Studies on design of 351  nm focal plane diagnostic system prototype and focusing characteristic of SGII-upgraded facility at half achievable energy performance

Abstract

In order to obtain the intensity distribution of a 351 nm focal spot and smoothing by spectral dispersion (SSD) focal plane profile of a SGII-upgraded facility, a type of off-axis imaging system with three spherical mirrors, suitable for a finite distance source point to be imaged near the diffraction limit has been designed. The quality factor of the image system is 1.6 times of the diffraction limit tested by a 1053 nm point source. Because of the absence of a 351 nm point source, we can use a Collins diffraction imaging integral with respect to λ=351 nm, corresponding to a quality factor that is 3.8 times the diffraction limit at 351 nm. The calibration results show that at least the range of ±10 mrad of view field angle and ±50 mm along the axial direction around the optimum object distance can be satisfied with near diffraction limited image that is consistent with the design value. Using this image system, the No. 2 beam of the SGII-upgraded facility has been tested. The test result of the focal spot of final optics assembly (FOA) at 351 nm indicates that about 80% of energy is encompassed in 14.1 times the diffraction limit, while the output energy of the No. 2 beam is 908 J at 1053 nm. According to convolution theorem, the true value of a 351 nm focal spot of FOA is about 12 times the diffraction limit because of the influence of the quality factor. Further experimental studies indicate that the RMS value along the smoothing direction is less than 15.98% in the SSD spot test experiment. Computer simulations show that the quality factor of the image system used in the experiment has almost no effect on the SSD focal spot test. The image system can remarkably distort the SSD focal spot distribution under the circumstance of the quality factor 15 times worse than the diffraction limit. The distorted image shows a steep slope in the contour of the SSD focal spot along the smoothing direction that otherwise has a relatively flat top region around the focal spot center.

Status and development of high-power laser facilities at the NLHPLP

“…As the pulse again propagates through the power amplifier, after passing the TSF and transport mirrors, it enters the final optics assembly (FOA) [50] and it is frequency converted to 351 nm light, before being focused to the target chamber using the wedged focus lens. In addition, the measurement system [51][52][53] , centralized control system [54,55] , and automatic alignment system [56,57] are all involved to enable the effective operation of the facility.…”

“…The total output energy of eight beams is in routine operation. We have developed a precision diagnostic system (PDS) based on a type of off-axis imaging system with three spherical mirrors [52] to measure the key parameters, such as the near and far fields. The near and far fields with a frequency and energy of 8.5 kJ are shown in Figure 11.…”