Tutorial: Residual Gas Analysis - PowerPoint Presentation

1. Tutorial: Residual Gas AnalysisReconstruction of mass spectra on the basis of partial pressures Paolo Chiggiato & Berthold Jenninger 06-16/06/2017CERN Accelerator School 20171

2. Tutorial: Residual Gas AnalysisReconstruction of mass spectra on the basis of partial pressures Why reconstructing ?- Visual: Analyser resolution, noise, offset and artefacts can be included- Check for suspected contaminants by adding fragmentation pattern of suspected component to library - Simulation allows training of personal on many cases without need for measurements.- Convincing, if residual gas composition is well identified. Discussions on acceptance can be more specific.- RGA are NOT ideal instruments:High uncertainties in gas the specific sensitivities and fragmentation .Reconstruction by assimilation to measured spectrum is an alternative to solving (by deconvolution) for partial pressures.06-16/06/2017CERN Accelerator School 20172

3. Some definitions used hereSensitivity ion gauge:Sensitivity residual gas analyser RGA:Normalised or relative sensitivity:Proportionality factor of the measured chande in collector current Ic,n as a reaction to a change in the partial pressure of a gas species n divided by the electron emission current Ie.Proportionality factor of the measured detector current at mass M as a reaction to a change in the partial pressure of a gas species n.If M is not indicated, it refers ot mass with maximum intensity in fragmentation pattern.Sensitivity of an RGA to a gas species with respect to the sensitivity for nitrogen (here)Reconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas AnalysisSensitivity of an Ion gauge to a gas species with respect to the sensitivity for nitrogen (here)06-16/06/2017CERN Accelerator School 20173

4. Mass spectrum:Measurement of Ion detector current as functin of m/zIon (detector) currents at mass m[ A ]Contribution of component b to ion current signal at mass mN2CO2CH4C28, N2 = 1C28, CO2 = 0.1C28, CH4 = 0Sensitivity of analyser to reference peak[ A / mbar ]Partial pressure[ mbar ]Reconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas Analysis06-16/06/2017CERN Accelerator School 20174

5. Composition of N gasesMass spectrumFragmentation patternRelative height to reference peakSensitivityto reference peakPartial pressureEquations for all massesReconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas Analysis06-16/06/2017CERN Accelerator School 20175

6. Fragmentation pattern matrixCSensitivity matrixαPartial pressur vector Ion current vector Whole spectrum expressed in simple mathematical form using matrices and vectors.Equation for spectra simulation with C as fragmentation pattern library and partial pressures in vacuum system.Reconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas Analysis06-16/06/2017CERN Accelerator School 20176

7. Bar graph spectrumAnalogue spectrumIon currentReference spectra library(fragmentation patterns)Gas specific sensitivitiesPartial pressuresFWMH = 0.5Reconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas AnalysisAssimilation of measured analogue spectrum with reconstructed simulated one andcross-check sum of partial pressures with ion gauge reading 06-16/06/2017CERN Accelerator School 20177

8. Cross-check: Comparison with ionization gaugeCollector current is sum of the contribution of all partial pressures multiplied by their respective sensitivityN2-equivalent pressure indicationBA-gauge collector currentBA-gauge emission currentSensitivity of BA gauge to species n [mbar-1]Partial pressure of species nRelative sensitivity of BA gauge to with respect to N2Cross-check RGAReconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas Analysis06-16/06/2017CERN Accelerator School 20178

9. Comparing relative sensitivitiesQMS for UHV applicationsIon gaugesGasfcHe0.18Ne0.3Ar1.29Kr1.94Xe2.87H20.46N21O21.01Air1CO1.05H2O1.12CO21.42CH41.4C2H62.6C3H84.2Uncertainty < 20 %Normalised sensitivity for different quadrupole mass spectrometers (QMS) at factory settings-> High dispersion at low massesRGA: Sensitivity to reference peaks of fragmentation patterns!mass withOpposite to ion gaugesWhen comparing with ion gauges sum up all fragments of a residual gas componentIon gauges collects all fragmentsIonisation cross-section + extraction + transmission probabilityReconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas AnalysisOnly indicativeRatio mainly determined by ionisation cross-section06-16/06/2017CERN Accelerator School 20179

10. Example of reconstruction of measured with simulated spectrumF-Contamination with CHF3 (Fluoroform), Confirmed ESD peaks O and FReconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas Analysis06-16/06/2017CERN Accelerator School 201710

11. Reconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas AnalysisInterpretation of the mass spectrum06-16/06/2017CERN Accelerator School 201711

12. Double-ionisation COF47 -> 23.5 (not simulated)Isotope 13COF (not simulated)Spectrum can be fully explained with presence of Fluorine + known composition in hot ion source (C, O, H, Fe, Cr)Reconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas AnalysisF-Contamination, ESD peaks O and F. Remaining traces of Fe and Cr just after degassing and deposition on hot filament. Cr and Fe disappeard after a several hours of operation.Measured / reconstructed mass spectrum06-16/06/2017CERN Accelerator School 201712SEM_Multipl.10000(B. Jenninger CERN TE-VSC) Sensitivity Ar (FAR) [A/mbar]1.00E-04Background dist4.00E-14FWHM0.5Background offs1.20E-13     Pi GasS [A/mbar][mbar]HydrogenH21.00E+006.00E-11HeliumHe1.00E+002.00E-13MethaneCH41.00E+001.00E-12WaterH2O1.00E+006.00E-12CarbonmonoxideCO1.00E+001.00E-11EthaneC2H61.00E+006.00E-13CarbondioxideCO21.00E+002.00E-12HydrogenFluorideHF1.00E+002.00E-12FerrumFe1.00E+004.00E-13ChromiumCr1.00E+002.00E-13Oxygen (atomic)O1.00E+003.00E-12Fluorine(atomic)F1.00E+003.00E-11TrifluoroAnhydridAceticAcidC4O3F61.00E+007.00E-13Carbonic DifluorideCOF21.00E+008.00E-12PerfluoroPropaneC3F61.00E+003.00E-12  P_tot =1.27E-10

13. ISO 14291: Vacuum gauges — Definitions and specifications for quadrupole mass spectrometersISO/NP TS 20175 (in preparation):Vacuum technology -- Vacuum gauges - Calibration of quadrupole mass spectrometers for partial pressure measurement ISO/NP TS 20177 (in preparation):Vacuum technology - Vacuum gauges -- Procedures to measure and report outgassing rates.Reconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas Analysis06-16/06/2017CERN Accelerator School 201713

14. Exercise: Reconstructing a measured mass spectrumReconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas AnalysisNOW06-16/06/2017CERN Accelerator School 201714

15. Intelligent Data Analysing Laboratory (IDAL)Collaboration:FEASIBILITY AND USEFULNESS OF THE USE OF NEURAL NETWORKS OR MACHINE LEARNING TOOLS TO HELP IDENTIFYING THE COMPOSITION OF RESIDUALS GAS SPECTRA (for UHV applications)Architecture and trainingSupervised trainingFundamental difference compared to sequencial programmingKnown inputCompare: Out – Exp OUTAdjust weights(small step)% H2% CH4% H2O...IdeallyUse method of random mass spectrum simulation for automated training.15Developments: Mass spectrum analysing toolMass spectrum analysis -> Pattern recognitionTypical application for Neural Networks and machine learning applicationsSpecialised in data mining, artificial intelligence, machine learning, image processing .... Reconstruction of mass spectra on the basis of partial pressures Tutorial: Residual Gas Analysis06-16/06/2017CERN Accelerator School 2017