Temperature effect and correction method for LHAASO KM2A timing synchronization node

Abstract

The Large High Altitude Air Shower Observatory (LHAASO) project aims to search the origin of galactic cosmic rays above 30TeV with high sensitivity and wide spectrum, it's a dedicated instrument made by 4 sub-detector arrays. The square kilometer complex array (KM2A) consists 5635 scintillation electron and 1221 muon detectors covering the area of 1.2Km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . To guarantee the angular resolution of reconstructed air shower event, a 500ps (rms) timing synchronization must be achieved among the spread few thousand detectors. White Rabbit (WR), a recent emerging technology providing a cost-effective solution that combines sub-nanosecond precision timing transfer and gigabit Ethernet data transfer over the same fiber media, is the most attractive solution to solve the synchronization requirement for the LHAASO KM2A detector array. A compact universal timing endpoint based on write rabbit (CUTE-WR) been designed as timing synchronization interface card to provide synchronized clock and time information to each KM2A detect unit. The test results from a prototype network with 4 White Rabbit switches and 8 CUTEWR nodes shows a 200ps (rms) synchronization precision in different connection topologies. The LHAASO instrument locates at wild field with an altitude of 4300m a.s.l. According to the local meteorological record, the daily and yearly temperature variation can be about 25 and 50 degrees respectively. The temperature dependency of white rabbit key components like VCO and integrated optical modules could significantly degrades the synchronization precision. This paper presents the measurement results of delay and synchronization variation of the WR switch, the CUTE-WR node and the link fibers under wide temperature range. The temperature effect on fiber propagation velocity, laser spectral drift, fixed delay on PCB trace and device are separately studied. The fitting value are applied to an asymmetrical link model which consists dedicated components for uplink/downlink media propagation delay and transmit/receive delay for both master/slave. According to the results, a quasi-linear temperature correction method is implemented for the KM2A timing synchronization node that reduce the synchronization variation from 300ps to 50ps in a temperature range of 50 degrees.