LEP and LHC double walled tubes experiences Sergio
LEP and LHC double walled tubes experiences Sergio Calatroni
LEP couplers Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 2
Double walled tube – coupler extension – “chemise” Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 3
LEP Coupler “crisis” – circa 1994 • Many people took part to the coupler study at that time • Problem: deconditioning – Traced back to desorption and re-adsorption of gases onto cold surfaces • Outgassing + multipacting: solutions – – – – Ti coating of all parts Fixed coupler Change impedance from 50 Ohm to 75 Ohm Smooth surfaces with less outgassing Bake the ceramic DC bias Clean room procedures • Factual review by R. Losito CERN-SL-2002 -057 ECT Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 4
Double walled tubes: fabrication • 316 LN stainless steel, forged blanks • Integral machining by turning • Advantages: – Lower Electron Stimulated Desorption – Smoothness – surface quality – Manufacturing cost in a series comparable to standard metalworking (F. Bertinelli) Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 5
Copper coating • Copper sputter coating preferred over electroplating • Advantages: – – Lower outgassing Lower Electron Stimulated Desorption (ESD) Better Thermal Desorption Spectrometry (TDS) Satisfactory RRR • Problem: – Difficult adhesion of copper on stainless steel • (C. Benvenuti et al. CERN LEP 2 Note 94 -21) Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 6
ESD measurements (500 e. V e- after 200°C / 24 h bakeout) Rolled+welded and Cu sputter coated Full machining and Cu sputter coated Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 7
Copper ESD measurements Cu sputter coated Cu electroplated Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 8
Copper TDS measurements Cu sputter coated Review of SPL RF couplers Cu electroplated Sergio Calatroni - 16. 3. 2010 9
RRR measurements Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 10
Thickness at 700 MHz: effect of RRR 10 Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 RRR 1 11
Copper thickness: room temperature, 700 MHz Ideal thickness = 4 m Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 12
Copper coating by sputtering Adhesion of Cu on s. steel is poor because it does not bind to Cr. O Idea: Ti underlayer, followed by Cu coating without breaking the vacuum. Metal-metal adhesion generally very good Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 13
Problem: BN • Stainless steel is vacuum fired at 950 °C – 316 LN steel contains some boron to increase hot workability (a few ppm) – If B > 9 ppm (Mn. S precipitates could act as possible nucleation sites) it may segregate at the surface as BN, with thickness of several nm (S. Sgobba, EST/SM/MB 00 -11 -25) – BN is hexagonal, called sometimes “white graphite”, hydrophobic – Cannot be removed by detergents – Film can peel-off S. Sgobba M. Taborelli Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 14
Solution: BN – Vacuum fire at 950 °C – Electropolish the surface (removes BN and becomes mirror like) – Second vacuum firing at 950 °C to remove H 2 introduced by electropolishing – Careful to use slow temperature ramp to avoid deformation of double wall Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 15
LHC couplers • 260 (? ) new LEP double wall tubes, very successful operation • LHC were made following LEP recipes • Total: 25 pieces (? ) Review of SPL RF couplers Sergio Calatroni - 16. 3. 2010 16
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