The gas system for the CMS-RPC detector

Roberto GuidaCollaborazione CMS-RPC

Riunione CSN1 Roma, 2 Aprile 2007

•Introduction to the CMS-RPC gas system

•R&D studies on the closed loop gas system: •Development of the purifier module

•Gas monitoring systems:•Gas quality monitoring system •Gas gain monitoring system

•Conclusions

Outlook

Characteristics and requirements

Total volume of the CMS-RPC detector: 18 m3

Gas mixture: 96.2 % C2H2F4, 3.5 % iC4H10, 0.3% SF6

Mixture relative humidity: 40%

Number of channels: 360

Nominal gas flow: 8 m3/h (0.5 volume change/h)

Gas recirculation factor (CLOSED LOOP MODE): ~95%

Fresh gas replenishing rate: 1 m3/h (~5%)

Humidity

SuppliersMixerHumidifier

Purifier module

Pre-distribution and pump

Final distribution

Layout of the system

Schedule

P um pM o d u le

P urifi erM o d u le

M ix er C ircu latio nM o d u le.

E x h aust

SG - B u ild in g

B uff er

P rim arySu p p ly

U SC - area

M ain P ip es / len gth = 70 mSu p p ly : 1 1 x Ø = 14/ 1 6 m mR etu rn : 11 x Ø = 20 / 22 m m

U X - C av ern

Pre-

Dis

trib

utio

nPr

essu

re R

egul

.

U X - D istrib u tio nR ack s

H u m id ityC o n d itio n in g

Apr-May 07(installation)

May 07(CL commissioning)

almost ready

• All gas racks are installed except purifiers and analysis

• All racks are leak tested and functionally tested in the lab

• Control software 95% ready• PVSS user interface ready

Gas racks layout

RPC Modules: Pre-distribution; Pump; Analysis; Control rack

RPC Modules:Mixer; Exhaust; Humidifier; Purifiers; Analysis; Control rack

SG5 (surface) USC55 (service cavern)

…at the end the RPC system will be the biggest

CMS UX cavern

The last step is the final distribution of the gas mixture on the experiment

11 gas distribution racks:•50 channels each •Gas flow read-out by means of flowcells•Manual adjustment between channels

A careful equalization of the gas flow is needed in order to exploit the full capacity of the system

-Between stations (i.e. wheel level): •Results independent on the number of channels connected•Optimized at the minimum pressure needed •The key parameter is the number of volume change/h

-Station level:When needed a needle valve has been inserted in order to equalize the flow between the two chambers

Final gas distribution

Present specificationsThe system described is the result of an intense R&D program. In fact,the results from the GIF ageing test carried out by the CMS-RPC and ATLAS-RPC collaborations (1999-2004) as well as the test of many RPC with cosmic have implied an important evolution in the requirement for the RPC gas system and mixture composition in CMS:

Today CMSin 1999/018

Gas volume 18 m3 18 m3

Gas mixture R134a/iC4H10/SF6

96.2/3.5/0.3

R134a/iC4H10

96.5/3.5

Tollerable contamination: H2O

Mixture RH 40%

(7,000-10,000 ppm)

<1000 ppm

Chamber pressure <3 mbar <3 mbar

Maximum flow rate 18 m3/h 2.2 m3/h

Flow rate at operating conditions

8 m3/h 1.2 m3/h

Fresh gas replenishing ≤1 m3/h <0.12 m3/h

• The CMS-RPC collaboration started an intense R&D program since 2003 using a closed loop gas system prototype, provided by the CERN gas group, connected with two final RB1 detectors under irradiation at the CERN-GIF.

• After, in a second phase (started in 2005) the system was moved to the ISR area and used for the mass production chamber test. The aim of this phase was a further optimization of the purifier effectiveness (Preprint LNF 06/27).

Phase 1: test at the GIF facility Phase 2: test at ISR

• Further studies of the materials properties: Bakelite, purifiers, gas mixture

R&D on the closed loop system

• The purifier module is a fundamental part of the closed loop gas system.

• 1999: Standard purifier using a twin column (6 l) filled with e.g. Molecular sieve and activated copper can be used to remove water and oxygen. Since the expected contaminant were H2O and O2

• 2003: first hints from the GIF first results. However the idea of an upgrade was not considered in detail.

• 2005: The final results from the GIF ageing test have confirmed that:– RPCs need a humidified gas mixture (RH about 40%)– The gas flow should be 0.5 – 1 volume change/hour– New component in the gas mixture is needed (0.3% SF6)– The outlet gas from a reference chamber (not irradiated) has a similar gas

chromatogram as the inlet gas (except air and H2O)– In the outlet gas from a chamber inside GIF 5-7 extra components are

visible in the gas chromatogram – The best configuration found for the purifier is:

1. Molecular Sieve 5A 2. Cu-Zn / Cu3. Ni – Al2O3

Purifier R&D on the closed loop system

Gas chromatogram: GIF results

Fresh mixture

purifiers

Closed loop recirculation (95%) after about one month: many small extra signals are visible especially at low retention time

Results obtained with a MicroGC Varian CP2002P (PoraPLOT Q and TCD detector).

Fresh mixture

Mixture in the closed loop circuit

Final RPCs in closed loop mode with cosmic ray

System stable from November 2005, but just before the purifier regeneration …

slight increase in the currents. Reversible after regeneration.

Log scale

Gas chromatogram: ISR results

Usually no trace of impurities and pollutants in GC analysis.Seldom something is visible only in log scale.

Upgrade of the Purifier Module

Fill

Empty

Ø=16

Mol.Sieve

5Å

Mol.Sieve

5Å

Fill

Empty

Ø=16

Cu-ZnBASFR3 12

CuBASF

R311-G

Cu-ZnBASFR3 12

CuBASF

R311-G

fromBuffer

to ExhaustModule

Buffer Volume

Free Rack placefor future upgrade of

third purifier.

First possibility: 2 purifiers, the second one including the two metallic filters In case of necessity a further upgrade including a third module will be considered for the LHC high luminosity phase

An upgrade of the purifier module is mandatory for both capacity and purifiers material reasons

• Purifier #1: Molecular Sieve 5, 24 l cartridge. Expected lifetime at operating conditions 1.5 day of running time

• Purifier #2: Cu-Zn/Cu and Ni-Al2O3 Adsorption capacity x 50 with respect to the GIF setup.

Expected lifetime at operating conditions 15 day

P #1 P #2 P #3

?

• 8 double gaps are now under test in ISR (they are equivalent to 4 RB1 detectors)

• We started with a open mode gas system and we will move soon in closed loop mode

• Results expected in 1-2 months

• New more gaps in 2 months (?)

ISR test: second phase, a dedicated test

Development of the gas system

Extra signals in the closed loop gas

Increase of the gas flow

New component (SF6)

Humidified mixtureHumidity

Gas monitoring for CMS-RPC

We foreseen two systems able to monitor the gas mixture for the RPC detector:

• Gas quality and composition monitoring system based on some chemical analyses of the gas: • F- electrode • pH electrode• Pick-up point for a possible use of a Gas Chromatograph (GC)

• Gas gain monitoring system based on three sets of small RPCs supplied with fresh mixture, input mixture to CMS, return mixture from CMS respectively. It will be also possible to scan all the gas lines.

Results from both the monitoring systems need be recorded in the DCS system.

Gas Monitoring Systems

USC55UXC55SG5

RPCs #1Fresh gas

Purifiers

Mixer

Humidifier

GCF-

pHsample

RPCs #2Before purifiers

RPCs #3After purifiers

Gas quality monitoring

Gas gain monitoringGC+others(half wheel pick-up points)

CMS

Specific for fluoride/HF concentration monitoring.

Analysis station with two independent input channels (possibility to monitor simultaneously different sampling points).

F- electrode

GIF results

Low-cost and very simple system, with PC readout, already used during tests at GIF, in our labs and in ISR … nevertheless quite sensitive

y = 5,29E-09x - 5,91E-06

R2 = 9,75E-01

0,E+00

5,E-06

1,E-05

2,E-05

2,E-05

3,E-05

0 1000 2000 3000 4000 5000 6000

time (min)

[HF

]

Serie1

Lineare (Serie1)

pH meter

Gas Chromatograph

purifiers

GC Results:Signals from impurities produced by RPCs working in Closed loop mode (95%) after about one month.

GC is the most flexible system it has already demonstrated at GIF its ability to detect produced pollutants in the gas mixture

A monitoring system of the RPC working point able to provide much faster and sensitive response than the CMS RPC system

Monitor of efficiency and charge continuously Monitoring of charge and efficiencies with cosmic rays in SG5 (surface

gas building) Three sub-sets of 45x45 cm2 Single Gaps

1 Reference RPCs, i.e. fresh mixture

2 Monitor “IN” RPCs, i.e. mixture for CMS-RPC

3 Monitor “OUT” RPCs, i.e. mixture downstream of CMS-RPC

In case work point changes– A warning goes on action needed

– The Gas quality monitoring system (i.e. GC,probes, etc) will understand the reason

Gas gain monitoring system

The R&D program carried out by the RPCs community since 1999 has allowed to found the proper solution in order to guarantee the detector functionality for a long term period (increase of the gas flow, humidified mixture, a more complex purifier module, ….)

However the results of this R&D program have produced many important changes on the CMS-RPC gas system

Now the system is much more complex, but the changes are effective

Further studies are needed especially for the purifier module (most of this work is already going on or well defined)

Conclusions