Backward light by stimulated Brillouin scattering (SBS) induces the final transport mirror laser damage, which has been considered a bottleneck problem in developing high-power and high-energy laser facilities. In this work, an improved continuous phase plate (CPP) model and a backward SBS model were established based on the G-S algorithm and the Fresnel diffraction transmission algorithm to simulate the backward SBS to the final transport mirror. This study discussed the propagation characteristics of backward SBS. The research results showed that the distribution of the backward SBS transmitted to the final transport mirror was determined by the phase structure of the continuous phase plate attached to the back-transmission process. By optimizing the CPP design, reducing the transmission distance, reducing the intensity modulation, and controlling the phase of the incident beam, the modulation of backward SBS can be suppressed, which has great significance in improving the laser-induced damage threshold to the final transport mirror.
Kilo-joule Laser System (KLS) is constructed as a X-ray backlighting resource, to provide X-ray for performing X-ray diagnostic experiments. As a crucial component of KLS, backlighting terminal system has such function as frequency conversion, color separation, laser transport, beam focusing, target alignment and debris shielding, enabling focal spot energy(2ω) of 500J in 1ns pulse, with target alignment accuracy of ≤±25 μm.
The Integration Test Bed (ITB) is a large-aperture single-beam Nd:glass laser system, built to demonstrate the key technology and performance of the laser drivers. The phase II designed output of the ITB at 1053nm is 18.2kJ with the peak power of 3.6TW. So it is important to keep a flat spatial intensity profile at the end of the system to avoid optical elements damage or small-scale self-focusing. Applying the Liquid Crystal Programmable Spatial Shaper (LCPSS) to compensate the beam non-uniformity related to amplification and transmission is an effective way at present. In this paper, we attempt to pre-compensate the beam nonuniformity by the LCPSS. Experiments were carried out to study the spatial fluence modulation and contrast improvement at the main laser output of the ITB laser facility. The results show that the peak-to-average fluence modulation in the near-field is typically 1.35:1; the contrast is about 0.08, at the designed energy and power, which meet the modulation less than 1.4:1 and the contrast under 0.1 design requirement.
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.
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 equipped on SG-III laser facility. After using these technologies, the focal spots of SG-III laser facility can be adjusted, controlled and repeated accurately. Experiments on SG-III laser facility indicate 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 the preamplifier and the main amplifier 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, and 0.84 with CPP and wedged lens. Polarization smoothing using flat birefringent plate in the convergent beam of final optics assembly (FOA) was studied.
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.
Multi-FM SSD and CPP was experimentally studied in high fluence and will be equipped on all the beams of SG-III laser facility. The output spectrum of the cascade phase modulators are stable and the residual amplitude modulation is small. FM-to-AM effect caused by free-space propagation after using smoothing by spectral dispersion is theoretically analyzed. Results indicate inserting a dispersion grating in places with larger beam aperture could alleviate the FM-to- AM effect, suggesting minimizing free-space propagation and adopting image relay. Experiments taken on SG-III laser facility indicate when the number of color cycles (Nc) adopts 1, imposing of SSD with 3.3 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of the preamplifier and main amplifier with 30-TDL pinhole size. The nonuniformity of the focal spot using Multi-FM SSD and CPP drops to 0.26, comparing to 0.84 only using CPP. The experiments solve some key technical problems using SSD and CPP on SG-III laser facility, and provide a flexible platform for laser-plasma interaction experiments. Combined beam smoothing and polarization smoothing are also analyzed. Simulation results indicate through adjusting dispersion directions of one-dimensional SSD beams in a quad, two-dimensional SSD could be obtained. The near field and far field properties of beams using polarization smoothing were also studied, including birefringent wedge and polarization control plate (PCP). By using PCP, cylindrical vector beams could be obtained. New solutions will be provided to solve the LPI problem encountered in indirect drive laser fusion.
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.
This paper is focused on the research of SSD and CPP carried out on TIL. A bulk phase modulator with 9.2-GHz modulation frequency is adopted in SSD. The output spectrum of the phase modulator is stable and the residual amplitude modulation is small. FM-to-AM effect caused by free-space propagation after using smoothing by spectral dispersion is theoretically and experimentally studied. Results indicate inserting a dispersion grating in places with larger beam aperture alleviates the FM-to-AM effect, suggesting minimizing free-space propagation and adopting image relay. Experiments indicate when the number of color cycles (Nc) adopts 1, imposing of SSD with 4.26 times diffraction limit (TDL) did not lead to pinhole closure in the spatial filters of the preamplifier with 20 TDL and main amplifier with 26 TDL. Experimental results also indicate SSD didn’t influence the load capacity of the laser facility. The contrast of the 440-μm diameter focal spot with 95% energy included using SSD and CPP drops to 0.47, comparing to 1.71 not using SSD and CPP. When the pulse width of the third harmonic wave is 1 ns and the energy is 1115 J, no damage is found in CPP and other final optics. The experiments solves some key technical problems using SSD and CPP on high-power laser facilities, and provides a flexible platform for the laser-plasma interaction experiments.
The multi-pass amplified scheme of SGII upgrading laser is similar as that of NIF. Large aperture plasma electrodes Pockels cell (PEPC) is the key unit of this amplified scheme. The transit time that laser beam passes through the PEPC for the first time and second time is about 270ns. PEPC should switch the state between ON and OFF in 270ns. The response time of the PEPC driven by positive-negative switching pulses can not satisfy the demand of SGII upgrading laser due to the higher generator impedance. In the single-pulse-process, the low-impedance high voltage generator based on double Blumlein pulse-forming line is used to drive the PEPC. The amplitude of single pulse is up to 21kV, while the impedance of the generator is only 6.25Ω. The theoretical charge time of the PEPC with 350mm×350mm aperture is about 54ns, and the response time of PEPC is less than 170ns in the single-pulse-process. The response time is reduced greatly. The switching efficiencies with full aperture are higher than 99.7%. The extinction contrast exceeds 381. The top width of the time window is larger than 160ns, and the bottom width is about 400ns. All the experimental results can meet the specification of SGII upgrading laser.
With the methods of time-division multiplexing in Frontend and angle detuning in FOA, each beam pulse on SG-III
prototype facility is controlled independently and so the systematic variations of power imbalance are eliminated
entirely.
A novel method based on diffraction theory to control the far-field irradiance profile by deformable mirror is presented.
Special near-field phase which determines the contour of the focal spot is obtained by a high spatial frequency
deformable mirror. Numerical simulations show that, we can control the far-field intensity envelope as CPP by
adopting adaptive optics technique when the spatial resolution of deformable mirror is high enough, here 16×16
actuators in 320mm×320mm aperture. The coupling coefficient is an important factor influencing control effect, and
its best value range is round 0.6.
KEYWORDS: Tolerancing, Lens design, High power lasers, Solids, Optical design, Optical amplifiers, Optical filters, Telescopes, Near field optics, Electronic filtering
The injection lens of the high power solid laser facility built soon was designed as double
lens. The tolerance of the double lens was analyzed, and the optical performance of which was
detected in experiment.
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