LEP and LHCdouble 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 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

RRR measurements

Review of SPL RF couplers Sergio Calatroni - 16.3.2010 10

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

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LHC couplers

• 260 (?) new LEP double wall tubes, very successful operation

• LHC were made following LEP recipes• Total: 25 pieces (?)

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