DRAFT summary. rev Review of SPL RF power couplers

1. DRAFT summary. revReview of SPL RF power couplers 16-17 March 2010 CERN W. Weingarten Review of SPL RF power couplers

2. Proposed choice for power coupler (PC) in view of basic fault events (1/8) Caveats in RED • RF window • Excessive mechanical stress (differential contraction) • RF window located unfavourably in presence of standing wave (high E/H field) • Poor cooling of ceramics • Ceramics with high dielectric losses ----------------------------------------------------------------------------------------- • The one cylindrical air-cooled window solution was not objected provided that all measures to avoid contamination of the cavity are taken during assembly. • Reliable operation for SPS at 550 kW CW in favor of one window solution • Spare components must be available - leak tightening and in situ repair forbidden • Not enough time to develop alternatives due to tight planning • Diagnostics equipment mandatory • Window position dictated by cryo-module design, not from RF considerations (DESY: window should be located in low E-field region) • Window (power coupler) to be conditioned and then stored in coupling box Review of SPL RF power couplers

3. Proposed choice for power coupler in view of basic fault events (2/8) Caveats in RED • Coaxial line and materials • Error in coating parameters /thickness wrong • Inadequate cleaning of substrate ----------------------------------------------------------------------------------------- • Convincing computations of cooling conditions were presented and experimentally validated (measurements with LHC in operation): 0.1 W instead of 21 W heat flow to lHe @ 2 K; g He flow 35 mg/s; but keep safety margin • Convincing arguments and recipes for materials choice and treatment were also presented (lesson learnt from “LEP power coupler crisis”): Cu sputter coating, RRR <30, 4 μm thickness, poor adhesion of Cu to 316LN, BN problem, ESD, Ti under-layer, EP, re-firing, … • No water at welds or brazings near vacuum (leak detection and repair impossible) • Diagnostics mandatory (in particular penning gauges, to be offside from any electron dark current) • Bake at 400 °C to check for blisters in Cu plating Review of SPL RF power couplers

4. Proposed choice for power coupler in view of basic fault events (3/8) Caveats in RED • Discharges, arcing, multipactor • Wrong geometry favouring resonant condition • Too high secondary coefficient /wrong material • Evaporation by RF power of particulate or gas deposits ----------------------------------------------------------------------------------------- • Microwave Studio has a new feature of particle tracking, but ad-hoc written codes are more flexible • Multipactor studies show that for 50 Ω only outer conductor is affected (for one point multipactor), but 75 Ω instead of 50 Ω more favorable for avoiding it • Conical line not worse than straight line, however some activity in the corner • DC voltage bias useful for long term operation to be provided Review of SPL RF power couplers

5. Proposed choice for power coupler in view of basic fault events (4/8) Caveats in RED • Cryogenic issues • Cooling fluid flow too high/low/non-turbulent • Wrong choice of cooling fluid (gas/liquid) ----------------------------------------------------------------------------------------- • We have 200 W (average) for whole cryo-module => 25 W per cavity • Cryogenic loss considerations on coupler double wall outer conductor are important • They can be reduced to negligible values – however at the expense of a outlet temperature below 273 K => isolation or heating needed Review of SPL RF power couplers

6. Proposed choice for power coupler in view of basic fault events (5/8) Caveats in RED • RF power issues and RF joints • Faulty design of joint or wrong choice of material • Joint located unfavourably in presence of standing wave (high E/H field) ----------------------------------------------------------------------------------------- • Fix power coupler is suitable • JLAB has very detailed specification for power coupler (Qext, vacuum pressure, VSWR, …) • Field flatness measurement and field flatness tuning on cavity is important for the power coupler to comply with specification Review of SPL RF power couplers

7. Proposed choice for power coupler in view of basic fault events (6/8) Caveats in RED • Clean assembly • Non dust-free assembly of PC /insufficient on-line monitoring of dust content in air ----------------------------------------------------------------------------------------- • There was a clear preference of horizontal assembly, vertical final position, but not so clear preference of whether up or down (should be decided from practicability considerations) • Concerning the assembly into the cryo-module, preference to the “pipeline solution” with the PC as supporting element => the distance to window determines the diameter of the cryostat; cavity position depends on temperature of coupler double wall • We heard very detailed recipes with precise quality assurance constraints from practice (DESY) that are already followed up in the procedures as planned at CERN (avoid silver coated bolts, …); DESY and JLAB mentioned strict Coupler testing and production procedures • We learnt about “cleanliness” strategies: nitrogen barriers, spray gun cleaning, on-line particle counting close to activity, correct choice of bolts, transportation bags, coupler after conditioning to be kept under dry protective gas/vacuum; all RF exposed surface to be kept under dry nitrogen • Warm parts not connected with the cavity vacuum shall be kept dust-free as well (helps to shorten conditioning times, but only valid when two RF windows are chosen) • Provide adequate tooling for all assembly actions Review of SPL RF power couplers

8. Proposed choice for power coupler in view of basic fault events (7/8) Caveats in RED • Excessive mechanical stresses • Excessive temperature (poor electrical conductivity) • Choice of thermally incompatible materials (differential contraction) ----------------------------------------------------------------------------------------- • Cantilever suspension of cavity is not possible due to excessive deformation of double wall => cavity needs to be supported at one end • The electrical axis of the cavity determines the adjustment constraints of the power coupler with regard to the cryostat vacuum vessel => adjustment elements and bellows needed between cryostat and coupler double wall; improves also acoustic decoupling of power coupler from ambient • Beware of waveguides being handles • Measures are mandatory to keep position of couplers during handling (< 0.5 g) Review of SPL RF power couplers

9. Proposed choice for power coupler in view of basic fault events (8/8) Caveats in RED • Beam stability issues • Beam breakup due to periodic field perturbation by power coupler ----------------------------------------------------------------------------------------- • Were not assessed/discussed ------------------------------------------------------------------------------------------------- • Additional items/hazards for manufacture and operation that were not mentioned • SPL Coupler manufacture is on critical path • Availability of test stands outside CERN must be checked • Gas barrier for waveguides are recommended • Multipactor in waveguides should be looked at Review of SPL RF power couplers

10. Acknowledgement We got useful, important and practical feedback on the technical choices for the SPL power coupler as presented so far. Our thanks go to all contributors/round table actors and especially to the guests from external labs including those from the US. We wish all of you a safe trip home. Review of SPL RF power couplers