Study On The Nitrogen Laser Calibration System And Cloud Cover Monitoring In LHAASO-WFCTA

It has been more than a century since Hess discovered the cosmic rays.The origin,acceleration and propagation of cosmic rays are still unsolved problem.To solve these problem,Chinese high-energy physicists have set up the Large High Altitude Air Shower Observatory(LHAASO)project.LHAASO consists of three parts: the Kilometre square array(KM2A),the Water Cherenkov detector array(WCDA)and the Wide Field of View Cherenkov telescope array(WFCTA).The nitrogen laser calibration system in LHAASO-WFCTA is an important part of LHAASO,which is mainly composed of the laser calibration room,the nitrogen laser,the temperature control subsystem,the mechanical subsystem,the Programmable Logic Controller subsystem,the standard zero subsystem,The light collection device subsystem,and time scale and monitoring subsystem.Several factors,such as laser stability,laser beam pointing accuracy and PLC stability,comprehensively affect the experimental results of laser calibration and air quality monitoring in LHAASO-WFCTA.Therefore,it is of great significance to study the construction of nitrogen laser calibration system and the performance of key subsystems.In the high-altitude and extremely cold environment,this experiment uses NL100 nitrogen laser as the light source.The home-made temperature control subsystem provides constant temperature conditions for the laser to ensure the stability of the laser.The warm-up experiment results show that it takes about 60 minutes for the laser energy to stabilize.A high-precision digital image processing technology was used to investigate the performance of the three-dimensional(3D)Rotating and Lifting Platform(RLP).The results show that the rotating and lifting positioning accuracies of RLP are 0.005° and 0.056 mm,respectively,while the corresponding repeatabilities are 0.003° and 0.075 mm,respectively,which fulfil the experimental requirements of the nitrogen laser calibration system.The main innovation of the nitrogen laser calibration system in LHAASO-WFCTA is that the standard zero subsystem is proposed and installed.The function of the sub-system is to calibrate and monitor the positioning accuracy of the RLP,and to realize remote calibration in the case of misalignment.As a result,we ensure an accurately table operation of the RLP over a long period of time.The RLP is controlled by the PLC(Programmable Logic Controller),which realizes the automatic cruise function of RLP and effectively minimizes the manual intervention.In addition,the qualitative research on cloud cover is based on the imager of infrared brightness temperature and the nitrogen laser,which provides real-time cloud cover information in LHAASO-WFCTA's observation.