The HADES data acquisition system was originally designed and built ten years ago. The performance was sufficient for all experiments run in recent years, but does not meet the requirements for the current experiments with heavy ions and higher beam energies at the FAIR accelerator complex. Here, the data rate is of special concern since the mean particle multiplicities are increased by a factor 40 compared to light ion systems and data rates were already a limiting factor in these experiments. Hence, the DAQ system was re-designed based on new technologies to cope with the new requirements.
The new DAQ system features Field Programmable Gate Array based electronic boards and optical data transmission. An FPGA-based custom network with optical links inside the detector and a commercial Gigabit Ethernet infrastructure transports data to the server farm. The custom optical network features high bandwidth, low latency bi-directional data transport. All data are transported via this network on dedicated virtual channels: trigger information, event data and slow-control information.
A comprehensive description of the new DAQ system can be found in "The Upgraded HADES Trigger and Data Acquisition System" (JINST 6 C12056 2011)
More than 7 billion Au+Au events have been recorded in the 2012 experimental run. The mean data rate during the flat top of the extracted beam was 150 MBytes/s at an event rate of 10 kHz. This corresponds to 20% of the peak performance of the DAQ system.