Dongxia Hu

Status of the SG-III solid-state laser facility

SG-III laser facility beam begins with a nanojoule energy laser pulse from the master oscillator and a fiber front-end system that can provide a variety of pulse shapes suitable for a wide range of experiments. The chirped pulse stacking method is used in the front-end system to generate arbitrarily shaped pulse with a rise time less than 50ps. The system stacks a set of 100-ps chirped pulses in fiber time-delay lines to obtain a 5-ns flat-top pulse with a spectral bandwidth of 1.2nm. The pulse is then transported to preamplifier modules under the middle of CSF for amplification and beam shaping. There is a total of 48 preamplifier modules on SG-III, each feeding a single laser beams. The main amplifier column of 4 high by 2 wide has been chosen as a module and the clear optical aperture is 40cm × 40cm. Small PEPC are chosen for system isolation and beam can be rotated by 90 degree in U-turn beam reverser located in the middle of TSF. After main amplifier, beams are subsequently redirected to final optics assembly in switchyard and are focused on the center of the target chamber with the diameter of 6m.

Status of prototype of SG-III high-power solid-state laser

We are currently developing a large aperture neodymium-glass based high-power solid state laser, Shenguang-III (SG-III), which will be used to provide extreme conditions for high-energy-density physical experiments in China. As a baseline design, SG-III will be composed of 48 beams arranged in 6 bundles with each beam aperture of 40cm×40cm. A prototype of SG-III (TIL-Technical Integration experimental Line) was developed from 2000, and completed in 2007. TIL is composed of 8 beams (four in vertical and two in horizontal), with each square aperture of 30cm×30cm. After frequency tripling, TIL has delivered about 10kJ in 0.351 μm at 1 ns pulsewidth. As an operational laser facility, TIL has a beam divergence of 70 μrad (focus length of 2.2m, i.e., 30DL) and pointing accuracy of 30 μm (RMS), and meets the requirements of physical experiments.

Research and construction progress of SG-III laser facility

SG-III laser facility is now the largest under-construction laser driver for inertial confinement fusion (ICF) research in China, whose 48 beams will deliver 180kJ/3ns/3ω energy to target in one shot. Till the summer of 2014, 4 bundle of lasers have finished their engineering installation and testing, and the A1 laser testing is undergoing. A round of physics experiment is planned in Oct. 2014 with 5 bundle of lasers, which means the facility must be prepared for a near-full-capability operation before the last quarter of 2014. This paper will briefly introduce the latest progress of the engineering and research progress of SG-III laser facility.

Research and construction progress of the SG-III laser facility

The under-construction SG-III laser facility is a huge high power solid laser driver, which contains 48 beams and is designed to deliver 180kJ energy at 3ns pulse duration. The testing ending up at September 2012 validated that the first bundle lasers of SG-III facility had achieved all the designed requirements. And shortly later in December 2012, the first round of running-in physics experiment provided a preliminary X-ray diagnostic result. In the testing experiment, detailed analysis of the laser energy, the temporal characteristics, the spatial distribution and the focusing performance was made by using the Beam Integrated Diagnostic System. The 25kJ 3ω energy produced by the first bundle lasers created the new domestic record in China. These great progresses in the laser performance and the physics experiment have already demonstrated that the facility is in excellent accordance with the designs, which establish a solid foundation for completing all the construction goals.

Optical zooming based on focusing grating in direct drive ICF.

In direct drive ICF, optical zooming is an effective way to mitigate cross-beam energy transfer and increase the hydrodynamic efficiency, by reducing the spot size of the laser beams while target compressing. In this paper, a novel optical zooming scheme is proposed, which employs a focusing grating to focus the broadband laser pulse, changing the spot size on the target within single beamlet. Experimentally, a focusing grating with clean aperture of 40-mm × 40-mm placed after the collimated light successfully realized the peak-valley of defocusing wavefront distribution of 0.73 µm as the wavelength ranging from 1052.43 nm to 1053.23 nm. Extended to the full-sized focusing grating with laser beam of 360-mm × 360-mm, it is derived that the focal spot reduction reaches to 21.8% with the 3rd harmonic light ranging from 350.81 nm to 351.08 nm, decreasing from 375 µm to 294 µm with 300 µm shaping continuous phase plate.

Research of beam smoothing technologies using CPP, SSD, and PS

Precise physical experiments place strict requirements on target illumination uniformity in Inertial Confinement Fusion. To obtain a smoother focal spot and suppress transverse SBS in large aperture optics, Multi-FM smoothing by spectral dispersion (SSD) was studied combined with continuous phase plate (CPP) and polarization smoothing (PS). New ways of PS are being developed to improve the laser irradiation uniformity and solve LPI problems in indirect-drive laser fusion. The near field and far field properties of beams using polarization smoothing were studied and compared, including birefringent wedge and polarization control array. As more parameters can be manipulated in a combined beam smoothing scheme, quad beam smoothing was also studies. Simulation results indicate through adjusting dispersion directions of one-dimensional (1-D) SSD beams in a quad, two-dimensional SSD can be obtained. Experiments have been done on SG-III laser facility using CPP and Multi-FM SSD. The research provides some theoretical and experimental basis for the application of CPP, SSD and PS on high-power laser facilities.

Process-oriented adaptive optics control method in the multi-pass amplifiers

In this talk, we propose and demonstrate the process-oriented adaptive optics (AO) wavefront control method, for optimizing the beam quality in the multi-pass amplifiers. Different from the conventional target-oriented wavefront control approach, the novel method divides the aberration correction process into several steps, to optimize the wavefront quality in time during the courses of the beams transport and amplification. The experimental results show that the proposed method can effectively prevent the beam quality from worsening and ensure the successful reality of multi-pass amplification, so it has obvious advantages both in efficiency and accuracy over the traditional target-oriented method.

Beam alignment based on two-dimensional power spectral density of a near-field image

Beam alignment is crucial to high-power laser facilities and is used to adjust the laser beams quickly and accurately to meet stringent requirements of pointing and centering. In this paper, a novel alignment method is presented, which employs data processing of the two-dimensional power spectral density (2D-PSD) for a near-field image and resolves the beam pointing error relative to the spatial filter pinhole directly. Combining this with a near-field fiducial mark, the operation of beam alignment is achieved. It is experimentally demonstrated that this scheme realizes a far-field alignment precision of approximately 3% of the pinhole size. This scheme adopts only one near-field camera to construct the alignment system, which provides a simple, efficient, and low-cost way to align lasers.

Polarization smoothing for single beam by a nematic liquid crystal scrambler.

Polarization smoothing (PS) is a key approach to suppress laser plasma instabilities (LPI) in inertial confinement fusion (ICF) experiments. Here, we propose a liquid crystal (LC) PS element to realize single beam smoothing and demonstrate its smoothing effect, in principle, with a 2×2 LC polarization checkerboard, which reduces the laser intensity variation in the focal spot to 78.4%. LC PS elements, which have potential applications in high-power ICF laser drivers, have many advantages because they are easy to fabricate, cost effective, flexible, and large.

Shannon entropy method of small-scale self-focusing assessment in high-power laser systems

Through analysis of near-field beam profiles, we propose a method using Shannon entropy to assess the development of small-scale self-focusing during laser propagation and amplification in high-power laser systems. In this method, the entropy curve that corresponds to increasing B integral displays an evident turning point at which small-scale self-focusing starts to rapidly develop. In contrast to classical methods using contrast, modulation, or power spectral density, the proposed method provides the B integral criterion more clearly and objectively. This approach is an optimization method that can be utilized in the design and operation of high-power laser systems.