The high voltage system for the CMS forward calorimeters (HF) is described and the measurements of the critical parameters are tabulated

Akchurin, Nural

Atramentov, Oleksiy

Dimitrov, Lubomir

Hauptman, John M

Kim, Heejong

Los, Serguei

Sergeev, Sergey

Vankov, Ivan

CERN Document Server

17 June 2006 Available on CMS information server CMS NOTE 2006/137The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandCMS NoteHigh Voltage System for the CMS Forward CalorimeterN. Akchurin1, O. Atramentov3, L. Dimitrov2, J. Hauptman3, H. Kim1, S. Los4, S. Sergeev4,5, I. Vankov21Texas Tech University, Department of Physics, Lubbock, TX, USA2Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria3Iowa State University, Department of Physics, Ames, IA, USA4Fermi National Accelerator Laboratory, Batavia, IL, USA5Joint Institute for Nuclear Research, Dubna, RussiaAbstractThe high voltage system for the CMS forward calorimeters (HF) is described and the mea-surements of the critical parameters are tabulated.1 IntroductionThe Compact Muon Solenoid (CMS) is a general purpose detector for precision detection of gammas,leptons, hadrons and jets at the Large Hadron Collider (LHC) at CERN [1]. The hadronic calorimeters[2] consist of three subsystems: Hadronic Barrel (HB), Hadronic Endcap (HE), and Hadronic Forward(HF), as shown in Figure 1. There are two identical forward calorimeters (HF+ and HF−) locatedat ±11 meters from the interaction point. The absorber structure is composed of 5-mm-thick stackedgrooved iron plates. The active medium consists of fused-silica core optical fibers inserted into thesegrooves. The signal is generated when charged shower particles above the Cherenkov threshold (E ≥ 190keV for electrons) generate Cherenkov light, thereby rendering the calorimeter mostly sensitive to theelectromagnetic core of hadronic showers. The absorber structure is azimuthally subdivided into 20-degree modular wedges (18 on either side of the interaction point). The fibers run parallel to the beamline and are bundled to form 0.175×0.175 in ∆η×∆φ towers. Each calorimeter is functionally subdividedinto two longitudinal depth segments. Half of the fibers run over the full depth of the absorber (165 cm)while the other half starts at a depth of 22 cm from the front of the detector. These two sets of fibers areread out separately by Hamamatsu R7525 photomultipliers (PMTs). This arrangement makes it possibleto distinguish showers generated by electrons and photons, which deposit a large fraction of their energyin the first 22 cm, from those generated by hadrons, which produce signals in both segments.Each calorimeter is housed in a hermetic radiation shielding consisting of layers of 40 cm thick steel, 40cm of concrete, and 5 cm of polyethylene. A large plug structure in the back of the detector providesadditional shielding (Figure 2).In this paper, we describe our design for supplying high voltage to 1728 PMTs, maintaining economyand ease of use without sacrificing performance. In Section 2, we elaborate on the design considerations.In Sections 3 and 4, we concentrate on the design of the power supply and high voltage units. Thedescription of the controller and monitoring is in Section 5. The software system is discussed in thefollowing section, Section 6. After a brief presentation of critical system parameters in Section 7, wesummarize in Section 8.2 Design Considerations2.1 Parallel Dynode High VoltageThe principle of a parallel dynode high voltage (HV) system is to use a common resistive divider to supplythe dynodes of a set of PMTs that are nearly identical in gain. This approach reduces the number ofhigh voltage channels, the dissipated power in the dividers, and the cost of the overall system. IndividualPMT gain adjustments are not possible, but this is easily addressed by selecting PMTs with similar gains.We chose to bias 8 PMTs with one printed circuit board (PCB) as depicted in Figure 3. The first sixdynodes are supplied from one HV channel (UK) by means of a common resistive divider since there isrelatively low current through the first six dynodes. Two separate channels (UD7 and UD8) are used tosupply dynodes D7 and D8. This arrangement makes it possible to provide the highest dynode currentsdirectly from the corresponding high voltage channels for D7 and D8, ensuring voltage stability anddrastically decreasing the power losses in the intermediate resistors. To decrease the total number ofHV channels, the cluster of three HV channels required per PCB is designed to supply nine PCBs withsimilar PMTs in gain.2.2 Photomultiplier PCB DistributionOne of the concerns in a calorimeter system of this kind is the PMT lifetime at high luminosity. Thecriterion is that the gain degradation should not exceed 20% over 10 years of LHC operation. We estimatethat approximately 1 kC of charge would be accumulated during this time. In order to remain under 1kC, the PMT gain must be about 5 × 104 for the region nearest to the beam (high rapidity), 1.5 × 105for the middle region, and 5× 105 for the outer region (low rapidity).The 24 PMTs are mounted in a readout box, and two readout boxes are needed per wedge (see Figure 4).In order to provide three groups of eight PMTs with different gains, the three PCBs in each readout boxmust use three different HV clusters. For this reason, nine PCBs supplied by each cluster are distributedamong the nine readout boxes of one quadrant (a 90-degree section of the calorimeter).2The three HV sets required for each quadrant are provided by one HV module. One 21-wire 165 m longcable (three extra wires are added for both LED control and Interlock circuits, and several spares areavailable) feeds the HV module output to a HV distributor, from which the nine readout boxes (throughabout 15 m cables) are powered. Thus, in the case of HF, eight HV power supply modules and eightcables (eight quadrants) are needed for the entire system.3 Power Supply SystemFour high voltage power supply modules for each calorimeter are housed in a 19 6U Eurostandard crate.A crate controller, a crate power supply module, and a monitoring and control unit also reside in thesame crate (Figure 5). The operation of this crate is similar to that of the crates of the CMS hadronbarrel and endcap calorimeter [3].Four trimmer potentiometers are installed in the monitor and control unit for presetting the upper limitvalues:1. One potentiometer for the output voltages of all channels: only the upper limit UK,lim for UK ofall three clusters (A, B, C) is adjusted, while the voltage limits for the remaining six channels arederived from UK,lim as (0, 4)UK,lim for UD7 channels and (0, 2)UK,lim for UD8 channels.2. Three potentiometers for the current limits in each three subsequent channels (Ilim1, Ilim2, Ilim3).These settings are common for all HV modules in the crate and are distributed by four analog lines inthe backplane of the crate.The crate controller provides data and instructions transfer between the host computer and the crate viaan RS-485 interface. The internal interface with the HV power supply modules is implemented by thecrate local bus using a custom protocol [4].The crate power supply module generates all voltages required for normal operation of the modules,+5,±8 and +40 V. They are read out by the crate controller and displayed on the main monitoringwindow of the host computer (see Figure 9). An alarm is generated if any of the supply voltages exceedthe preset limits. A photograph of the power supply crate is shown in Figure 6.4 High Voltage Power Supply ModuleThe block diagram of the HV module is shown in Figure 7. As already mentioned, it contains threeclusters (A, B, and C) of three channels. The output voltage of the first channel in each cluster (A1, B1,and C1) is intended to supply the resistive dividers in the PCBs and can be regulated from 0 to −2000V. The second channel produces the voltage for the D7 dynodes and can be varied from 0 to −800 V.The third channel generates the D8 voltage , and it ranges from 0 to −400 V. Each high voltage channelincludes one 12-bit serial digital-to-analog converter (DAC), a DC-DC converter [4], and two comparators(CMPs) for overvoltage and overcurrent protection.A LOCAL CONTROL block in the module [4] receives all the output voltage set values from the cratelocal bus in a serial mode sent by the crate controller. This block transfers them to the DACs thatprovide the reference voltages for each output. The data are stored only in the DAC output register ofthe corresponding channel.All analog signals corresponding to the output voltage and load current values are fed consecutivelythrough a multiplexer (MUX) to a 12-bit analog-to-digital converter (ADC) to be read (in serial mode)by the local control block. These signals are also sent to the individual channel comparators (CMPs),where they are compared with the corresponding voltage or current limit values received by the cratelocal bus. In case of overvoltage or overcurrent in any channel, the corresponding comparator sends asignal to the local control block, which immediately clears the DACs in all three channels of this cluster,dropping the output voltage to zero. An alarm signal is also sent to the crate controller.In order to maintain the integrity of all HV lines, a daisy chained interlock circuit is added in all cablesdistributing the high voltage from the module to the readout boxes. In case of interruption of the interlock3circuit, the local control block immediately disables all HV channels, activates the module front panelLED (INTERLOCK), and communicates the interlock error condition to the crate controller.Ten LEDs (one in the HV distributor and nine in the readout boxes) indicate that at least one group ofHV channels is on and that readout boxes are powered.A photograph of the HV power supply module is shown in Figure 8. All high voltage circuits are protectedby an aluminum shielding.5 Crate Controller Module and Monitor and Control UnitThe internal structure and the principle of operation of the crate controller module and the monitorand control unit (Figure 9) are the same as in the HCAL power supply system [3]: a standard RS-485interface is used for the link with the host computer; the same control logic block performs the dataexchange with the HV modules and the monitor and control unit; and the unit displays the status of thesystem using seven front panel LEDs. The main difference between the HV system for HB/HE and HFis that the former subsystems use hybrid photodiodes (HPD) for light detection whereas HF uses PMTsFirmware is capable of controlling both HPD and PMT power supply systems while recognizing thedifferent types of modules by their unique ID numbers.6 System SoftwareEssentially the same software package as in the HB/HE power supply system [3] is used. The maindifference is in the client program which controls and monitors the entire system. The program isadopted to monitor and display the parameters of nine channels, as shown in Figure 10.7 Technical ParametersThe technical parameters of the system are shown in Table 1. The performance values marked with anasterisk have been verified by measurements.Table 1: The specifications of the high voltage system for the HF are summarized below. The asterisksindicate tested values.Parameter Channels Channels ChannelsA1, B1, C1 A2, B2, C2 A3, B3, C3Max. Operating Voltage (V) −2000 −800 −400Voltage Adjust Step (V) 0.5 0.2 0.1Ramp Rate (V/s) 5− 500 2− 200 1− 100Voltage Ripple (mVpp) < 100∗ < 40∗ < 20∗Voltage Monitor Precision (%) < 0.1∗ < 0.1∗ < 0.1∗Long Term Stability (%) < 0.1∗ < 0.1∗ < 0.1∗Max. Current Out (mA) 0.8 0.8 0.8Current Monitor Precision (%) < 1∗ < 1∗ < 1∗8 Conclusions and SummaryAn economical, robust, and reliable multi-channel power supply system is developed, based on a custompower supply module and containing three clusters of three high voltage outputs for the CMS hadronicforward calorimeter photomultipliers. The long term and beam tests confirm the stable operation of thesystem and the reliability of its user interface.4HE HB HF+ HF    -Figure 1: Three subdetectors make up the hadronic calorimeter system in CMS, hadronic barrel (HB),hadronic endcap (HE) and hadronic forward (HF).5Figure 2: The back view of the quarter of the CMS hadron forward calorimeter (HF) is shown duringassembly. Nine readoutboxes are installed per one quarter of the detector. The high voltage cables arein red and the signal cables are in black.6 K UK UD7   A S ID =     50 ì A K A S A B C D E F K A S A B C D F E PMT Nr 1 LG LG UD8 LG D1 D2 D3 D4 D5 D6 D7 D8 D1 D2 D3 D4 D5 D6 D7 D8 D1 D2 D3 D4 D5 D6 D7 D8 A B C F D E LG LG PMT Nr 2 PMT Nr 8 Figure 3: The electrical diagram of the PMT voltage distribution shows that there are three voltagesprovided to PMTs in a group of eight in addition to ground. The photocathode voltage is set at UK andthe seventh and the eighth dynodes are set at UD7 and UD8, respectively.7 HV Dis- tributor Cable 1 Cable 2 Cable 9 9 HV cables L≈15 m 1 HV cable L≈165 m PS3 PS2 PS1 HV MODULE Nr 1 ROBOX Nr 1  ROBOX Nr 2 ROBOX Nr 9  UD7,1 UK,1 UD8,1   LG  UD7,2 UK,2 UD8,2   LG  UD7,3 UK,3 UD8,3   LG  K1 D7,1 D8,1 LG  K2 D7,2 D8,2 LG  K3 D7,3 D8,3 LG  LED LED LED Figure 4: One quadrant high voltage distribution 9 readout boxes are biased using one high voltagemodule. For the voltages delivered to individual PMT dynodes see also Figure 3.8   RS485  HOST    PC 220 V CRATE POWER SUPPLY CRATE  CONTROLLER  HV POWER  SUPPLY  MODULE 1    MONITOR AND CONTROL UNIT  HV POWER  SUPPLY MODULE 2    HV POWER  SUPPLY MODULE 4      CRA T E  L O C AL  B U S A1 A2 A3 B1 B2 C1 B3 C2 C3 A1 A2 A3 B1 B2 C1 B3 C2 C3 A1 A2 A3 B1 B2 C1 B3 C2 C3 Figure 5: The block diagram of the high voltage supply module is depicted above.9Figure 6: The high voltage crate normally houses 4 power supply modules, one per HF quadrant.10  CHANNEL C3 CHANNEL A3 CHANNEL A2 INTERLOCK       M U X       ADC   LOCAL CONTROL C R A T E  L O C A L  B U S  DAC CHANNEL A1 DC-DC CONVERTER -2000 V/0,8 mA CMPs DAC DC-DC CONVERTER -800 V/0,8 mA CMPs DAC DC-DC CONVERTER  -400 V/0,8 mA CMPs DAC DC-DC CONVERTER  -400 V/0,8 mA  CMPs INTERLOCK LED  LED CONTROL Figure 7: The block diagram of the high voltage module (see Figure 8).11Figure 8: The high voltage supply module is shown above and it is housed in the HV crate (see Figure 6).The high voltage output is through the connector shown on the panel.12  CPU    CONTROL LOGIC           MONITOR AND        CONTROL UNIT   POWE R  O N   CHANNE L O N VOLT AG E PR OT    CURREN T P RO T   O VE R HEA T  IN TER LOCK   POWE R F AILUR E             A[13..15]                    A[8..12]                               D[0..7]        RxD     TxD                                                     A[13..15]                         A[0..7]     D[0..7]    CNTRL[0..3]                                                   KILL                TxD                            RxD  INTERFACE   RS485 OUT  RS485 IN   ST  MD[0..3]  CLK  D-WR   D-RD   RES  U Kl im    I I   I     MEMORY   CNTR[0..3]   D[0..7]  A[0..12]  S[0]   MD[0..3]   CLK  D-WR   RES    D-RD   KILL  lim1  lim2  lim3       Figure 9: The block diagram of the crate controller module with the monitoring and control unit showsthe fundamental elements of the system. The communication to the computing system is through theRS-485 interface.13Figure 10: Control and the monitoring functions for the high voltage are carried out through a graphicaluser interface. The monitoring system reports back the actual voltages and currents in all nine outputsfor a given HF quadrant.14References[1] CMS Collaboration, CMS Technical Proposal, CERN/LHCC 94-38, LHCC/ P1, 15 December 1994.[2] CMS Collaboration, The Hadron Calorimeter Project, Technical Design Report. CERN/LHCC 97-31,CMS TDR 2, 20 June 2003.[3] L. Dimitrov, J. E. Elias, B. Kunov, S. Sergueev, A. Ronzhin and I. Vankov, Photodetector PowerSupply System for the CMS Hadron Calorimeter, Nucl. Instr. Meth. A 2005 (in print).[4] L. Dimitrov and I. Vankov, High Voltage Module for the Photomultiplier Power Supply System ofthe CMS Forward Hadron Calorimeter, Proc. of the 14-th Int. Conf. ELECTRONICS ET2005, Sept.2005, Sozopol, Bulgaria, Book 2, p. 179.15

High Voltage System for the CMS Forward Calorimeter

High Voltage System for the CMS Forward Calorimeter

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