Lanqin Liu

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.

SILEX-I: 300-TW Ti:sapphire laser

Based on chirped pulse amplification technology, we have built a Ti:sapphire laser system, called SILEX-I (superintense laser for experiments on extremes), at CAEP, which consists of three stages with 5-, 30-, and 300-TW outputs, respectively. The first and the second stages work at 10 Hz, while the third works at single shot. Pulse durations of 30 fs have been obtained by installing an acousto-optic programmable dispersive filter (AOPDF) to compensate for the spectral gain narrowing in the regen. By taking a number of advanced measures for spatial beam control, such as spatial beam shaping, relay-imaged propagation, precise alignment of compressor gratings, and OAP, near-diffraction limited focal spots (FWHM) have been obtained. Focused intensities are calculated at (1–3) × 1020 W/cm2 with an f/2.2 OAP.SILEX-I has shown an excellent stability and reliability in operations for applications since its completion and will soon be able to operate at 500 TW.

High-power solid-state lasers for high-energy-density physics applications at CAEP

High-power solid-state laser programs at China Academy of Engineering Physics have made great progresses in recent years. A three-stage Ti:sapphire laser system, SILEX-I, was completed early in 2004 which could deliver 26-fs pulses at 5TW, 30TW, and 300TW to the corresponding target chambers for diverse applications. SILEX-I has been working very stably since its completion for experiments, demonstrating that it is the most powerful femtosecond Ti:sapphire laser for exploring strong-field phenomena in the world. The SG-III Nd:glass laser facility has been under conceptual design to meet the requirements from laser fusion applications. The SG-III facility is planned to have sixty-four beamlines divided into eight bundles with an output energy more than 100kJ at 0.35μm for 3- to 5-ns pulses. The eight-beamline TIL (Technical Integration Line), the prototype of the SG-III laser facility, has been installed in the new laboratory in Mianyang. The commissioning experiments have been conducted and one of the eight beams has produced 1-ns pulses of 3.0kJ and 1.2kJ at 1.053μm and 0.35μm, respectively. All the eight beamlines will be activated by the end of 2005 and completed in 2006 for operation. Meanwhile, the eight-beam SG-II laser in Shanghai Institute of Optics and Fine Mechanics has been operated for the experiments since 2001 and an additional beam, built in 2004, has been used for plasma backlighting experiments.

Backward Raman amplification in plasmas with chirped wideband pump and seed pulses

Chirped wideband pump and seed pulses are usually considered for backward Raman amplification (BRA) in plasmas to achieve an extremely high-power laser pulse. However, current theoretical models only contain either a chirped pump or a chirped seed. In this paper, modified three-wave coupling equations are proposed for the BRA in the plasmas with both chirped wideband pump and seed. The simulation results can more precisely describe the experiments, such as the Princeton University experiment. The optimized chirp and bandwidth are determined based on the simulation to enhance the output intensity and efficiency.

Production of Single Pulse by Landau Damping for Backward Raman Amplification in Plasma

The effect of plasma wavebreaking has been proposed to obtain single-pulse output for backward Raman amplification in plasma. However, some experiments indicate that the scheme may not be effective. In this paper, we propose the effect of Landau damping for this purpose. According to the theoretical analysis, the single pulse is generated by setting a proper plasma temperature to fully Landau damp the secondary spikes. Although the main pulse will also be partly suppressed by the damping, the effective efficiency can be enhanced. Moreover, the scheme is numerically demonstrated feasible by using the parameters of Princeton experiment [1], [2]. Additionally, the temperature to produce single-pulse output can keep below the critical temperature at various ratios of the plasma frequency to the pump pulse frequency.

Experimental research on the influences of smoothing by spectral dispersion on the Technical Integration Line

We describe one-dimensional smoothing by spectral dispersion (SSD) in high fluence on the Technical Integration Line. The experimental results indicate that SSD did not influence the load capacity of the laser facility. The near- and far-field analysis prove that adopting SSD could smooth the high-frequency modulations in the near field and dramatically suppress 10 μm-100 μm spatial modulations in the far field. The focal spot contrast decreases from 2.58 to 0.80 after using SSD and adaptive optics. Adopting a 0.31 nm bandwidth frequency-modulated laser pulse and a 1200 l/mm dispersion grating, experimental results proved that 97% 3ω energy passed through the laser entrance hole using a 4 m focal length wedged lens and gold foil target with an 1100 μm hole.

Introduction of SILEX-I Femto-second Ti:sapphire laser Facility

We have built a Ti:sapphire laser system, referred to as SILEX-I, with a peak power of 286TW for a pulse duration of 30fs using chirped-pulse amplification technique. A number of spectral and spatio-temporal beam control measures have been taken and near-diffraction limited focal spots have been obtained which, to our knowledge, are the best far fields ever measured for any existing high-power Ti:sapphire laser system without deformable mirror corrections.

Focal spot measurement in ultra-intense ultra-short pulse laser facility

A peak power of 286-TW Ti:sapphire laser facility referred to as SILEX-I was successfully built at China Academy of Engineering Physics, for a pulse duration of 30 fs in a three-stage Ti:sapphire amplifier chain based on chirped-pulse amplification. The beam have a wavefront distortion of 0.63μm PV and 0.09μm RMS, and the focal spot with an f/2.2 OAP is 5.7μm, to our knowledge, this is the best far field obtained for high-power ultra-short pulse laser systems with no deformable mirror wavefront correction. The peak focused intensity of ~1021W /cm2 were expected.

Amplifying modeling for broad bandwidth pulse in Nd:glass based on hybrid-broaden mechanism

In this paper, the cross relaxation time is proposed to combine the homogeneous and inhomogeneous broaden mechanism for broad bandwidth pulse amplification model. The corresponding velocity equation, which can describe the response of inverse population on upper and low energy level of gain media to different frequency of pulse, is also put forward. The gain saturation and energy relaxation effect are also included in the velocity equation. Code named CPAP has been developed to simulate the amplifying process of broad bandwidth pulse in multi-pass laser system. The amplifying capability of multi-pass laser system is evaluated and gain narrowing and temporal shape distortion are also investigated when bandwidth of pulse and cross relaxation time of gain media are different. Results can benefit the design of high-energy PW laser system in LFRC, CAEP.