Reduction of Backward Scatterings at the Low-Coherence Kunwu Laser Facility.

We report the first experimental observation on the reduction of backward scatterings by an instantaneous broadband laser with 0.6% bandwidth in conditions of interest for inertial confinement fusion at the low-coherence Kunwu laser facility. The backscatter of stimulated Brillouin scattering (SBS) was robustly reduced by half at intensities of 1-5×10^{14} W/cm^{2} with the 0.53-µm broadband laser in comparison with the monochromatic laser. As SBS dominates energy loss of laser-plasma interactions, the reduction of that demonstrates the enhancement of laser-target coupling by the use of broadband laser. The mitigation of filamentation leads to the reduction of stimulated Raman backscattering at low intensities. In addition, the three-halves harmonic emission was reduced with the broadband laser as well.

The generation of a fourth-harmonic probe and its application in Nomarski interferometry at Shengguang-II.

In this work, a design for the generation of a 4ω (263-nm) probe converted from a 1ω (1053-nm) laser is presented. The design is based on a beta-barium borate and potassium dihydrogen phosphate two-step frequency-conversion process. A suitable configuration for Nomarski interferometry based on the 4ω probe is proposed, for measuring the electron density of laser-produced plasmas. The signal-to-noise ratio of the output 4ω probe to 1ω and 2ω light after frequency quadrupling and harmonic separation is 103 with a 0.5 GW/cm2 1ω input but decreases to ∼102 at intensities below 0.1 GW/cm2. Additional noise suppression by a factor of 104 is achieved using filters before the interferometer recording camera. The spatial resolution of the diagnostic can reach 5.2 µm for a 10% modulation transfer function. An experiment validating the probe diagnostic system is conducted at the Shengguang-II laser facility. A clear interferogram of an aluminum plasma is obtained with 0.1 GW/cm2 input, suggesting a maximal electron density of about 2.5 × 1020 cm-3 as retrieved through an inverse-Abel transform. The design proposed in this paper is appropriate for a small laser device or a large laser facility that lacks a separate diagnostic beam, and it is an inexpensive solution as it requires small-aperture 1ω input at a relatively low intensity. All the key parameters necessary to implement the design are provided in detail, making it straightforward to reproduce or transplant the system for specific uses.

Design and calibration of hot-electron spectrometer array for angle-resolved measurement.

A hot-electron spectrometer array with two-dimensional distribution has been designed with a wide-angle range and high-energy resolution to measure the spatially resolved electron spectra for high-power-laser plasma interaction experiments. It consisted of 19 identical electron spectrometers set in three directions with an interval of 10°. Each electron spectrometer was designed with a uniform magnetic field to detect electrons in the range from 20 to 500 keV. The spectrometers were calibrated using electrons from an accelerator. In an experiment, the spatially resolved electron energy spectra, which approximately had a Maxwell distribution, were obtained from an aluminum foil target irradiated by a 0.53-µm laser pulse.