High Power Laser Synchronous Measurement Technology at the Target
Objective Laser inertial confinement fusion(ICF)achieves controllable nuclear fusion to produce clean and safe energy.The ICF experiment has stringent energy,power balance,and waveform consistency requirements for pulses arriving at the target point.Uniform driving of the target surface requires accurate beam synchronization to achieve an accurate power balance.Therefore,synchronous measurement and adjustment technology for multibeam lasers is critical.Methods To achieve synchronous measurements of multiple beams at the target point,the testing method and principle employed are shown in Fig.4.The seed light was sampled and coupled as the reference light after passing through the regenerator and then connected to a 1053 nm single-mode fiber with a length of approximately 130 m.After transmission through the fiber,it was converted into an electrical signal using a photodiode and entered an oscilloscope.The central target sphere in the target chamber is replaced by an～800 μm diameter alumina scattering sphere positioned at the center of the target chamber using the target positioning system.In this experiment,a single beam of ultraviolet light from any direction is scattered isotropically at a solid angle of 4π,covering the entire target chamber.A fused quartz nonspherical mirror was placed on a flange in the direction of non-transmitting light to capture the scattered light.The scattered light is detected using a fast photomultiplier tube and converted into an electrical signal,which is input into another oscilloscope channel.The synchronization time delay between the measured beam and the time reference can be measured.Results and Discussions The time delay between the first signal light and the infrared reference light of the Shenguang-Ⅱ device is measured.Fig.6 shows the track data obtained using an oscilloscope.The blue curve represents the infrared reference light,and the red curve represents the ultraviolet signal light.The infrared reference and ultraviolet signals are fitted with second-and first-order Gaussian functions.The vertex of the fitted Gaussian pulse is selected as the characteristic point to measure the difference in the delay between the two pulses.The experiment measures the delay between the signal light and the reference light eight times,and the average time difference is obtained.Fig.8 shows the eight test results of the relative delay measurement of the first signal light and the infrared reference light,where the red line is the average of the eight signals.The average of the eight signals was 852.208ps,the peak valley(PV)value was 3.660 ps,and the root mean square(RMS)was 1.213 ps.An off-line experiment was performed to analyze the inter-beam jitter caused by the method.A scattering sphere combined with an oscilloscope was used for the measurements.The experimental optical path is shown in Fig.10.The interbeam jitter of the relative delay of the dual-channel signal under 10,20,50,100,150,and 200 shots was tested.The jitter test results are listed in Table 1.When the test involved 10 shots,the jitter PV value was 2.53 ps,and the RMS value was 0.821 ps.Under the maximum test of 200 shots,the jitter PV value was 5.98 ps,and the RMS value was 1.124 ps.Subsequently,the two optical signals do not pass through the scattering spheres.The optical path diagram is shown in Fig.11.The jitter test results are shown in Fig.2.The jitter PV value of the two pulses was 2.51 ps,and the RMS value was 0.771 ps for 10 shots.The relative jitter PV value at 200 rounds was approximately 6.19 ps,and the RMS was approximately 1.10 ps.By comparing Tables 1 and 2,introducing the scattering sphere does not affect the measurement of inter-beam jitter.Comparing offline jitter test results obtained from the scattering sphere and oscilloscope dual-channel with online jitter measurements using a single pulse from the SGⅡdevice,the PV value was 3.660 ps,and the RMS value was 1.213 ps.These data can be considered the inherent jitter of online synchronous measurement devices.Subsequently,the relative delay between the multiple signal lights to be measured and the reference light was tested and averaged for the SG Ⅱ device's second,fifth,and sixth channels.Fig.9 shows the final test results for the relative delay of the final four beams.The PV value between the beams was 3.144 ps,and the RMS was 1.476 ps.In multichannel laser synchronous measurements,delay errors primarily stem from the geometric structure of the scattering sphere,positioning accuracy,oscilloscope indication errors,and collimation drift.In this experiment,the error in interbeam synchronous measurement was determined to be 762 fs.Conclusions A method is proposed for time synchronization measurement of multibeam laser targets using a scattering sphere tailored explicitly for large laser devices like SG-Ⅱ.Through a verification experiment based on the target synchronization measurement of the SGⅡ device,the final synchronization measurement results of the four beams were found to be 3.144 ps(PV)and 1.476 ps(RMS).While the maximum delay error in interbeam synchronization measurement due to this scheme reached 5.06 ps,the interbeam synchronization error for the simultaneous measurement of four beams with identical scattering angles was approximately 762 fs.The final experimental measurements and analysis concluded that the time-synchronization jitter and delay based on the target achieved precision at the picosecond level for both nanosecond-long pulses and picosecond-short pulses.In addition,the inherent jitter of the method was obtained by comparing the offline and online experiments.
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