CMS NOTE 2003/000

March6, 2006

High Voltage and Low Voltage system for the Resistive Plate Chamber detectors

Pierluigi Paolucci

Istituto Nazionale di Fisica Nucleare, Napoli (ITALY)

Abstract

The CMS Resistive Plate Chamber (RPC) system consists of two main parts, the Barrel formed by a total of 480 chambers and the End-Caps subdivided in two halves and consisting of 756 chambers in total.

The chambers are formed by two or three bigaps and two or three rows of 6 front-end boards (FEB)

The first design of the High Voltage and Low Voltage system, made in the 2002, was developed on the idea to have a sub-system made by independent detectors and so it was configured to have an high voltage channel per gap and two low voltage channels (analog and digital) for each FEB row.

After a long set of test and different design and taking into account a strong budget reduction of about 50% the system was completely redesigned as is described here.

Now, after having tested three prototype versions, the system is under construction and the first 10% of it has been tested at the I.N.F.N. laboratory of Napoli. The whole production and test will finish in June 2006 when the installation and commissioning phase at SX5 of the HV and LV system will begin.

Preliminary version

  1. The CMS RPC detector
  2. Introduction

The RPC system is divided in two regions: barrel (0 <││< 1.2) and endcap (0.9 <││< 2.4). The barrel is made by 5 wheels each with 12 sectors, equipped with 4 muon stations. The two innermost stations are made by a sandwich composed by a Drift Tube chambers (MB1 and MB2) and two RPCs (RB1-in, RB1-out and RB2-in, RB2-out).The two outer muon stations are made by a DT chamber (MB3 and MB4) coupled with two RPCs (RB3 and RB4). In some special sectors there are one RB4 (sector 9 and 11) or four RB4 (sector 4). In total there are 4 muon station and 6 RPC layers used for triggering and detect muons.

Each endcap is made by 3 iron disks and 4 muon stations of Cathode Strip Chamber and RPC. The muon station contains one layer of double-gap RPC and is divided in R into 3 chambers (REs/1, REs/2 and REs/3, where s=1,…,4 is the station number). Chambers RE2/1, RE3/1 and RE4/1 cover 20o in , whereas all the other RE cover 10o.

1.2.RPC chamber description

An RPC gap is made by two parallel bakelite plates (1-2 1010 Ω·cm) placed at a distance of 2 mm and filled with a gas mixture of 96% C2H2F4, 3.5% i-C4H10 and 0.5% of SF6. High voltage is applied to the outer graphite coated surface of the bakelite plates in order to have an electric field inside the gas gap able to generate a charge avalanche along the track of an ionizing particle. The avalanche induces a signal on the aluminum strips placed outside the gap and isolated from the graphite.

Figure 1. Abarrel RPC chamber schema, with two double-gaps and a strip plane in the middle, is shown here.

All the barrel RPC chambers (fig.1) consist of two double-gaps unless the RB2-out made by three double-gaps, with a read-out strips plane placed in between. Chambers are equipped with two or three (RB3-out) rows of 6 front-end boards [9] each connected to 16 readout strips.The total number of FEBs per chamber is always 12 or 18 (RB2-out) and 10 in some special smaller RB4. Chamber is equipped with a “distribution board” that distributes the LV power and the I2C control signal to all the boards.

The high voltage connection of the 2 or 3 upper and lower gaps are joint together (see fig. 1) in order to reduce the number of HV channels keeping the possibility to have different gap voltages in every chamber. The chambers are equipped with a “custom” tri-polar HV connector housing the two HV channels and the common ground reference.

2The High and Low Voltagearchitecture

2.1Introduction

2.2General requirements

The main requirement of the LHC power system is that they have to work in a very “unusual” hostile environment due to the high magnetic field and high radiation flux. For the muon system the idea is to have a large part of the power system close to the detector and in particular on the racks on the balconies placed around the barrel wheel and the endcap disk. In this area the magnetic field can reach up to 1 Tesla while the radiation is around 5·1010 proton/cm2 and 5·1011 neutron/cm2. Starting from our experience in the L3 and BaBar experiments, where the condition were much more safe than in CMS, and after a very deep market surveys we decided to begin a new design of a power system able to work at LHC in cooperation with the ATLAS, ALICE and LHCb Italian colleagues working on RPCs. This phase of design and study began in the 2001 and after some very preliminary tests we asked to some companies to produce a prototype in order to test it very carefully in laboratory and at the radiation facilities used by the CERN experiment.

The specific requirements for the HV and LV power supply are reported in table 1

Power supply / High voltage / Low voltage
Voltage / 12 KVolts / 7 Volts
Current / 1 mA / 3 A
Ripple / < 100 mV pp at load
(freq < 20 MHz) / < 10 mV pp at load
(freq < 20 MHz)
Programmable voltage / from 0 to 12 KVolts / from 0 to 7 Volts
Voltage step / 10 Volts / 100 mV
Voltage precision / < 10 Volts / 100 mV
Voltage/Current monitoring / yes / yes
Trip setting / from 0 to 100 sec / from 0 to 100 sec
Maximum voltage / hardware/software / hardware/software
Maximum current / hardware/software / hardware/software
Errors led / red led / red led
Power led / green led / green led

Table1: The high voltage and low voltage power supply requirements are reported here.

2.3The RPC power system architecture

In the past experiments the high voltage power system for RPC detectors was always designedwith a central system, called mainframe, containing both the power supplies than the control and monitoring. The mainframes were placed in the electronic room and the high voltage channels were connected to the detectors through a very long cable (up to 100 meters) and some patch panels or high voltage distributors in order to reduce the number of channels.

Vice versa the low voltage power supply was always placed in the detector area in order to minimize the noise pickup and the voltage drop due to long cables.They were controlled and monitored using serial communication protocol and remote hardware switches.

At LHC, taking into account the hostile environments and the requirements described before, the RPC community decided to design a system based on a master/slave architecture for both the high and low voltage systems. The master, called mainframe, is supposed tocontrol and monitoring on ore more slaves and is placed in a safe and accessible area as the electronic room. The slaveis where the power is generated and was designed to be modular and multifunctional in order to be able to have mixed systems containing both HV and LV power supplies. It is based on a crate with a dedicated backplane housing a certain number of different boards going from the power supplies to others. The slave system can beplaced around the detectors, in a hostile and not accessible area and for this reasonhas to be modular, redundant and based on a radiation tolerant electronics.

In the 2000 some LHC I.N.F.N. groups and the CAEN Company produced a first rudimental prototype (SASY 2000) [ref] based on the master-slave idea and after a long test period the CAEN produced a complete design of such a system called EASY project [ref.]

In a second moment we decided to keep the master/slave architecture for both the high and low voltage systems but to move all the high voltage slaves in control room in order to be more safe with a so crucial part of the RPC system and also to reduce the cost of the project to a budget reduction. Keeping all the HV system in a safe and accessible area give us the possibility to reduce the number of HV channels of a factor two and improve the system time to time when it will be necessary or when a larger budget will be available.

Low Voltage Architecturefor RPC detectors

In order to minimize the noise pickup and the high voltage drop and to reduce the cost of the Low Voltage project, the CMS collaboration decided to design and develop a common LV project based on the master/slave architecture described before.

The Muon collaboration, after having analyzed the general requirement and where to place the LV power supplies decided to have a LV slave system placed on the balconies around the detector, in a hostile regionbut at a maximum distance of about 15 meters from the detectors instead of the 120 metersthat separates the electronic room from the detectors.

High Voltage Architecture for RPC detectors

Since the 2000 the CMS RPC collaboration decided to use the master/slave architecture for the High Voltage system but without a very clear idea regarding the geographical distribution of the system. For this reason the collaboration began to test very careful the first prototypes produced (SASY 2000) and to work on two different solutions; the first very similar to the LV, where the power supplies are on the detector and the master in electronic room and a second one with the full HV system is in electronic room.

The experience made in the past experiments with the RPC detectors suggest us that it is very important to have the power supplies in an accessible area to easily fix any problem regarding the connection and the distribution of the HV. Sparks, generated in HV connections or in a chamber, can create a fail of one or more power supply and in some special case of the whole HV mainframe and so in this case it is very important to access as soon as possible at the incriminated HV channel to disconnect it and repair the problem. Another situation in which is important to have a fast access to the power supplies is when one or more chambers begin to drawn too much current. In this case it needs to move the “bad” chamber to a different HV channel where it is possible to study and fix the problem without disturbing the operation of the whole RPC sub-detector.

Taking into accounts the arguments discussed before and the cost reduction due to the possibility to have the same number of HV channels in a reduced number of slave crates, the RPC collaboration decided to adopt the second solution or to have the whole HV system in an accessible and not hostile area as the electronic room.

3From the SASY 2000 to the EASY 3000

3.1The SASY 2000 system

3.2The EASY 3000 system

Embedded Assembly SystemEASY3000 is the new CAEN power supply solution for operation in magnetic field and radioactive environment. CAEN had been involved for more than a decade in developing different solutions for the main LHC experiments, where the electronic equipment of the experiment is dealing with high dose radiation and intense magnetic field. Moreover, though designed for harsh environment, the EASY3000 modules can work also in normal condition with excellent performance. In the new architecture, the powersupply can be located directly in the hostile area (see Table 1.1), where the EASY3000modules provide a wide variety of output voltages to satisfy the requirements of mostdetectors and front-end electronics.The control of the EASY3000 power supply system is done remotely using a Branch Controller (Mod. A1676A) plugged in a SY1527 or SY2527 mainframe located in the control room.

Fig 3. CAEN SY1527 Mainframe / Fig 4. EASY 3000 / Fig 5. A1676A Branch Controller

Table 2. – EASY3000/4000 Crates and Boards hostile areas tolerances

3.3The mainframe SY1527 (master)

3.4The EASY crate (slave)

The EASY crate and all the EASY boards have been designed to work in an hostile area with a magnetic field up to 2kGauss and a radiative environment up to 1·1011 p/cm2 TID, 2·1012 n/cm2 TID and 15 kRad TID

3.5The High Voltage hardware

The Model A1526 is a double width board houses 6 H.V. channels with either positive or negative polarity. It is Standard High Voltage Board for the SY1527. The channels share a common floating ground. The output voltage can be programmed and monitored in the range 0-15 kV with 1 V resolution. The Model A 1526 offers 100 mA / 1 mA dual current FullScaleRange (dip switch selectable). If the output voltage differs from the programmed value by more than 3% of voltage full scale range, the channel is signaled to be either in OVERVOLTAGE or UNDERVOLTAGE condition. Moreover, for each channel, a voltage protection limit SVMAX can be fixed via software with 1 V resolution and the output voltage can not be programmed beyond this value.

Figure 6. High voltage connector for CAEN HV power supply boards

Polarity: / Positive (A3512P) or Negative (A3512N), with Floating return
Output Voltage: / 0 ÷ 12 kV (connector output)
Max. Output Current: / 1 mA
Voltage Set/Monitor Resolution: / 1 V
Current Set/Monitor Resolution: / 100 nA
VMAX hardware: / 0 ÷ 12 kV
VMAX hardware accuracy: / ± 2% of FSR
VMAX software: / 0 ÷ 12 kV
VMAX software resolution: / 1 V
Voltage Ripple: 2 / < 50 mV pp
Voltage Monitor vs. Output Voltage Accuracy: 3 / typical: maximum: / ± 0.3% ± 3 mV ± 0.3% ± 5 mV
Voltage Set vs. Output Voltage Accuracy: 3 / typical: maximum: / ± 0.3% ± 3 mV ± 0.3% ± 5 mV
Current Monitor vs. Output Current Accuracy: 3 / typical: ± 2% ± 0.05 A maximum: ± 2% ± 0.1 A
Current Set vs. Output Current Accuracy: 3 / typical: maximum: / ± 2% ± 1 μA ± 2% ± 2 μA

Table 3. – Channel characteristics of the Mod. A 3512 HV Board

The CAEN A3512 houses 6 Channel 12 kV / 1 mA EASY Subsystem Power Supply Module, developed for operation in magnetic field and moderate radioactiveenvironment. One A3512 houses 12 floating (i.e. with independent return) 12 kV / 1 mAchannels; the board is available with either positive or negative polarity.The connector output voltage range is 0 ÷ 12 kV with 1 V monitor resolution.The output current is monitored with 100 nA resolution.

Fig. 7. A1526 CAEN High Voltage Standard Board / Fig. 8. A3512 CAEN High Voltage EASY Board

3.6The Low Voltage hardware

The Model A1513 is a single width (5 TE wide) board housing 6 LV floating (reversible polarity) channels for the SY1527 mainframe. The connector1 output voltage range is 0÷10 V (2.7 A maximum output current) with 10 mV monitor resolution. The board is provided with Remote Sensing Lines to compensate for the voltage drop over the connection cables.

Fig. 9. A1513 CAEN Low Voltage Standard Board / Fig.10. A3009 CAEN Low Voltage Standard Board

The CAEN A3009 12 Channel 8V/9A Power Supply Board for the EASY Crate. It is developed for operation in magnetic field and moderate radioactive environment. One A3009 houses 12 floating (reversible polarity) 8 V / 9 A / 45 W maximum output power channel.

The connector output voltage range is 1.5 ÷ 8 V with 5 mV monitor resolution; channel control includes various alarms and protections. The board is provided with Remote Sensing Lines to compensate for the voltage drop over the connection cables. If the output voltage differs from the programmed value by more than 3% of voltage full scale range, the channel is signalled to be either in OVER VOLTAGE or UNDER VOLTAGE condition.

Moreover, for each channel, a voltage protection limit SVMAX can be fixed via software with 5 mV resolution and the output voltage can not be programmed beyond this value.

The output current is monitored with 10 mA resolution; if a channel tries to draw a current larger than its programmed limit it is signalled to be in OVERCURRENT condition; the SY 1527 system detects this state as a fault and reacts according to the setting of the TRIP parameter, which can be programmed in 0.1s steps from 0 to 1000s. Actually TRIP = 1000 s means infinite: in case of TRIP infinite the output current is permitted to keep the programmed limit; if the maximum output current value is reached the channel behaves like a constant current generator. In case of TRIP < 1000 s, the output current is permitted to keep the limit only for programmed time interval and then is switched off.

The maximum output voltage (VMAX) and the maximum output current (IMAX) can be fixed for each channel, through trimmers located on the front panel.

Polarity: / Floating
Output Voltage3: / 1.5 ÷ 8 V (connector output)
Max. Output Current: / 9 A
Voltage Set/Monitor Resolution: / 5 mV
Current Set/Monitor Resolution: / 10 mA
VMAX hardware: / 1.5 ÷ 8 V
VMAX hardware accuracy: / ± 2% of FSR
VMAX software: / 1.5 ÷ 8 V
VMAX software resolution: / 5 m V
Voltage Ripple: 4 / < 20 mV pp
Voltage Monitor vs. Output Voltage Accuracy: 5 / typical: maximum: / ± 0.3% ± 30 mV ± 0.3% ± 50 mV
Voltage Set vs. Output Voltage Accuracy: 5 / typical: maximum: / ± 0.3% ± 30 mV ± 0.3% ± 50 mV
Current Monitor vs. Output Current Accuracy: 5 / typical: ± 2% ± 0.05 A maximum: ± 2% ± 0.1 A
Current Set vs. Output Current Accuracy: 5 / typical: ± 2% ± 0.05 A maximum: ± 2% ± 0.1 A
Load Regulation: 5 / ± 0.3 % (with sense wires) ± 2 % (without sense wires)
Output power: / 45 W per channel

Table 4. – Channel characteristics of the Mod. A3009 Power Supply Board.

Figure 2. Low voltage Molex Microfit-Fit 3,0 (43025-1200)(left side) and high voltage CPE tri-polar (right side) connectors used for both the barrel and endcap RPC.

4The prototype tests (Pigi)

4.1The High Voltage…..

5The test station in Naples (David)

5.1System description

5.2First results

6The test station at CERN (David)

6.1System description

6.2First results

7Commissioning (Giovanni)

7.1System description

7.2First resul

8Conclusion (Pigi)

References should be placed at the end of the note (see example [1]).

References

[1]CMS Note 2005/000, X. Somebody et al., "CMS Note Template".

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