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 ECTReview 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 LEP2 Note 94-21)
Review of SPL RF couplers Sergio Calatroni - 16.3.2010 6
ESD measurements (500 eV e- after 200°C / 24h bakeout)
Review of SPL RF couplers Sergio Calatroni - 16.3.2010 7
Rolled+welded and Cu sputter coated
Full machining and Cu sputter coated
Copper ESD measurements
Review of SPL RF couplers Sergio Calatroni - 16.3.2010 8
Cu sputter coated
Cu electroplated
Copper TDS measurements
Review of SPL RF couplers Sergio Calatroni - 16.3.2010 9
Cu sputter coated Cu electroplated
Thickness at 700 MHz: effect of RRR
Review of SPL RF couplers Sergio Calatroni - 16.3.2010 11
RRR 10 RRR 1
Copper thickness: room temperature, 700 MHz
Review of SPL RF couplers Sergio Calatroni - 16.3.2010 12
Ideal thickness = 4 m
Copper coating by sputtering
Review of SPL RF couplers Sergio Calatroni - 16.3.2010 13
Adhesion of Cu on s.steel is poor because it does not bind to CrO
Idea: Ti underlayer, followed by Cu coating without breaking the vacuum. Metal-metal adhesion generally very good
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 (MnS 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
Review of SPL RF couplers Sergio Calatroni - 16.3.2010 14
S. SgobbaM. Taborelli
Solution: BN
– Vacuum fire at 950 °C– Electropolish the surface (removes BN and becomes mirror
like)– Second vacuum firing at 950 °C to remove H2 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
Top Related