E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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BEAM LOSS MONITORS FOR THE LHC

E.B. Holzer

LHC Specification Committee

July 15, 2004

• Monitor types

• Monitor positions

• Cleaning procedures for monitor production

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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Monitor Types

Location Area of use Monitor typeNumber of monitors

BLMCCollimation sections

Ionization chamber (parallel type)

~ 200

SEM ~ 200

BLMSBLMS*

Critical aperture limits or critical positions

Ionization chamber (parallel type)

~ 500

SEM ~ 100

BLMAAll along the rings (ARC, …)

Ionization chamber coaxial type or parallel type

~ 3500

• Ionization chamber with parallel plate electrodes: faster and more expensive.

• Ionization chamber with coaxial electrodes: slower but less expensive.

• Secondary emission monitors: gain is ~ 30000 times smaller.

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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Ionization Chamber - Parallel Plate Design

New LHC chamber design:Diameter = 8.9 cm,Length 60 cm, 1.5 litre,Filled with Ar + C02 or N2 at 1.1 bar,61 Al disks of 0.5 mm,Bias voltage: 1500 V.

SPS ChamberGas: N2, Volume: ~ 1 Liter,

30 Al disks of 0.5 mm,Typical bias voltage: 1500 V.

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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Ionization Chamber - Coaxial Electrode Design

2 or 3 electrodes from inox or Al,Diameter = 8.9 cm,Length 60 – 150 cm,Filled with Ar + CO2 or N2 at 1.1 bar,Bias voltage: 1500 V

Tube diameters: 8/10 mm

(inner/outer) 35/38 mm

66/70 mm

86.9/88.9 mm

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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Secondary Emission Monitor

Diameter = 8.9 cm,Length 15 cm,2 electrodes from Al or Ti,10-5 mbar.

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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Monitor Positions in Arc

Installation with a small support and straps or cables on the cryostats

Chamber (89 mm) + fixation (8 mm) just fits between the cryostat and the transport space (2 mm space left).

Precise longitudinal position not defined.

Installation of BLMAs on a SSS quadrupole

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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Monitor Positions for BLMS and BLMC

Collimator interconnect with ion pump and BLM (possible positions)

BLMS and part of BLMC will be installed on separate stands. The other part of BLMC will be installed on the collimator interconnects:

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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Chamber Gas Purity

Possible problems with ageing:

• Electronegative gases (O2, H2O, NH3, …) capture electrons - drift velocity reduced by a factor ~1000 change in signal height and length.

• Polymerization (should not pose a problem when properly cleaned and organic materials are avoided in the production process.)

• Impurities/additives change the chamber operation characteristics: • Drift velocity,• Recombination losses,• Gain (ionization/cm of charged particle),• Onset of gas amplification,• Flatness of signal vs voltage in the ionization region …

– These effects are often used intentionally:

• 2% H2 in He to suppress gas amplification,

• 5-10% CO2 in Ar to increase drift velocity …

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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• Estimate that electronegative impurities should stay below ppm level.• Calculated for cleaned/baked Al:

• Thermal desorption acceptable if cleaned according to CERN

standard for UHV application and backed in vacuum before filling.• Particle induced desorption: for BLMC glow discharge cleaning

during filling process.• Avoid organic material in production process.• Avoid closed volumes in design (from welding, from tubes, …).

Levels of electronegative impurities in the chamber after 20 year of LHC operation

thermal Electron induced Photon induced

BLMA/BLMS 1.6 10-6 10-8 4 10-11

BLMC 1.6 10-6 10-3 4 10-6

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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Cleaning and Filling Procedure

• CERN standard cleaning for UHV procedure– ultrasonic bath of the alkali detergent NGL 17.40 Alu from NGL Cleaning

Technologies at 60 degree C

– rinsing with cold demineralised water jet (conductivity < 5 uS cm-1)

– immersion in hot ultrasonic demineralised water bath

– drying with compressed dry nitrogen and afterwards in a hot (80 degree) air oven

• Mount (several chambers at same time)• Pump • He leak detection• Bake• He leak detection• For BLMC: glow discharge (Ar at ~10-3 mbar)• Pump• Fill

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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Cleaning and Filling

• Detailed procedures for pumping, baking, glow discharge and filling will be written after the tests on the prototype chambers.

• The system for pumping, baking, glow discharge and filling will be built at CERN.

E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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E.B. HolzerLHC Specification Committee , CERN July 15, 2004

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