as less energy is coupled into the ion-acoustic wave in Brillouin scattering than the Langmuir wave associated with Raman scattering. This makes the Brillouin mechanism particularly useful for applications such as laser amplification techniques[6, 9] and induced energy transfer between adjacent laser beams at facilities such as the National Ignition Facility[10, 11].
3. Experimental setup
The experiment was conducted on the Vulcan Nd:glass laser facility at the Rutherford Appleton Laboratory[12]. This facility provided two linearly polarized laser pulses of central wavelength with a bandwidth. The two laser beam diameters were reduced to 20 mm using pierced plastic plates, in order to have the correct spot size on the target. Each laser pulse was focused onto the target using off-axis parabolic mirrors, with focal length, giving focal spots of diameter. The pump beam contained between 570 and 860 mJ of energy, with a pulse duration , giving a pump intensity on the target of around . The seed beam contained between 38 and 477 mJ, with a pulse duration , giving a seed intensity on the target of between and . The laser pulses were injected into the target from opposite directions, with an angle of between the two counter-propagating beams. This angle was used for safety reasons; while it led to a small reduction in pulse growth, this was deemed acceptable. A 1.65 mm long overlap distance was achieved in this geometrical setup. The temporal delay between the pump and the seed was adjusted so that the two ascending edges of the pulses crossed in the center of the gas target in order to maximize the duration of the interaction. This was achieved by using a streak camera looking at the overlap region. The laser pulses were focused in the center of a 5 mm long supersonic gas jet target, using either argon or deuterium. The gas target produced uniform plasmas when ionized, with background electron densities varying between and . The plasma density was controlled by adjusting the backing pressure of the supersonic gas jet. The plasma is created by the interaction pulses themselves – without any ionization pulse needed – triggering multiphoton ionization of the gas and collisions between electrons and atoms.
The light transmitted through the plasma in the direction of propagation of the seed beam was collected and collimated using a 600 mm focal length lens. The collimated beam was then steered out of the target chamber using flat silver mirrors, and focused onto the entrance slit of an optical spectrometer, equipped with a 150 lines/mm diffraction grating coupled with an Andor 16-bit CCD camera recording the spectra with a 0.1 nm resolution. A schematic diagram of the experiment can be seen in Figure
4. Experimental results
The results of the experiment are shown in Figure
Fig. 2. Experimental frequency spectra of a 1 ps laser pulse recorded after propagation through a supersonic gas jet (normalized intensity versus normalized angular frequency). A reference spectrum, recorded with the gas jet turned off, has been included in each plot. Graph (a) is the spectrum recorded with only the seed beam at an intensity of interacting with the gas jet at , without a counter-propagating pump beam. Graph (b) was recorded with the two counter-propagating beams interacting, the seed at an intensity of and the pump at , at . Graph (c) was recorded with the seed at an intensity of and the pump at , at . The generation of a downshifted peak can be observed through the interaction of the laser pulses and the gas jet, with its relative intensity compared to the fundamental peak strongly depending on the plasma density and the presence of a counter-propagating pump pulse.
Fig. 3. Simulated spectra corresponding to each of the experimental regimes presented in Figure 2 (normalized intensity versus normalized wavevector). The electron density varied from to . Graph (a) is the spectrum simulated with a single laser of intensity interacting in a neon-like argon plasma with an electron temperature of about 20 eV and density of . Graph (b) was calculated with the two counter-propagating beams interacting in a deuterium plasma of density , the seed at intensity and the pump at , with an electron temperature of 120 eV. Graph (c) was simulated with the seed at intensity and the pump at , in an argon plasma with an electron temperature of 5 eV and a density of .
The energy transfer efficiency is calculated as follows. For the laser shot depicted in Figure
5. Numerical simulations
The numerical simulations were conducted in 1D using the fully relativistic particle-in-cell (PIC) code OSIRIS[5] and were constructed to mirror the experimental parameters as closely as possible. For these simulations, the plasma electron temperature and the effective ionization degree of the atoms are needed. These quantities were calculated using the laser plasma simulation code MEDUSA, in 1D planar geometry, using a corrected Thomas–Fermi equation-of-state model and an average-atom model, and assuming collisional ionization. For the shots shown in Figure
Three sets of simulation results corresponding to each of the three experimental regimes examined are presented, and were set up as follows. In simulation (a) a single laser of intensity was injected into an argon plasma of density with a mass ratio of ions to electrons of . The plasma temperature ratio was set such that , where and , assuming neon-like argon with the majority of the outer shell of electrons depleted. For simulation (b) two counter-propagating pulses were launched into a plasma of density , in this case comprising deuterium, with a mass ratio of ions to electrons of , with the plasma ion and electron temperatures kept constant at 20 eV for the ions and 120 eV for the electron species. Laser intensities of and for the pump and seed, respectively, were used, where the seed pulse was launched at the instant the pump laser had traversed the length of the plasma. In the case of simulation (c), two counter-propagating beams were used and their intensities were both set to and propagated through an argon plasma with a configuration such that , , where and , and a density of . The following parameters are consistent throughout each of the three simulations presented: the pulses propagate through a plasma column of length , with the pump pulse traveling from right to left through the simulation box; the pump pulse has a duration of 1.5 ps and the seed pulse a duration of 100 fs; each of the pulses is from a laser of wavelength ; the time step for integration is , where is the plasma electron frequency; the spatial resolution of the simulations is of the order of the Debye length, with 100 particles per cell. Due to computational limitations, the pulse lengths and plasma column were scaled down by a factor of ten from the parameters used to obtain the experimental results.
Upon examination of the spectra presented in Figure
6. Conclusions
These experimental observations of Brillouin scattering using two beams at the same wavelength are a promising confirmation of the observations by Lancia
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Article Outline
E. Guillaume, K. Humphrey, H. Nakamura, R. M. G. M. Trines, R. Heathcote, M. Galimberti, Y. Amano, D. Doria, G. Hicks, E. Higson, S. Kar, G. Sarri, M. Skramic, J. Swain, K. Tang, J. Weston, P. Zak, E. P. Alves, R. A. Fonseca, F. Fiuza, H. Habara, K. A. Tanaka, R. Bingham, M. Borghesi, Z. Najmudin, L. O. Silva, P. A. Norreys. Demonstration of laser pulse amplification by stimulated Brillouin scattering[J]. High Power Laser Science and Engineering, 2014, 2(4): 04000e33.