China is constructing SG-III(48 beams,3ns,3ω,200kJ) super laser facility, which to be in operation in 2012. And the new Ignition facility (3ns,3ω,1.4MJ) will be finished in 2020.
The SG-III laser facility, which is one of the most important parts of the China ICF Program, is now under construction in the Research Center of Laser Fusion (LFRC) of China Academy of Engineering Physics (CAEP). The engineering design of the facility has already been completed and the
construction of building housing the laser system will be completed in late 2007. SG-III will be used to investigate target physics before ignition for both direct-driven and indirect-driven ICF and will be operating in 2012. The facility is designed to provide up 48 energetic laser beams (six bundles) and laser energy output of 150-200kJ (3ω) for square pulse of 3ns. If fast ignition is workable, SG-III will couple with a PW laser of tens KJ to demonstrate fast ignition.
The SG-III laser facility is shown schematically in Figure 1. The entire laser system and target area are housed in an environmentally controlled building. The building is mainly divided into three parts, which are main experimental area, target area and optics assembly area. The main experimental area, approximately 24 meters wide and 121 meters long, is the core area where the main control room, front-end system, preamplifier, main amplifier and capacitor reside. The optics assembly building is located at one side of building for assembling and installing the precision optical and opto-mechanical components that make up the SG-III laser system. All areas are one level except for the target area,
which have five floors and in the central core where elevated floors provide space for facility utility equipment.
SG-III’s laser system, the heart of the facility, is comprised of 48 high-power laser beams. For inertial fusion studies, these laser beams will produce 180,000 joules (approximately 60 trillion watts of power for 3 nanoseconds) of laser energy in the near-ultraviolet (351-nanometer wavelength). It consists of a number of subsystems including front-end, preamplifier, main amplifier, target diagnostic unit, beam control and diagnostic unit, and the integrated computer control system.
The front-end system mainly consists of four parts: 100-ps standard pulse unit, pulse shaping unit, fiber transport/amplify unit and power amplifier unit.
The pulse begins in a passively mode-locked Yb-fiber laser which generates a train of stable 200-fs pulses at the central wavelength of 1053 nm with a bandwidth of 10 nm. A polarization- independent waveguide modulator is used as the pulse selector to decrease the pulse repetition rate from 20 MHz to 1 Hz. The selected pulses are coupled into an Yb-fiber amplifier with a ~1 nm fiber grating filter centering at 1053 nm, and after passing through a chirped fiber grating the amplified pulse is stretched to ~100 ps called standard pulse.
These ~100 ps standard pulses are input into a compact pulse shaping system based on temporal stacking of pulses. The system can generate shape-controllable pulses with fast rise time of 50 ps with a spectral bandwidth of 1.2nm. The pulse stacker, the main part of the system, is made up of eight 4-channel modules, each of which consists of a 1×4 divider, a 4×1 multiplexer and four variable optical attenuators (VOA). By using another two pairs of 1×4/ 4×1 and one pair of 1×2/ 2×1 fiber couplers, these modules are arranged in a parallel configuration which constructs a 32-channel pulse stacker, as shown in Figure 3. Since a number of fiber dividers, multiplexers and VOAs are used in the stackers that introduce a relatively high loss, two successive Yb-fiber amplifiers are adopted to amplify the shaped pulse.
Each of the 48 pulses from the Front-end enters the Preamplifier system on a single-mode optical fiber, where it is amplified first by a repetition amplifier module, then by a LD-array pumped thin-slab amplifier and a double-pass rod amplifier. After aligned and isolated, the pulse from the Front-end system is first injected into the high-gain amplifier where experiences a gain that raises its energy from ~10 uJ to ~10 mJ. After being switched out of the high-gain amplifier, the pulse traverses a spatial shaping module that transforms the Gaussian spatial shape to a profile that is designed to compensate for the spatial nonuniformity of the gain throughout the rest of the laser. The ability to accurately shape the spatial profile allows the SG-III to produce beams at the output of the system that have a flat irradiance distribution across the central part of the beam.
After passing through the beam-shaping module, the pulse is injected into thin-slab amplifier pumped by LD array, where the pulse makes ~12 round trips and improves its energy from ~10 mJ to ~1J. Then the pulse makes double pass through a flashlamp-pumped rod amplifier, yielding a nominal net energy gain of 5. The overall energy gain of the preamplifier system is of the order of 106.
The SG-III facility/ beampath has been successfully designed, and the construction housing the laser system is going to completed at the end of this year. During the next four years, a relatively small staff of engineers, designers, and construction staff will assist in completing this project. Based on the success of checking the key systems and unit techniques, such as multi-segment amplifiers, the whole solid-state fiber front-end technique etc on the technical integrated line of SG-III, we have high confidence in the project.