A vacuum tolerant high voltage system with a low noise and low power Cockcroft-Walton photomultiplier base

We developed a high voltage system for the electromagnetic calorimeter of the KOTO detector. The system is designed around a low noise, low power Cockcroft-Walton (CW) photomultiplier tube base with a high gain preamplifier. The low power makes it suitable for operations in vacuum. The low noise and high gain allow detecting signals in the 1 MeV range. We achieved a final noise level below 180μVrms for a preamplifier gain of more than 40. A vacuum tolerant control system for the CW bases power distribution was also designed. This system is able to control and monitor the high voltage of each individual base.