1 / 11

Cleaning

Cleaning. Chamber gases and impurities Tolerable impurity levels Thermal desorption Particle induced desorption M. Taborelli’s conclusions and suggestions Open questions. Chamber gases and impurities. Chamber gases - advantages: N 2 :

atira
Download Presentation

Cleaning

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Cleaning • Chamber gases and impurities • Tolerable impurity levels • Thermal desorption • Particle induced desorption • M. Taborelli’s conclusions and suggestions • Open questions

  2. Chamber gases and impurities • Chamber gases - advantages: • N2: • Same as SPS chambers (experience, 2004 tests for ageing). • In case of leak – small changes in ionization properties. • Noble gas (e.g. Ar): • Cleaning with glow discharge done with noble gases. • Cleaning of gas with getter pumps before filling. • Possible problems with ageing: • Electronegative gases (O2, CO, CO2, H2O, …) capture electrons - drift velocity reduced by a factor ~1000  reduction in signal height. • Polymerization (should not pose a problem when properly cleaned and organic materials are avoided in the production process.) • Changes in ionization properties through change of gas composition.

  3. Chamber gases and impurities • Impurities/additives (like e.g. H2, CO2, CH4) 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: • NuMi chambers add 2% H2 to He to suppress gas amplification, • CO2 additives to increase drift velocity …

  4. Tolerable impurity levels • After some literature search we have not found data which tells us exactly, what kind of impurities we can tolerate in which chamber gas. • We came up with an estimate that electronegative impurities should stay below ppm level to avoid electron attachment (hep-ex/0212011 (2002), NIM B 187 (2002) 535 – 547, NIM B 179 (2001) 412 – 435). • But no estimate for other impurities.

  5. Thermal desorption (M. Taborelli) • Calculation for the SPS ionization chamber layout and 1 bar • Thermal desorption (after 20 years) baked Al: • Impurity level: 1.6 10-4, mainly H2, 1% CO (1.6 10-6), < 1% CO2, O2 and H2O

  6. Particle induced desorption • Estimation of number of charged particles at the chamber locations (from the showers induced by local proton losses running at quench level): • Cleaning efficiency: 104, • Required life time of operation: 20 years, 4000 hours / year, • 10 % of time at 450 GeV and 90 % of the time at 7 TeV. • The drift velocity of electrons and ions created by ionization in the chamber stay around/below the thermal velocity  do not induce desorption.

  7. Particle induced desorption (M. Taborelli) • Calculation for the SPS ionization chamber layout and 1 bar • Particle induced desorption chemically cleaned Al (data not available for baked Al): • Maximum impurity level (all releasable gas): 7 10-3, mainly CO and CO2;H2 levels are higher (?) • Assume all “MIPs” are electrons (1 - 10 MeV)  impurity level • BLMA/BLMS: 10-8 (negligible) • BLMC: 10-3 (almost all desorbed) • assume all “MIPs” are photons (10 MeV)  impurity level • BLMA/BLMS: 4 10-11 • BLMC: 4 10-6

  8. M. Taborelli’s conclusions and suggestions • Thermal desorption acceptable if cleaned according to CERN standard for UHV application and backed in vacuum before filling. • Particle induced desorption: for BLMC possibly further cleaning necessary, for instance glow discharge during filling process. • Systematic He leak test of all chambers before baking. • Glow discharge: • needs to be done in a noble gas (use Ar as chamber gas?) • Insulators needs to be shaped especially to avoid metal coating by sputtering. • Cleaning of working gas before filling: advantage of nobel gas because NEG pumps can be used. • Rest gas (and outgasing) is dominated by H2. • Avoid closed volumes in design (from welding/brazing, from tubes, …) • Avoid threaded rod (use rod with treaded ends). • Feed through in one piece (no brazing).

  9. M. Taborelli’s conclusions and suggestions • Foresee baking of filling hoses and tubes and getter cleaning during filling. • Clean before weld. • Rather weld than braze (e.g. feed through). Define procedures for welding (brazing). • Avoid organic material in production process. • Completely penetrating welding to avoid pockets.

  10. M. Taborelli’s conclusions and suggestionsCleaning 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 (1-3 hours) • Leak detection (He from outside – for example in the oven) • Bake • Possibly: glow discharge (Ar or He at ~10-3 mbar) • Pump • Fill

  11. Open questions • Particle species and energy spectrum  GEANT simulations – not the same for BLMA/BLMS and BLMC. • Suggest to measure the signal characteristics with the LHC prototype chambers for Ar and N2 (possibly with additives) and defined levels of impurities (the ones from M. Taborelli’s estimate). • Cleaning procedure (and chamber gas) for the two chamber types. • Glow discharge necessary for BLMC?

More Related