DAQ1000 Project: Upgrade of STAR TPC's Detector Electronics & DAQ
by Tonko Ljubicic
The STAR experiment at RHIC is undergoing a series of major detector upgrades which should be completed in the following years. One of these is the upgrade of the electronics and the data acquisition system (DAQ) of its most important detector, the Time Projection Chamber (TPC). The goal is to be able to record a 1000 Hz event rate with minimal detector deadtime which represents a factor of 20 increase from today. At the same time the large aggregate data rate of 70 GB/s will be further compressed to a more manageable 400 MB/s. The project, colloquially known as "DAQ1000", is hoped to be completed in time for the RHIC FY09 physics run.
As the STAR experiment gathered more knowledge of heavy ion and spin physics in the preceding years it has become more and more crucial to record many different types of events (triggers) at the same time. These event types generally have different requirements. Some are common (i.e. minbias events) and would benefit from high data & event rates while some are rare (i.e. high Pt triggered samples) and would benefit from low deadtimes. The current TPC electronics became the bottleneck for both since it can only record events at 100 Hz while each event contributes full 10 ms of deadtime regardless of data content. Thus at typical 50Hz the TPC has 50% deadtime resulting in a factor of two loss of rare triggers.
The design and construction of new frontend electronics is generally a very costly and time consuming process but what made this project possible on a short timescale with limited resources was the availability of electronics chips developed for the TPC of the ALICE experiment at LHC in CERN, Geneva.
The PASA is an analog preamplifier and shaper matched to the TPC pad readout while the ALTRO is what makes the high rates and low deadtimes possible because it offers 16 channels of fast 10 bit ADCs, pedestal subtraction, tail cancellation, zero suppression, on-chip multiple event buffering and fast off-chip transmission, all in one package.
Each DAQ1000 FEE houses 32 channels while 36 FEEs feed one larger electronics board, the Readout Board (RDO) via fast digital electrical links. 144 RDOs send their data over 2.5 Gbaud optical links to STAR's DAQ Room where they are fed into a farm of powerful PCs. The data are further compressed via online cluster finding and the result is about 1/200th of the original size.
During this FY07 RHIC run we have successfully demonstrated one slice of the chain (1/144th of the whole) and we expect to produce and further test one full TPC sector (1/24th of the detector) in the next FY08 run. The project is hoped to be completed in time for the FY09 RHIC run at which point it is expected that the full complement of STAR's detectors (including the new TOF) will be able to record data at 1 kHz.