Donghui Lin

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.

286-TW Ti:sapphire laser at CAEP

We have built a three-stage Ti:sapphire laser system at CAEP which could deliver 5-TW, 30-TW and 286-TW pulses to the corresponding target chambers for diverse applications with innovative high-power Ti:sapphire crystal amplifiers. Pulse durations of 30fs have been obtained by installing an acousto-optic programmable dispersive filter (AOPDF) before the stretcher to compensate for the spectral gain narrowing. By taking a number of advanced measures for spatial beam control, near-diffraction limited focal spots (FWHM) have been obtained which, to our knowledge, are the best far fields ever measured for the existing high-power Ti:sapphire laser systems without deformable mirror correction. Focused laser intensity is about 1021W/cm2 measured with an f/1.7 OAP. The laser system has the potential to operate at 500TW and even higher and laser intensities of 1022W/cm2 are expected with deformable mirror for wavefront correction and small f-number fine OAP for tighter focus added to the system in the near future.

Progress on developing a PW ultrashort laser facility with ns, ps, and fs outputting pulses

A petawatt laser facility with three beams for fast ignition research and strong-field physics applications has been designed and is being constructed. The first beam (referred as SILEX-I) is a Ti:sapphire femto-second laser which pulse width is 30 fs, and till now, output power has reached to 330 TW. The other two beams are Nd3+:glass lasers which output energy are larger than 1kJ and pulse width are about 1ps and 1ns respectively. By using the technology of OPA pumped by 800nm femtosecond laser and seeded by super-continuum spectrum white light, the three beams are synchronized with each other without jitter time. By using the seeds from OPA pumped by femtosecond laser, and by using the pre-amplification stage of OPCPA, the signal to noise ratio of the Nd3+:glass petawatt laser will reach to 108. Active methods are taken to control the gain narrowing effect of the Nd3+:glass amplifiers, giving the option to compress the chirped pulse to ultrashort pulse with width less than 400fs. Tiled multilayer dielectric coating gratings are used for the compressor of the PW beam, which has been successfully demonstrated on a 100J picosecond Nd3+:glass laser system.

Matched wavelength and incident angle for the diagnostic beam to achieve coherent grating tiling

Design and operation of a practical, accurate alignment diagnostic system is important for the grating tiling technology, which is supposed to be applied in a chirped-pulse amplification system to increase the output power. A diagnostic method is proposed and demonstrated for grating tiling. Provided that the wavelength and incident angle of the diagnostic beam are properly set, the far-field of the main laser beam and that of the diagnostic beam can vary in the same way with the tiling errors between the sub-aperture gratings. Therefore, rotational and translational errors can be controlled and compensated according to the far-field of the diagnostic beam. The real-time monitoring and alignment can be achieved without disturbing the main beam.

Design and performances of prototype laser amplifiers for technical-integration-line facility

We present work on the design and physical performances for a large-aperture multisegment amplifier, which is an engineering prototype amplifier for our technical-integration-line facility. The amplifier consists of 4×2 apertures with each clear aperture about 300×300 mm. Each amplifier module consists of eight laser slabs, arranged four high, two wide, and one long, and twenty high-pulse power flashlamps with an input electric energy of about 23 kJ per lamp. These twenty flashlamps are arranged in a 6-8-6 scheme. The central eight flashlamps will irradiate their energy to the laser slabs in two directions to increase the pumping transfer efficiency. Experimental results indicated that the energy stored in the upper laser level is about 0.24 J/cm 3 , which corresponds to an energy storage efficiency of about 3.0%. Finally, with an 1.053-μm laser probe we obtained a small-signal gain coefficient of 5.0% cm –1 .

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.

Development of large-aperture electro-optical switch for high power laser at CAEP

Large-aperture electro-optical switch based on plasma Pockels cell (PPC) is one of important components for inertial confinement fusion (ICF) laser facility. We have demonstrated a single-pulse driven 4×1 PPC with 400mm×400mm aperture for SGIII laser facility. And four 2×1 PPCs modules with 350mm×350mm aperture have been operated in SGII update laser facility. It is different to the PPC of NIF and LMJ for its simple operation to perform Pockels effect. With optimized operation parameters, the PPCs meet the SGII-U laser requirement of four-pass amplification control. Only driven by one high voltage pulser, the simplified PPC system would be provided with less associated diagnostics, and higher reliability. To farther reduce the insert loss of the PPC, research on the large-aperture PPC based on DKDP crystal driven by one pulse is developed. And several single-pulse driven PPCs with 80mm×80mm DKDP crystal have been manufactured and operated in laser facilities.

Single-pulse driven plasma Pockels cell with 350mm×350mm aperture

Large-aperture plasma Pockels cell is one of important components for inertial confinement fusion laser facility. We demonstrate a single-pulse driven PPC with 350mm×350mm aperture. It is different to the PPC of NIF and LMJ for its simple operation to perform Pockels effect. With optimized operation parameters, the PPC meets the optical switching requirement of SGII update laser facility. Only driven by one high voltage pulser, the simplified PPC system would be provided with less associated diagnostics, less the maintenance, and higher reliability.