In traditional indirect-drive inertial confinement fusion (ICF) research, dozens or hundreds of laser beams are injected in a cylindrically symmetric gas-filled high-Z hohlraum, so as to generate a high-temperature (~300 eV) and uniform X-ray radiation source. Before reaching the hohlraum wall, the high-intensity (>10¹⁴ W/cm²) laser beams have to pass through a mm-scale underdense (<10²¹ cm^-3) plasma. This process could excite backward stimulated scattering, which is harmful to the quality of the X-ray radiation source.

It is generally believed that under the cylindrically symmetric configuration, backward stimulated scattering processes are identical for laser beams with the same incident angle, thus their scattered light energy should thus be equivalent. However, experimental results from the Shenguang-100 kJ laser facility revealed a several-fold difference in scattered light energy among laser beams with identical incident angles, indicating a significantly asymmetric distribution of backward stimulated scattering.

To investigate this phenomenon, two types of experiments were conducted on Shenguang-100 kJ by altering the polarization direction of the laser beams, which revealed the underlying physical mechanism. Cross-beam energy transfer (CBET) occurs among the overlapping laser beams. Due to the asymmetric distribution of laser polarization directions, beams with the same incident angle gain or lose different amounts of energy via CBET from or to neighboring beams, resulting in subsequent differences in the level of backward stimulated scattering. This discovery provides new insights for understanding stimulated scattering, CBET, drive deficit, and drive asymmetry in an ICF hohlraum.

[1] Liang Hao, Tao Gong, Wenyi Huo, et al., Nature Communications 16, 10343 (2025).

Stimulated scattering,gas-filled hohlraum,polarization