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The SG-Ⅲ laser facility (SG-Ⅲ) is the largest laser driver for inertial confinement fusion (ICF) researches in China, which has 48 beamlines and can deliver 180 kJ ultraviolet laser energy in 3 ns. In order to meet the requirements of precise physics experiments, some new functionalities need to be added to SG-Ⅲ and some intrinsic laser performances need upgrade. So at the end of SG-Ⅲ's engineering construction, the 2-year laser performance upgrade project started. This paper will introduce the newly added functionalities and the latest laser performance of SG-Ⅲ. With these function extensions and performance upgrade, SG-Ⅲ is now fully prepared for precise ICF experiments and solidly paves the way towards fusion ignition. Keywords: Inertial confinement fusion, Laser driver, SG-III, Power balance, Beam smoothing, PACS Codes: 52.57.-z, 42.60.-v, 42.55.-f, 28.52.-s

In the research of inertial confinement fusion, laser plasma interaction (LPI) is becoming a key problem that affects ignition. Here, multi-frequency modulation (Multi-FM) smoothing by spectral dispersion (SSD), continuous phase plate (CPP) and polarization smoothing (PS) were experimentally studied and implemented on the SG-III laser facility. After using these techniques, the far field distribution of SG-Ⅲ laser facility can be adjusted, controlled and repeated accurately. The output spectrums of the cascade phase modulators used for Multi-FM SSD were stable and the FM-to-AM effect can be restrained. Experiments on SG-III laser facility indicate that when the number of color cycles adopts 1, imposing SSD with 3.3 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of preamplifier and main amplifiers with 30-TDL pinhole size. The nonuniformity of the focal spots using Multi-FM SSD, CPP and PS drops to 0.18, comparing to 0.26 with CPP+SSD, 0.57 with CPP+PS and 0.84 with only CPP and wedged lens. Polarization smoothing using flat birefringent plate in the convergent beam of final optics assembly (FOA) was studied. The PS plates were manufactured and equipped on SG-III laser facility for LPI research. Combined beam smoothing and polarization manipulation were also studied to solve the LPI problem. Results indicate that through adjusting dispersion directions of SSD beams in a quad, two dimensional SSD can be obtained. Using polarization control plate (PCP), polarization on the near field and far field can be manipulated, providing new method to solve LPI problem in indirect drive laser fusion.

In this paper, we give a review of our theoretical and experimental progress in octahedral spherical hohlraum study. From our theoretical study, the octahedral spherical hohlraums with 6 Laser Entrance Holes (LEHs) of octahedral symmetry have robust high symmetry during the capsule implosion at hohlraum-to-capsule radius ratio larger than 3.7. In addition, the octahedral spherical hohlraums also have potential superiority on low backscattering without supplementary technology. We studied the laser arrangement and constraints of the octahedral spherical hohlraums, and gave a design on the laser arrangement for ignition octahedral hohlraums. As a result, the injection angle of laser beams of 50°–60° was proposed as the optimum candidate range for the octahedral spherical hohlraums. We proposed a novel octahedral spherical hohlraum with cylindrical LEHs and LEH shields, in order to increase the laser coupling efficiency and improve the capsule symmetry and to mitigate the influence of the wall blowoff on laser transport. We studied on the sensitivity of the octahedral spherical hohlraums to random errors and compared the sensitivity among the octahedral spherical hohlraums, the rugby hohlraums and the cylindrical hohlraums, and the results show that the octahedral spherical hohlraums are robust to these random errors while the cylindrical hohlraums are the most sensitive. Up till to now, we have carried out three experiments on the spherical hohlraum with 2 LEHs on Shenguang(SG) laser facilities, including demonstration of improving laser transport by using the cylindrical LEHs in the spherical hohlraums, spherical hohlraum energetics on the SGIII prototype laser facility, and comparisons of laser plasma instabilities between the spherical hohlraums and the cylindrical hohlraums on the SGIII laser facility. PACS codes: 52.70.La, 52.35.Tc, 47.40.Nm, Keywords: Indirect-drive inertial confinement fusion, Novel spherical hohlraum with 6 LEHs, High and robust radiation symmetry, High energy coupling efficiency, Theoretical study, Experiments

The polarization smoothing (PS) of the focal spot on target is a key technology for inertial confinement fusion (ICF) laser. A mathematical model is presented to analyze the polarization smoothing in a convergent beam. The relation between the separations (both transverse and longitudinal) of focal spots and the parameters of the crystal are established. Via numerical simulation, the three-dimensional distributions of the far-field with and without PS are demonstrated. The relation between the property of the focal spot and the crystal’s thickness and tilt angle are obtained. Best smoothing can be achieved with the optimized thickness and tilt angle of the PS crystal.

Advanced an online low 1ω drive irradiance tuning technique of frequency conversion crystals of high power solid-state laser facility, which can acquire the best match angle of frequency conversion crystals through online low 1ω drive irradiance tuning curve test, and achieve fast and high precision angle correction to assure the frequency conversion crystals to achieve the highest energy conversion efficiency in shot experiments. Analyzed the possibility of online low 1ω drive irradiance tuning technique of frequency conversion crystals, researched the technical scheme of online low 1ω drive irradiance tuning of frequency conversion crystals, and applied this technique on SG facility, which achieved 60%~70% frequency conversion efficiency in high energy shots.

The dependence of the phase matching (PM) angle on temperature in type-I second harmonic generation (SHG) and type-II third harmonic generation (THG) of KDP is experimentally investigated. The experimental results show that as the crystal temperatures vary by 1 °C, the external PM angles of the KDP doubler and tripler change 100 μrad and 220 μrad, respectively. The PM angle of KDP tripler is more sensitive to temperature variation than is that of the KDP doubler. Comparing the numerical calculations and the previous reports, a discrepancy has been found among the different reported results. Based on the analytical solution of the temperature dependence of the SHG process it is shown that a small error in the thermo-optic coefficients of the crystal is the main factor source of the discrepancy.

A novel composite partially deuterated KDP crystal used for frequency tripling is experimentally demonstrated for the first time in this paper. The deuteration level is used as a degree of freedom to alter the phase matching wavelength so as to increase the bandwidth acceptance. The results show that the acceptance bandwidth based on such crystal can be increased by a factor of 3 comparing with the conventional method. The obtained results are very useful to develop frequency tripling components in broadband third harmonic generation.

In this paper, it is the first time to experimentally demonstrate the second harmonic generation (SHG) of temporal phase modulated broadband laser. The SHG conversion efficiency and the characteristic of SHG spectrum are investigated in detail and the relation between SHG conversion efficiency and incident fundamental light intensity is obtained. The highest efficiency of nearly 70% is achieved at the fundamental pulse intensity of 1.6 GW/cm2. The experimental results show that the SHG spectrum widths under high and low intensity are both half of the incident fundamental lights, and don’t result in the spectrum narrowing. The obtained results are very useful for operation at the second harmonic wavelength in inertial confinement fusion (ICF) laser facility.

Based on the study and manufacture of the technical integration experiment line (TIL) prototype laser facility, and using the method that theoretical simulations guide experiments and experimental results check programs, the factors affecting third harmonic generation (THG) efficiency for a large-aperture, high-intensity inertial confinement fusion (ICF) laser facility have been analyzed and studied in detail. Furthermore, in the year 2004–2005, the output ability of a THG system for a TIL driver has been discussed adequately in a first beam eligibility experiment. As shown by experimental results, when the average drive irradiance of fundamental light, I1ω, is 2.7 GW/cm2, the measured value of THG efficiency is 50–55% and the theoretical value is 60%, which indicates that the experimental result agrees well with the theoretical value.

Precise physical experiments place strict requirements on target illumination uniformity. A new smoothing technology aimed at indirect-drive laser fusion is studied, which uses linearly modulated light and angular spectral dispersion (ASD). Simulation indicates that the best smoothing effect could be obtained when number of color cycles ( N c ) is chosen to be 0.5 at 1.2 nm bandwidth. A table-top setup aimed at the experimental study of the beam smoothing method has been established. When the linearly modulated light generated by pulse stacking is dispersed by a grating with N c =2.6, periodical undulations are found on the far-field focal spot, which agrees with the simulation results. To improve illumination uniformity, the N c adopted should be decreased to 0.5. Simulation also indicates that the combination of linearly modulated light, angular spectral dispersion and a continuous phase plate could considerably improve the irradiation nonuniformity.

A multi-beam alignment method is proposed to reduce the total time for aligning at the target area all the laser beams of an ICF laser facility. A number of sub-areas with invariant size and position are extracted from the image acquired by the alignment sensor. An alignment route is comprised of a certain part of those sub-areas, and several alignment routes can cover all the sub-areas. The invariant layout of the sub-areas and the alignment routes is called an invariant sub-area configuration of the alignment sensor. The focused spots of the alignment beams are adjusted in a specific sequence along the alignment routes, and finally reach the desired position on the alignment sensor. The adjustment of all the spots inside each sub-areas is carried out concurrently, and the adjustment along one route for a spot moving from one sub-area into the next sub-area is carried out consecutively. The estimated total time for aligning all the laser beams at target area shows that the proposed multi-beam alignment method has a much higher efficiency.

The target position system (TPS) is one of the important subsystems of an ICF laser facility. However, TPS shows to have kinematic coupling problem in practice. This necessitates iterative adjustment of the Stewart 6-DOF manipulator to make the pose of the target as expected. In every iteration, the pose of the target must be measured, making TPS incompetent in some scenarios which call for only one step to position a target. To handle this problem, this paper proposes a target positioning method focusing on translational kinematic coupling. This method have a significant advantage that it has no relation with both the geometric parameters and the mounting of the target. This makes the proposed positioning method featured by a good practicality. Experiment results show that the proposed method can greatly reduce the position error when positioning a target by only one step.

To improve the performance of SG-III prototype facility (TIL-Technical Integration Line), final optics assembly (FOA) is re-designed. It contains that stray light and focusing ghosts are optimized, operational performance and environments are improved and the total thickness of optics is reduced. With the re-designed FOA, Some performance advantages are achieved. First, the optics damages are mitigated obviously, especially crystals and Focus lens; Second, stray light and focusing ghosts are controlled better that organic contamination sources inside FOA are eliminated; Third, maintenance and operation are more convenient for the atoms environment; Fourth, the focusable power on target is increased for lower B-integral.

In ICF laser driver, the technique of precise target locating and guiding with multiple-beam is the core technique to couple beams and target on experiments. Its performance superiority will decide operation success or failure of the experiment on the whole facility. This paper will describe basic configuration of multiple-beam guiding and target locating system in detail, emphasize on the technique route, which makes the whole system to carry out the full closed loop control guiding beams and auto program process of shooting target, and makes the system shooting preparation time less than 30 minutes and shooting accuracy better than 30μm (RMS). The technique is successfully applied on ShenGuang-III prototype facility for two years.

The analysis of Bespalov and Talanov concerning small scale self-focusing or filamentation effects for plane waves is generalized to a tapered beam case in this paper. A model of nonlinear phase and amplitude perturbations different from Siegman’s is suggested. The previous conclusion that self-focusing is very ineffectual for strongly tapered, both divergent and convergent, beams in optical Kerr media as compared to small-scale self-focusing in plane waves is found to be inappropriate. Moreover, the different conditions of phase matching in cases of cylindrical and spherical waves are explicated, which could throw some light on the issue.

This paper will introduce in detail how to adopt the technique of the match between light beams and the target on optoelectronics and control system to realize the full closed loop control of light beam leading and target precisely fixed position, auto program flow of shooting target practice process, make the system shooting practice preparative time less than 30 minutes and shooting target practice accuracy better than 30µm (RMS). Its performance superiority will decide operation efficiency and success or failure of physics experiment on the whole ICF laser driver facility.

The influence of laser beam size on laser-induced damage performance, especially damage probability and laser-induced damage threshold (LIDT) is investigated. It is found that damage probability is beam size dependent when various damage precursors with different potential behaviors are involved. This causes damage probability and LIDT are different between tested under large-aperture beam and under small-aperture beam. Moreover, fluence fluctuations of large-aperture laser beam bring about hot spots moving randomly across the beam from shot to shot. Thus it leads to the most probable maximum fluence after many shots at any location across components is several times the average beam fluence. These two effects result in difference of damage performance of components in large-aperture lasers and in small-aperture lasers.

By third-order nonlinear coupling and tightly focusing the beams in a single crystal of BBO, high intensity broad-bandwidth femtosecond pulses are used to generate 50μJ of light at 270nm (up to 1% THG efficiency). It is proved that the major contribution to the THG observed is the third-order process, not the cascaded second-order process. And the primary reasons why THG efficiency is about 1% is that 1ωlight is frequency-chirp beams and BBO crystal isn't tuned to optimized angle. At last, the methods to improve the third harmonic conversion efficiency have been put forward.

By third-order nonlinear coupling and tightly focusing the beams in a single crystal of BBO, high intensity broad-bandwidth femtosecond pulses are used to generate 50mJ of light at 270nm (up to 1% THG efficiency). And it is proved that the major contribution to the THG observed is the third-order process, not the cascaded second-order process.

We report an experimental demonstration of polarization smoothing by using a specially cut KDP plate in the convergent ultraviolet (351 nm wavelength) section of a high-power laser system. A 410 mm×410 mm×10 mm sized KDP plate placed after the final focus lens successfully reduced the focal spot intensity contrast to 75.6%, which is in good agreement with the theoretical expectation. In our experiment, no obvious transverse stimulated scattering was observed with the maximum 351 nm laser energy of 4032 J/3 ns, thus indicating the operation safety margin of our laser system.

To realize multi-beam positions and crystals alignment, the 3Ω target alignment laser of SG-III Prototype Facility has been designed as a reflection optical system, which avoids the effect of chromatic aberration.

The design of the Large Laser Facility incorporates a type-I-type-II third harmonic generator to convert the 1.053μm fundamental wavelength of the laser amplifier to a wavelength of 0.351μm for target irradiation. To understand the design of frequency conversion system, we have carried out a series of theory researches and experiments, including parameters optimization of the third harmonic converter, experiments on high-intensity third harmonic generation (THG), researches on Transverse Stimulated Raman Scattering (TSRS) in the converting crystal and on broad-band third harmonic generation.

To measure the temperature and density of electronics in laser plasma accurately, two probe laser systems are built on XG-II laser facility. The one, called UV probe laser system, which can provide the UV laser pulse with duration of 30 ps by cascade Raman compressor after fourth harmonic conversion is used for density measurement of laser plasma. The other one, Thomson scattering system, which can provide the 4 ω laser pulse with energy of 3 - 5J, is now routinely operated for electronics temperature diagnostic in laser plasma. It is the first time that the density and temperature of laser plasma are measured directly by probe laser at the same shot.

Stray light in high-power laser facilities not only decreases the gain capacity of the main amplifier, but can also cause permanent damage to optical components, greatly threatening the safety operation of the SG-III. This Letter reports a technique for quickly pinpointing the source of stray light in the main amplification optical path of the SG-III. The achieved results indicate that the accuracy of our method is up to 1 m in an optical path of 100 m. The judging method is effective for examining and removing harmful stray light in the SG-III laser facility and it can be promoted in huge laser facilities of this kind.

A 2ω wedge design is proposed with KDP crystal to disperse the unconverted light away from the target in a high power laser facility for inertial confinement fusion. The ultraviolet B-integral problem is released, and about 1.2 times in color separation angle is achieved according to both theoretical and experimental investigations when compared with conventional 3ω wedge. The frequency conversion efficiency is unaffected when the wedge is along the non-sensitive axis of the tripler.

Temporal, spectral and spatial characters of 0.3-nm-bandwidth high-energy laser pulse propagating through a long path are studied in detail in one newly constructed beamline of our laser facility. The evolution of propagation, pulse energy and near-field deterioration are analyzed theoretically and experimentally. Substituting argon for air is demonstrated effectively to suppress stimulated rotational Raman scattering and the experimental result provides operating criterion, and engineering parameters for the under-constructed beamlines.

The influence of laser beam size on laser-induced damage performance, especially damage probability and the laser-induced damage threshold (LIDT), is investigated. It is found that damage probability is dependent on beam size when various damage precursors with different potential behaviors are involved. This causes the damage probability and the LIDT to be different between cases under a large-aperture beam and a small-aperture beam. Moreover, the fluence fluctuation of the large-aperture laser beam brings out hot spots, which move randomly across the beam from shot to shot. Thus this leads the most probable maximum fluence after many shots at any location on the optical component to be several times the average beam fluence. These two effects result in the difference in the damage performance of the optical component between the cases under a large-aperture and small-aperture laser.

The dark spot effect downstream from the nonlinear hot image is accounted for in this paper. The conditions for the formation of dark spot are carefully discussed. The explanation is based on analytical analysis, and the results are verified by numerical simulations. The dependence of the location of the dark spot on the nonlinear phase delay seems to suggest a probable method for measuring the nonlinear refractive coefficient of materials.

Based on the existing design in our prototype laser facility, the third-harmonic generation of the phase modulated broadband of 0.3 nm and narrowband lasers are theoretically analyzed and experimentally studied in this paper. The output third-harmonic generation (THG) laser characteristics such as the conversion efficiency, the spectrum distribution and the intensity modulation are compared in detail. It is found that the THG conversion efficiencies for the narrowband and such broadband lasers are fundamentally identical without spectrum narrowing. In addition, the interrelationship between THG conversion efficiency and incident fundamental light intensity for both lasers is experimentally demonstrated and the experimental results agree with the theoretical analysis. The broadband THG spectrum with ideal repetition and without loss of spectrum is obtained in the experiment.

Laser induced damage experiment is carried out on a large aperture laser facility. Severe damage is observed on a large-aperture fused silica grating which presents dense craters on the front surface and six cracks alternatively located at the front and the rear surface. The bizarre fact about the damage on the grating is that, unlike other optics, the damage craters are almost on the front surface. According to observation, damage phenomenon is due to the stimulated Brillouin scattering (SBS) effect occurring in the grating, which includes the transverse SBS, the back SBS and the zigzag SBS.

The cascaded KDP crystal scheme with different deuteration levels for enhancing the bandwidth of third-harmonic generation (THG) is experimentally demonstrated in this paper. The deuteration level of a partially deuterated KDP crystal is used as a degree of freedom to alter the phase-matching (PM) wavelength. The experimental results show that this scheme can increase the bandwidth acceptance in broadband frequency tripling. In addition, the spacing between the two cascaded crystals has significant influence on the THG spectrum as well as THG conversion efficiency, and a relatively ideal THG spectrum can be obtained by the optimized parameters. Meanwhile, nonlinear-coupled wave equations are used to theoretically analyze the experimental results, and the experimental results are consistent with the numerical simulation.