It is well known that stimulated Brillouin scattering (SBS) and stimulated Raman scattering SRS are two kinds of parametric instabilities in laser–plasma interaction (LPI) of inertial confinement fusion (ICF)[
High Power Laser Science and Engineering, Volume. 1, Issue 2, 02000094(2013)
Experimental observation of backscattered light based on different coherence between incident laser beams
Recent experimental results on NIF revealed a much higher stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) backscatter than expected; one possible reason was due to the coherence between incident laser beams. In our research, two laser beams (,
in each one) with different coherent degrees between them from the SG-II facility were employed to irradiate an Au plate target; the backscatter of SBS and SRS in the range of the given solid angle had been measured. The results showed that it could change dramatically corresponding to the difference of the coherent degree between the two laser beams, and there was usually more intense backscatter the higher the coherent degree between the incident beams.
1. Introduction
It is well known that stimulated Brillouin scattering (SBS) and stimulated Raman scattering SRS are two kinds of parametric instabilities in laser–plasma interaction (LPI) of inertial confinement fusion (ICF)[
2. Experimental condition and setup
The experiment was carried out on the SG-II laser facility; Figure ,
in each one), with an angle of
between them, were employed to irradiate an Au plate target (thickness of
); beam 1# was perpendicular to the target’s surface, and the diameter of the focal spot was about
. Due to change-regard of the backscatter along with different coherence between the incident laser beams, a measure of partial and relative backscatter is enough. So the backscatter of SRS and SBS in the given solid angle was obtained by sampling in a small aperture during the experiment. Two sampling mirrors (M1 and M2 shown in Figures
were used to collect a part of the backscatter from beam 1# and beam 3#, respectively.
Because the wavelengths of SBS and SRS are very different (the former is similar to the wavelength (527 nm) of the incident laser, while the latter has a broad band (covers 1–2 times the wavelength of the incident laser)), with peak wavelength near 800 nm, it is feasible to separate the backscatter of SBS and SRS by a spectroscope. Figure is better than 98%, except their reflectivity is over 99% at the wavelength of 527 nm. The partial backscatter (sampled by M1) from laser beam 1# was separated into SBS and SRS by S1, then recorded by energy meters T1 and T2, respectively (see Figure
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In order to study the influence of the coherent condition on backscatter in our experiment, the key is to form different coherent degrees between the incident beams. Fortunately, the driving laser beams 1# and 3# are all well polarized linearly, so it is possible to achieve different coherence degrees between them by altering their polarization directions. In fact, it was realized by rotating the frequency doubling crystal. The frequency doubling efficiency of the crystal remains as it rotates , but the polarization direction of the driving laser appears a quarter turn.
Figure and 34°, shown in Figure
, and this means that he coherent degree can be characterized by
. For Figure
and 0.83, respectively.
3. Experimental results and analysis
Table , but there is from the same
. By comparing the counts from the same
with different
, there obviously is a degree of change in counts with different coherent conditions; moreover, the counts from
display a consistent growth along with the increase of coherent degree between incident beams.
Of course, two factors may have affected the experimental result somewhat. On the one hand, the experimental data are insufficient due to the limited shots. On the other hand, sampling in a small aperture may be vulnerable to the difference in distributions of laser intensity from different shots. However, the experiment data from different beams reveal a consistent trend. That is, although insufficient data and sampling in a small aperture may have some effect on the experimental result, it is credible that the backscatter of SBS and SRS becomes more intense along with the increase of coherence degree between incident beams.
|
For beam 3#, the spectrum of SRS was measured with a grating spectrometer, as shown in Figure , it appears as a unimodal spectrum; when
, it appears as a bimodal spectrum. It is well known that the shapes of spectra are closely related with the physical condition of plasma. According to the matching condition between frequency and wave number, and also the dispersion relation of SRS, the frequency of SRS backscatter decreases with the increase of electron density. And generally speaking, the size of the plasma area with a certain electron density is proportional to the strength of the corresponding spectrum. So the unimodal spectrum directly reflects a more gentle distribution of electronic density compared with the bimodal spectrum. In other words, the distribution of laser intensity on the target is more uniform with low coherence degree, and this is consistent with the result discussed above. Furthermore, the shift of the peak wavelength to a shorter wavelength suggests a lower average electron density under the condition of lower coherence. With the shortwave cut-off of SRS spectra[
and
were preliminarily estimated to be about
and
, respectively.
4. Conclusion
Experimental results prove that the impact of coherence between laser beams on backscattered light is real, and that there is usually more intense backscatter the higher the coherent degree between the incident beams. Meanwhile, the conclusion above suggests a possible way to understand the increase of SBS and SRS backscatter in recent related experiments on NIF.
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Xiangfu Meng, Chen Wang, Honghai An, Guo Jia, Huazhen Zhou, Sizu Fu. Experimental observation of backscattered light based on different coherence between incident laser beams[J]. High Power Laser Science and Engineering, 2013, 1(2): 02000094
Received: Jun. 27, 2012
Accepted: May. 3, 2013
Published Online: Nov. 19, 2018
The Author Email: Xiangfu Meng (mengxiangfu07@163.com)