Properties and performance of nitrogen laser systems for calibration of LHAASO-WFCTA

Abstract

The Wide Field of view Cherenkov Telescope Array (WFCTA) of the Large High Altitude Air Shower Observatory (LHAASO), located in Haizishan National Natural Reserve, Sichuan Province, China, is designed to perform nearly calorimetric measurements of extensive air showers induced by cosmic rays with energies between 1013eV–1018eV. The end-to-end calibration method will be employed in the WFCTA and nitrogen lasers have been proven to work properly in our application at the altitude of 4410 m a.s.l. for the first time.Due to extremely harsh environment, especially outdoor temperature as low as about −30 °C during operation of WFCTA, a high-stability thermotank for lasers was designed to stabilize temperature and humidity, with a thermal stability of 1 °C. In order to precisely control the beam pointing, a high-precision 3D Lifting Rotating Platform (HiRoP) with a home-made standard zero monitoring system is developed. To eliminate the error of the beam pointing by manual controls, an automatic scanning program was adopted to control the beam pointing when scanning the full field-of-view (FOV) of WFCTA. Both the on-site and remote controls are applied for the nitrogen laser systems and ∼13 million laser shots have been recorded by WFCTA from Sept. 2020 to May 2022. In this paper, the design, properties, operation and the on-site performance of the nitrogen laser systems at LHAASO are reported.