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Vacuum Technology for Particle Accelerators

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Vacuum Technology for Particle Accelerators Vacuum Technology for Particle Accelerators Marek Grabski (Max IV laboratory) CERN Accelerator School: Introduction to Accelerator Physics 8th October 2016, Budapest, Hungary Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 1 Marek Grabski Contents • What is vacuum and why do we need it in particle accelerators? • Basics of vacuum technology, • Gas sources, • Pumping technology, • Pressure profile evaluation, • Outlook of accelerator vacuum systems. Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 2 Marek Grabski What is vacuum? Vacuum is a space with no matter inside. Vacuum in engineering and physics is a space in which the pressure is lower than atmospheric pressure and is measured by its absolute pressure. Low pressure High pressure Force [N] 2 Pressure is Area [m 2] measured in Pascals, 1 [Pa] = 1 N/m Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 3 Marek Grabski What is vacuum? The force exerted on the walls of an evacuated vessel surrounded by atmospheric pressure is: 1 kg/cm 2 Atmospheric pressure Vacuum Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 4 Marek Grabski What is vacuum? Conversion table: units of pressure In vacuum technology: mbar or Pa Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 5 Marek Grabski Why do we need vacuum in particle accelerator? Less beam-gas interaction: • Increase beam lifetime , • Prevents to increase beam size, • Reduces radiation hazard, • Lowers background in detectors. The total beam lifetime in a particle accelerator is given by: The interaction between beam particles and residual gas molecules consist of two main mechanisms: elastic and inelastic scattering which contribute to total beam lifetime. Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 6 Marek Grabski Elastic scattering Elastic scattering with residual gas molecules alter transverse motion of particles increasing their betatron oscillations (energy of particles is conserved). • The particles are lost when the oscillation amplitude exceeds physical acceptance aperture. Not only the absolute pressure is important but also what are 1 the gas species in the system ~ Gas e- molecule where: nucleus Z - atomic number of the residual gas (careful which gas is dominant) - average vertical beta function - vertical beta function at the limiting vertical aperture γ - relativistic factor of the particles in the stored beam ay - limiting vertical aperture ng - residual gas density Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 7 Marek Grabski Inelastic scattering Inelastic scattering with residual gas molecules decreases speed of particles causing photon emission called Bremsstrahlung (energy of particles is not conserved). • The particle are lost if the energy loss exceeds the momentum acceptance of the accelerator. 1 1 ~ / Not only the absolute pressure is important but also what are the gas species in the system where: Z - atomic number of the residual gas (careful which gas is dominant) / - momentum acceptance ng - residual gas density Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 8 Marek Grabski Vacuum ranges 1 Atm. = 1013 mbar =~ 1 bar Pressure range [mbar] Low Vacuum 10 3 - 1 Medium Vacuum 1 - 10 -3 High Vacuum (HV) 10 -3 - 10 -9 Ultra High Vacuum (UHV) 10 -9 - 10 -12 Extreme High Vacuum XHV < 10 -12 Storage rings Storage Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 9 Marek Grabski Ideal gas law Ideal gas equation: P V = N moles R T P V = N molecules kb T macroscopic microscopic P - the pressure of the gas, V - the volume of the gas, Nmoles - the number of moles of the substance, Nmolecules - the number of gas molecules, R - the universal gas constant, T - the absolute temperature of the gas, kb - the Boltzmann constant 1 drop of water contains: 10 20 molecules Each cubic centimeter of gas at room temperature contains: ~10 19 molecules/cm 3 - at 1 atm (~1 Bar), ~10 6 molecules/cm 3 - at 10 -10 mbar (similar pressure as on the moon) Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 10 Marek Grabski Maxwell-Boltzmann model Mean speed of gas molecules: - impingement rate, n - gas density (molecules/volume), k - the Boltzmann constant 1.38*10 -23 [J/K], 8 b T - the absolute temperature of the gas [K], !" m0 - molecular mass [kg], - the average gas molecules speed [m/s], Number of molecules striking unit surface in unit time (Impingement rate): 1 1 8 4 4 !" Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 11 Marek Grabski Flow regimes - Mean free path [m] $ % - Diameter of flow channel [m] % $ - Knudsen number dimensionless d FLOW REGIME VISCOUS TRANSITIONAL MOLECULAR $ & 0.01 0.01 & $& 0.5 *+ , -. Mean free path: Low vacuum Medium vacuum High/ultra high vacuum At atm. Pressure = 6.5x10 -8 m -3 At 10 -9 mbar (storage ring) = 66 km Typically: >1 mbar < 10 mbar Vacuum Technology Know how by Pfeiffer Vacuum GmbH Increasing pressure Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 12 Marek Grabski Flow regimes Gas molecules collide with each other Pipe diameter [cm] diameter Pipe Pressure [mbar] Gas molecules collide with chamber walls Particle accelerators operate in molecular flow regime. Vacuum Technology Know how by Pfeiffer Vacuum GmbH Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 13 Marek Grabski Gas flow in molecular regime In molecular flow regime the gas flow (Q) from one point to the other is proportional to the pressure drop: Conductance Gas flow Pressure difference D /EF0 GFH !IJ Slot of area A [DA B 0 Conductance depends on the gas / 2 /32 molecule velocity thus its molar mass 1 and temperature (not on pressure). 3 1 / Conductance C at 295 K for nitrogen 4 B C! (N 2 - molecular mass 28): 3 / - conductance of unit surface area for given gas / /32 11.8 2 A 5 slot Area [cm 2A B C! - the average gas molecules speed [m/s] Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 14 Marek Grabski Gas flow in molecular regime Combination of conductances: a). For components in series: In steady state Q 1=Q 2 -> -> b). In parallel: Paolo Chiggiato, Vacuum Technology for Ion Sources, CAS 2012 proceedings Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 15 Marek Grabski Gas flow in molecular regime In vacuum technology a pump is an obJect that permanently removes gas molecules from the gas phase. Pumping speed S of a pump is defined as the ratio between the pump throughput Q p and the pressure P at the entrance to the pump. DL K F B Pump Gas removal rate can be written as: 0 3 DL 2LM 2L M 2L/ F M 1 N impingment rate 2 Ap 5 is the area of the pump aperture [cm A /3 - is the conductance of the unit surface area From the definition of pumping speed: for given gas 3 n – gas density S 2L/ M M – is the capture probability Paolo Chiggiato, Vacuum Technology for Ion Sources, CAS 2012 proceedings Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 16 Marek Grabski Gas flow in molecular regime Introduced limitation between pump and pumped vacuum volume limits the nominal pumping speed of chosen pump. Example: Beam path Pump Pump 300 l/s ion pump connected to a vacuum chamber 0 0 0 + through a 90 deg elbo_ of given dimensions can VWXX V Y result in ~30 % pumping speed reduction. Paolo Chiggiato, Vacuum Technology for Ion Sources, CAS 2012 proceedings Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 17 Marek Grabski Generic vacuum system P D F Processes -> Q Kaa b 5 gas pressurec d 5 gas load EoutgassingHc eeff 5 fffective pumping speed. Pump Seff Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 18 Marek Grabski Sources of gases Sources of static gas loads in vacuum system: Processes -> Q Vacuum chambers are source of gas Courtesy of Eshraq Al-Dmour Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 19 Marek Grabski What process defines pressure What process defines the pressure over time? Outgassing : Material – Binding energy Surface condition – As delivered – Machined Outgassing ~t -1 – Polished Diffusion ~t -(1/2) – Cleaning – Heat treatment… Difussion : - Material - Heat treatment (Vacuum firing) - Inner surface barrier (Air baking, Film deposition) http://web.utk.edu/~prack/Thin%20films/VACUUM-3.pdf Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 20 Marek Grabski Thermal outgassing Thermal outgassing (static outgassing) For metals: • If not baked (heated to 200 0C) in-situ • If baked (heated to ~200 0C) in-situ water is the dominant gas specie. hydrogen H 2 is the dominant gas ghJJ Outgassing rates q at 20 0C: B C! Austenitic stainless steel not baked, 3x10 −10 (H O) after 10 h pumping 2 Austenitic stainless steel baked in- 2x10 -12 (H ) situ for 24 h at 150 0C 2 OFS copper baked in-situ for 24 h at ~10 -14 (H ) 200 0C 2 Paolo Chiggiato, Vacuum Technology for Ion Sources, CAS 2012 proceedings Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 21 Marek Grabski Dynamic outgassing In particle accelerators energized particles impinging on vacuum surfaces induce desorption of molecules. Usually such dynamic gas load dominate over thermal outgassing. Beam stimulated desorption is characterised by i - the desorption yield: j!IkJ hl %kBhJIk% !hkCjkB i j!IkJ hl JgmCk m!mnmn gok BjJlCk i – depends on many parameters: • incident particle: type and energy, The desorption may be • material, stimulated by: • surface roughness, • electrons, • cleanliness of the surface, • ions, • history of the material (dose), • synchrotron radiation (photons). • Particle flux. Vacuum Technology for Particle Accelerators, CAS Budapest, Hungary, October 2016 22 Marek Grabski Photon Stimulated Desorption When charged particles (moving at relativistic speeds) are accelerated they emit synchrotron radiation in a narrow cone.
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