Upgrade of the CMS ECAL Barrel HV System for HL-LHC

The electromagnetic calorimeter (ECAL) of the CMS experiment at the Large Hadron Collider is a homogeneous calorimeter made of 75848 lead tungstate crystals. The scintillation light is detected by avalanche photodiodes (APDs) in the barrel and vacuum phototriodes in the endcaps. One of the drawbacks of the APDs is the challenging requirement on the stability of the high voltage source. Indeed, for the CMS ECAL APDs, which are operated at a gain of 50, the variation of the gain as a function of the voltage is 3%/V. The excellent ECAL energy resolution is ensured thanks to the achieved stability of high voltage, temperature and crystal transparency monitoring, whose individual contribution are required to not exceed 0.2%. Therefore, the high voltage system stability must be within 60 mV during a period of one month, the typical period over which calibration of the crystals with physics channels can be performed. The legacy high voltage system was developed by CAEN s.p.a. and it is based on the board A1520PE. The system has provided excellent stability and reliability. The APDs are silicon devices and therefore are subject to radiation damage and particularly their leakage current will increase during the high luminosity phase of the LHC as much as 10 times the present value. Therefore, a new HV system has been developed based on the new-type board A7420PE. Stability tests were performed on these new HV boards, as well as noise measurement, using prototypes of the front-end electronics mounted on a spare ECAL supermodule.