Tutorial Residual Gas Analysis Reconstruction of mass spectra

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

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

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures Some definitions used here Sensitivity 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 Ion gauge to a gas species with respect to the sensitivity for nitrogen (here) Sensitivity of an RGA to a gas species with respect to the sensitivity for nitrogen (here) 06 -16/06/2017 CERN Accelerator School 2017 3

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures Mass spectrum: Measurement of Ion detector current as functin of m/z Ion (detector) currents at mass m [A] C 28, N 2 = 1 N 2 06 -16/06/2017 Sensitivity of analyser to reference peak [ A / mbar ] Contribution of component b to ion current signal at mass m C 28, CO 2 = 0. 1 Partial pressure [ mbar ] C 28, CH 4 = 0 CO 2 CERN Accelerator School 2017 CH 4 4

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures Equations for all masses Mass spectrum Fragmentation pattern Relative height to reference peak Sensitivity to reference peak Partial pressure Composition of N gases 06 -16/06/2017 CERN Accelerator School 2017 5

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures Whole spectrum expressed in simple mathematical form using matrices and vectors. Fragmentation pattern matrix C Sensitivity matrix α Equation for spectra simulation with C as fragmentation pattern library and partial pressures in vacuum system. 06 -16/06/2017 CERN Accelerator School 2017 6

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures Bar graph spectrum Ion current Analogue spectrum Partial pressures Gas specific sensitivities Reference spectra library (fragmentation patterns) FWMH = 0. 5 Assimilation of measured analogue spectrum with reconstructed simulated one and cross-check sum of partial pressures with ion gauge reading 06 -16/06/2017 CERN Accelerator School 2017 7

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures Cross-check: Comparison with ionization gauge Partial pressure of species n Collector current is sum of the contribution of all partial pressures multiplied by their respective sensitivity BA-gauge collector current BA-gauge emission current Sensitivity of BA gauge to species n [mbar-1] Relative sensitivity of BA gauge to with respect to N 2 -equivalent pressure indication 06 -16/06/2017 Cross-check RGA CERN Accelerator School 2017 8

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures Comparing relative sensitivities QMS for UHV applications Ion gauges 4 3 Rel. sensitivity to N 2 with mass QMA 125 (QMG 422) QMA 125 (Hiqh. Quad) Prisma VGQ Hiden 3. 5 2. 5 Opposite to ion gauges Ionisation cross-section + extraction + transmission probability 2 Only indicative 1. 5 ! 1 RGA: Sensitivity to reference peaks RGA: of fragmentation patterns 0. 5 0 H 2 He CH 4 N 2 Ar Kr Ion gauges collects all fragments Normalised sensitivity for different quadrupole mass spectrometers (QMS) at factory settings -> High dispersion at low masses When comparing with ion gauges sum up all fragments of a residual gas component 06 -16/06/2017 CERN Accelerator School 2017 Gas He Ne Ar Kr Xe fc 0. 18 0. 3 1. 29 1. 94 2. 87 H 2 0. 46 N 2 1 O 2 Air CO 1. 01 1 1. 05 H 2 O CO 2 1. 12 1. 42 CH 4 C 2 H 6 C 3 H 8 1. 4 2. 6 4. 2 Uncertainty < 20 % Ratio mainly determined by ionisation cross-section 9

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures Example of reconstruction of measured with simulated spectrum F-Contamination with CHF 3 (Fluoroform), Confirmed ESD peaks O and F 06 -16/06/2017 CERN Accelerator School 2017 10

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures Interpretation of the mass spectrum 06 -16/06/2017 CERN Accelerator School 2017 11

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures Measured / reconstructed mass spectrum SEM_Multipl. (B. Jenninger CERN TE-VSC) F-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. Double-ionisation COF 47 -> 23. 5 (not simulated) Background dist Background offs 4. 00 E-14 1. 20 E-13 Hydrogen Helium Methane Water Carbonmonoxide Ethane Carbondioxide Hydrogen. Fluoride Ferrum Chromium Oxygen (atomic) Fluorine(atomic) Isotope 13 COF (not simulated) Trifluoro. Anhydrid. Acetic. Acid Carbonic Difluoride Perfluoro. Propane Sensitivity Ar (FAR) [A/mbar] FWHM CERN Accelerator School 2017 1. 00 E-04 0. 5 Pi Gas S [A/mbar] H 2 He CH 4 H 2 O CO C 2 H 6 CO 2 HF Fe Cr O F 1. 00 E+00 1. 00 E+00 C 4 O 3 F 6 COF 2 C 3 F 6 1. 00 E+00 P_tot = [mbar] 6. 00 E-11 2. 00 E-13 1. 00 E-12 6. 00 E-12 1. 00 E-11 6. 00 E-13 2. 00 E-12 4. 00 E-13 2. 00 E-13 3. 00 E-12 3. 00 E-11 7. 00 E-13 8. 00 E-12 3. 00 E-12 1. 27 E-10 Spectrum can be fully explained with presence of Fluorine + known composition in hot ion source (C, O, H, Fe, Cr) 06 -16/06/2017 10000 12

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

Tutorial: Residual Gas Analysis Reconstruction of mass spectra on the basis of partial pressures NOW Exercise: Reconstructing a measured mass spectrum 06 -16/06/2017 CERN Accelerator School 2017 14