ALICE is a dedicated heavy-ion experiment at CERN LHC aiming to study the properties of the quark–gluon plasma. A lead– lead collision might produce several ten thousand new particles. Detailed study of the event requires precise measurements of the particle tracks. A 90 m3 Time Projection Chamber (TPC) with more than 500 000 read-out pads was built as the main central barrel tracker. Collisions can be recorded at a rate of up to about 1 kHz. The front-end electronics, designed from FPGAs and custom ASICs, performs shaping, amplification, digitisation and digital filtering of the signals. The data is forwarded to DAQ via 216 1.25 Gb/s fibre-optical links. Configuration, control and monitoring is done by an embedded Linux system on the front-end electronics. First results on the performance of the front-end electronics and the distributed detector control system are presented

Larsen, T D

CERN Document Server

ALICE TPC control and read-out systemD. T. Larsena for the ALICE TPC collaborationa Department of Physics and Technology, University of Bergen, Bergen, Norwaydagtl@ift.uib.noAbstractALICE is a dedicated heavy-ion experiment at CERN LHC aim-ing to study the properties of the quark–gluon plasma. A lead–lead collision might produce several ten thousand new particles.Detailed study of the event requires precise measurements of theparticle tracks. A 90 m3 Time Projection Chamber (TPC) withmore than 500 000 read-out pads was built as the main cen-tral barrel tracker. Collisions can be recorded at a rate of up toabout 1 kHz. The front-end electronics, designed from FPGAsand custom ASICs, performs shaping, amplification, digitisa-tion and digital filtering of the signals. The data is forwarded toDAQ via 216 1.25 Gb/s fibre-optical links. Configuration, con-trol and monitoring is done by an embedded Linux system onthe front-end electronics.First results on the performance of the front-end electronicsand the distributed detector control system are presented.I. TIME PROJECTION CHAMBER (TPC)The A Large Ion Collider Experiment (ALICE) [1] is using aTPC [2] as the main track-finding detector. A TPC is a gaseousdetector. It is shaped like an horizontal barrel and positioned inthe same direction as the beam pipe, which is passing throughthe centre of the barrel. The overall length is 500 cm, dividedby a 100 kV Central Electrode (CE) into two identical drift vol-ume. The diameter is 494 cm, though the innermost 170 cm isnot part of the TPC to make room for the beam pipe and innertracking detectors. A schematic view of the TPC can be seen inFigure 1.Collisions will take place in the beam pipe in the centre ofthe TPC, allowing the particles produced to traverse the TPCand leave tracks of ionised gas along their paths. A strong elec-tric field of from the CE will make the electrons drift towardsthe end planes, where data read-out is performed.Each end plane is divided into 18 azimuthal sectors,which again are divided into two Multi-Wire ProportionalChambers (MWPC), the Outer and Inner Read-Out Chamber(OROC/IROC). The OROC has four Read-out Partitions (RPs);the IROC two. A RP is an electronic entity for reading out datafrom read-out pads. The ionistic signal will be amplified bythe space charge around the wires of the MWPC. The inducedcharge on the read-out pads is forwarded to the read-out elec-tronics. In total for both sides there are 557568 pads.The drift volume is filled with counting gas composed of85.7 % Ne, 9.5 % CO2 and 4.8 % N2. A cold, light gas is usedto assure low diffusion and low multiple scattering. Field dis-tortions are minimised because of the high ion mobility and fewionisation electrons per unit length. The electronics design noisefigure is 1000 RMS e− (700 actually achieved); not limiting theposition resolution will require a signal/noise ratio of at least 20.Apart from tracking—measuring the charged particle mo-mentum and having a good two-track separation—it also pro-vides Particle IDentification (PID). The TPC is expected to per-form well at multiplicities as high as dNch/dη=8000 in the par-ticle momentum range [0.1, 100] GeV/c and within |η| <0.9.Tracking efficiency is required to be >90 %, and the dE/dx res-olution better than 10 %. Further, the TPC alone will have amomentum resolution of about 1 % at 2 GeV/c and 10 % at50 GeV/c. For p–p collisions a read-out rate of ≈1 kHz is ex-pected, while for central Pb–Pb collisions ≈0.2 kHz.Figure 1: Schematic view of the TPC. To the left a singe Read-outPartition (RP) is enlarged for visibility. The support for the sectors isshown on the two end planes. Between them is the Central Electrode(CE). The TPC allows space around the centre of the length axis forbeam pipe and inner silicon detectors.II. DATA READ-OUT DESIGNAs already mentioned, each sector has six RPs. A RP con-sists of a Read-out Control Unit (RCU) with up to 25 Front-EndCards (FEC), depending on the radial location. The innermostRPs have the highest number of FECs, as a smaller size for thereadout-pads is used to increase the resolution to take into ac-count the higher track density close to the collisions. The elec-tronics for one RP, as well as its connection to the central sys-tems is shown as a block diagram in Figure 2.Eight ALICE TPC Read-Out (ALTRO) [3] chips aremounted on a FEC, each capable of reading out 16 read-outpads. The FECs are attached to the RCU via buses; one fordata transfer and one for control/monitoring. Once on the RCU,the data is forwarded to Data Acquisition system (DAQ) and theHigh Level Trigger (HLT) via a 1.25 Gb/s optical fibre. A De-tector Control System (DCS) board equipped with an embedded586ARM processor running Linux is attached to the RCU for con-trol and monitoring. The board is equipped with a standard Eth-ernet network interface. Radiation tolerant electronics is neededto sustain the radiation from the collisions.On the FECs, the pad signal passes through a shaping am-plifier before it is forwarded to the ALTRO, which will digitiseand digitally filter it. The ALTRO is using a 10-bit Analogue-Digital-Converter (ADC) capable of 10 million samples per sec-ond. The digital filtering is performed in four stages. First, sys-tematic effects and low frequency perturbations are removed aspart of a base-line correction. Tail cancellation removes the tailof the pulses within 1 µs of the peak. Fully programmable filtercoefficients allow for removal of a wide range of tail shapes.Next, non-systematic perturbations of the base-line superim-posed on the signal is removed by applying a base-line correc-tion moving average filter.The RPs will read out data from a collision when they re-ceive an external trigger. Before a new trigger is issued, it mustbe ascertained that all RPs have finished reading out data asso-ciated with the previous trigger. This is handled by a BUSYsystem. The Busy Box has a direct link to each of the DAQcomputer nodes receiving data from a RP. Once the node hasreceived all data from a certain RP, it will flag this to the BusyBox. When the Busy Box detects that the read-out is done, itwill inform the central trigger system, which can now issue anew trigger to the RPs.DCSSIUTPCRCUD-RORCBusy BoxLTUBusyChannel A / Channel BTTCrxReceived Event IDsFECFECFECFECFECFECFECFECFECFECDCSTTCrxCTPLeve l 0 / Level 1Detector Counting RoomTrigger, Timing & Control SystemData Acquisition SystemChannel AChannel BEventsL0/L1/L2for readoutTPC Front-end ElectronicsDIUHigh Level TriggerH-RORCEventsFigure 2: Block diagram of TPC read-out and control electronics. Leftside is embedded on the detector, right external system in the count-ing room. Data is collected from FECs, forward to DAQ/HLT via theRCU. Control is achieved via the associated DCS board. The Busy Boxindicates when read-out is finished, and a new trigger may be fired.III. DETECTOR CONTROL SYSTEM (DCS)Control and monitoring of the RPs is mainly done via aspecial software, the FeeServer (FS), running on the embed-ded Linux system on the DCS board. Communication is viastandard IP/TCP network. Functionally, the FeeServer has twomain functionalities: monitoring and command handling. Mon-itoring will publish the values of important hardware registersto external clients. Command handling allows an instructionset to be built for configuring the Front-End Electronics (FEE).The handling of the fundamental network interface and infras-tructure for monitoring and command handling is implementedin FeeServer Core, whereas the specific implementation for ofhardware access for monitoring and command handling is donein a module called Control Engine (CE).The InterComLayer (ICL) acts as a hub in the system. Itmaintains contact with the FS’ of all 216 RP, as well as thePVSS-based GUI for the operator and a configuration databasecontaining pre-defined configurations for the FEE.Config.DB216 TPC read-out partitionsSCADA (PVSS II)FED ClientFED ServerInterComLayerFEE ClientCommandCoder........RCUFeeServerControlEngineServices Acknowledge,Commands,MessagesRCUFeeServerControlEngineServices Acknowledge,Commands,MessagesAcknowledge,Commands,MessagesServicesOperatorField layerControl layerSupervisory layerFigure 3: Structure of the control hierarchy for the Detector ControlSystem (DCS). From bottom: “field layer” (FEE); “control layer” (FSand lower part of ICL); “supervisory layer” (upper part of PVSS andGUI).A three-layer hierarchy is defined for the DCS: “field layer”is the FEE itself; “control layer” is the FS on each RP, as well asthe lower part of the InterComLayer (ICL); “supervisory layer”is the upper part of the ICL and the GUI the shifter is operating.This structure is shown in Figure 3.Configuration of the FEE is accomplished by sending binaryconfiguration data blocks to the FS. Values of registers of specialimportance, such as FEC temperatures, voltages and currents, aswell as states of the state machine, are being published. Uponreceiving a high-level configuration command from the GUI,ICL assembles configuration blocks for the FS by retrieving587configuration parameters from the DB. ICL also collects datapoints published by FS, and forward them to the GUI. There isa full integration with the Experiment Control System (ECS),enabling operation of the TPC by the ALICE shifter.IV. NOISE LEVELThe background noise level is obtained regularly frompedestal runs. Figure 4 shows the distribution for pairs of ROCsfor one of the end planes. The IROC constitutes one pair, whilethe four chambers of the OROC is divided into two pairs. InFigure 5 the same data is plotted on the corresponding read-outpad.Figure 4: Noise distribution from pairs of ROCs: IROC and twoOROC. The peak is around 0.7 ADC counts.Figure 5: Typical noise level (in ADC counts) for the TPC from a recentpedestal run. As in previous figure, the noise level is around 0.7 ADC.The noise figure is required to be less than 1000 e− RMS ofbase-line, corresponding to 1 ADC count. The noise levels fromthe pedestal runs, showing that the noise figure is ≈0.7 ADCcount (700 e−), well within the requirement. This is close tothe natural limit, and do not change much with time. Also, it al-lows for zero-suppressed empty events less than 70 kB (noise);without zero-suppression 10 000 times larger.V. DATA READ-OUT PERFORMANCERPs have a varying number of FECs depending on radial po-sition in the sector, from 25 (innermost) to 18 (outermost). Re-serving the same amount of bandwidth for each FEC regardlessof radial location implies only RPs with 25 FECs can utilise thefull bandwidth of the optical fibre, hence effective read-out rateper 6-RP sector is limited to 770 MB/s. Benchmark tests (Figure6) show that this is indeed achievable for high-occupancy eventswhere zero-suppression has been applied. Considering the caseof low-occupancy events, read-out is possible at an event rateof 595 Hz (0 % occupancy) using full readout. The electronicsalso supports sparse read-out, in which case empty channels areentirely stripped, including headers. Applying this technique,the read-out rate increases to 1386 Hz. The respective data ratesare 70 MB/s and 927 kB/s.occupancy [%]-110 1 10 210rate [events/s]0100200300400500600700data rate [MB/sector/s]0100200300400500600700800900Figure 6: Event rate (black, left scale) and data rate (red, right scale) asfunction of occupancy, for full read-out. At 100 % occupancy the theo-retical maximal data rate of 770 MB/s is reached. At 0 % occupancy thedata rate is 595 Hz, however applying sparse read-out increases this to1386 Hz (not shown, as it only significantly departs at low occupancy).REFERENCES[1] The ALICE Collaboration, K. Aamodt et al., “The ALICEExperiment at the CERN LHC”, JINST 3 (2008) S08002.[2] The ALICE Collaboration, “ALICE TPC Technical De-sign Report”, CERN/LHCC 2000-001, ALICE TDR 7, 7January 2000.[3] L. Musa et al., “The ALTRO chip: a 16-channel A/D con-verter and digital processor for gas detectors”, IEEE Trans.Nucl. Sci., November 2003.588

ALICE TPC control and read-out system

Abstract

Similar works

Full text

Available Versions

CERN Document Server