Development of the readout system for the LST telescope of CTA using the DRS4 waveform digitizing chip

The next generation VHE gamma-ray observatory, the Cherenkov Telescope Array (CTA), will feature tens of telescopes of different sizes. We have developed a readout system for the Large Size Telescope (LST) that will use photomultiplier tubes (PMT). The PMTs along with their readout systems are arranged on the focal plane of each telescope, with one readout unit per 7-PMT cluster. The signal from a PMT detecting Cherenkov light from an air shower is amplified, and the waveform is then digitized at a sampling rate of the order of GHz using an analog memory ASIC developed at Paul Scherrer Institute (PSI), called Domino Ring Sampler (DRS4). The sampler has 1024 capacitors per channel and the ability to cascade channels for increased buffer depth. After a trigger is generated in the system, the charges stored in the capacitors are digitized by an external slow sampling ADC and then transmitted via Gigabit Ethernet. An on-board FPGA controls the DRS4, trigger threshold, Ethernet transfer as well the control and monitoring of the Cockcroft-Walton circuit that provides high voltage for the 7-PMT cluster. The readout system is called Dragon and samples the PMT pulses at 2 GSPS with the capability of single photoelectron resolution.