Go/no-go test for the acceptance of components for unbaked vacuum sectors Paolo Chiggiato (Thanks to Sophie Meunier for the preparation of the presentation)
Introduction CERN policy: All vacuum components must be tested before installation in particle accelerators, even those already installed in accelerators before modification Points to take into account to define the acceptance test: The tested elements could be only a part of a mechanical assembly. The outgassing rate of unbaked components change with pumping time. Hundreds of components could be regularly tested. Technicians prefer easy-to-apply go/no-go acceptance criteria. 4/6/2014 Document reference 2
Acceptance criteria for components of unbaked vacuum sectors The acceptance test of components for unbaked vacuum sectors relies on: 1. The pumpdown curve. AND 2. Relative comparison of RGA spectra. Typical all-metal system used for acceptance tests of unbaked components 4/6/2014 Document reference 3
Acceptance criteria: pumpdown curve The pressure reading is monitored with and without (background) the sample with a strict procedure: The vacuum system is exposed to air overnight. The duration of the pumpdown is at least 10 h. The slope a of the P=P(t) function in log-log scale is calculated. The outgassing rate after 10 h of pumping is calculated and converted in square meter of regular stainless steel. The acceptance condition is: a 0. 85 Virtual leaks, presence of organic materials or contamination are detected Q(10h): compatible with the application: If a 0. 85, unexpected high roughness or open porosity are detected 4/6/2014 Document reference 4
Pressure [mbar] Acceptance criteria: example of non-acceptance Electrodes for PS strip liner monitor BTM a=-1.02 a=-0.97 background Pumping time[s] The estimated outgassing rate after 10 h of pumping of the four electrodes is: Q (10h) = (9.1 10-7 - 6.6 10-8 )*63 = 5.32 10-5 mbar.l/s. The data is reported as equivalent m 2 of stainless steel. For the example: 18 m 2 of stainless steel for the 0.5 m 2 of electrode surface area 4/6/2014 Document reference 5
4/6/2014 Document reference 6
Pressure [mbar] Acceptance criteria: example of non-acceptance a=-0.5 background Pumping time[h] 4/6/2014 Document reference 7
Acceptance criteria: RGA spectra The aim of this measurement is to discriminate between acceptable and non acceptable components on the basis of relative thresholds. The procedure is repeated twice with exactly the same parameters and procedure, with and without the component to be tested. 1. After 11 h of pumping, The RGA filament is switched-on. 2. Three hours after the ignition of the filament, mass scans are recorded (in Faraday or electron-multiplier mode depending on the attained pressure). 4/6/2014 Document reference 8
Acceptance criteria: RGA spectra Two criteria for the acceptance of the components based on RGA spectra: 1. All mass-peak between 18 amu and 44 amu shall be 100 times lower than the 18-amu mass-peak, except for 28 amu and 44 amu. AND 2. All peaks for masses higher than 44 amu shall be 1000 times lower than the 18 amu mass-peak. The purpose is to detect contamination due to ineffective (or lack of) cleaning, post-cleaning manipulation, packaging, etc. 4/6/2014 Document reference 9
Passed the RGA test 4/6/2014 Document reference 10
Did not pass the RGA test 1 100 1000 4/6/2014 Document reference 11
Are the acceptance criteria too stringent? A clean system, which comprises several components found in vacuum sectors, was exposed to N 2, air, and water vapor and then re-pumped. Sector 1 Sector 2 Sector 3 Residual gas analyser All-metal sector valves Pumping group for training lab Pumping group for Injection line 4/6/2014 III. Document Systèmes reference Utilisés 12
Venting and re-pumping procedure Venting of the test system with 4 different gas : nitrogen, dry air (80% N 2, 20% O 2 ), water vapor (25 mbar), ambient air Bake-out after water vapor and air venting Mass scan analysis after different pumping time : 5h, 7h et 24h. 16 h00 9 h00 12 h00 14 h30 16 h00 9 h00
Pump-down curves CO and CO 2 generation on the RGA s filament Pump-down measurement after venting: P 1 t as expected The pressure spikes are due to the ignition of the RGA filament. 4/6/2014 Document reference 14
RGA spectra Ion Current (A) 1.00E-06 1.00E-07 1.00E-08 Mass spectrum test system After bakeout N2 Dry Air (N2+O2) Water Air 1.00E-09 1.00E-10 1.00E-11 1.00E-12 1.00E-13 0 10 20 30 40 50 60 70 80 90 100 m/z RGA used: Pfeiffer Prisma QMA200 (SEM 1200V ) 4/6/2014 Document reference 15
Ion Current [A] Ion Current [A] 4.00E-12 Peak 55 2.00E-12 Peak 57 3.00E-12 1.50E-12 2.00E-12 1.00E-12 1.00E-12 5.00E-13 0.00E+00 0.00E+00 Hydrocarbon peaks are most likely generated by chemical reactions involving the decomposition of water vapor onto the hot RGA s filament. Such hydrocarbon peaks are several orders of magnitude lower than those regularly measured during acceptance tests. 4/6/2014 Document reference 16
Conclusions 1. Acceptance tests for components of unbaked vacuum sectors are now mandatory at CERN. 2. The acceptance criteria are based on: a. The slope of the pumpdown curve and the outgassing rate after 10h of pumping. b. The relative intensities of the RGA mass spectrum. 3. Such criteria allow vacuum technicians to easily identify non-conformities. They are now part of the normal practice at CERN. 4. Clean all-metal systems fulfil easily the requirements. 5. Part of the hydrocarbon peaks could be originated by the interaction of surface carbon contamination and dissociated water on hot filaments. 6. The validity of the criteria will be soon checked by XPS and FTIR analysis. 4/6/2014 Document reference 17