Legnaro Variola

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High Power Couplers and… pushing the limits of SC cavities Alessandro Variola Laboratoire de l’Accélérateur Linéaire, IN2P3 -CNRS and the Université de Paris-Sud 11, Centre Scientifique d’Orsay, B.P. 34 F-91898 Orsay Cedex France Variola@lal.in2p3.fr

What is the connection between high power couplers and thin films and pushing the limits of SC cavities ? I will try to persuade you that the two fields are close…. : 

What is the connection between high power couplers and thin films and pushing the limits of SC cavities ? I will try to persuade you that the two fields are close….

What is a coupler? Its functions:: 

What is a coupler? Its functions: Transferring RF power from the source (klystron) to the accelerating structure Matching the “external world” with the “cavity world” as far as the e.m field, the thermal and the vacuum transition are concerned

How does it looks like?: 

Example: TTFIII Pulsed Cylindrical window How does it looks like?


Scenario As far as SC cavities are concerned power couplers are not only a ‘coupling device’. Due to the strong cavity constraints we have to consider the coupler, or at least the cold part, as a integrated part of the cavity itself. So to assure the cavity to reach its “limits” a considerable attention must be put on the coupler and its integration


Pushing the cavities limits: High power (Pulsed or CW) beam current - accelerating gradients. As far as the coupler is concerned,to reach this goal the design phase is crucial. A multidisciplinary approach must be followed taking into account e.m, thermal and material science aspects i.e. - impedance matching design, Qext flexibility (105-107), mechanical adjustements (bellows), thermal transitions (300-70-2 K) & heating, materials choices,dynamic and static heat load, cool down stresses (300 – 2 K), cavity performances (cleanliness), integration, vacuum requirements beam dynamics effects (p mm mrad), windows coating………and COST $$$$$$$$ => CONDITIONING TIME……… Often increasing in one performance affects other aspects. The coupler design is a multi-parameter optimisation process . HIGH POWER & COUPLERS Before production……

Design : Two Basic Options: 

Design : Two Basic Options Coaxial (SC cavities) Aperture & sizes (integration) Thermal heat leak Matching & Flexibility Waveguide (RF-GUNS, Sources, Injectors) Simplicity of the design Lower Surface E-field Cooling system Pumping efficiency No transition WG / Coax

To reach higher and higher power performances we have to take into account the possible limiting factors:: 

To reach higher and higher power performances we have to take into account the possible limiting factors: 1 ) MULTIPACTOR (resonant electron cascade in HF) -ceramic window (coating) -diameter -impedance -fields -frequency -Preparation (cleanliness) 2) Matching to the cavity => Q ext Frequency shift Transients Transition (Doorknob) Feedthrough isolation RF contact 3) THERMAL - Loads Materials (copper plating) Heat sinks


Reduce the dynamic and static losses (optimisation) -Choice of the Material -Cu plating thickness -Windows (again) -Hot spots (ceramics, bellows (again)….) -Flanges and gaskets -Simulations Thermal design (extremely important in CW)

Production Thins films : ceramic window coating: 

Production Thins films : ceramic window coating Alumina windows & TiN coating Alumina windows = critical component Thermal, Vacuum, Multipacting (High SEY) High purity alumina TiN coating (evaporation, sputtering)

Ti – TiN Coatings. Windows and couplers parts: 

Ti – TiN Coatings. Windows and couplers parts Lower the SEY reducing the multipactor activity. Constraints : loss tangent, reflectivity => Thickness ~ 10 nm (DESY) Windows and coaxial parts Evaporation / Sputtering Examples : Evaporation [DESY]. Ti vapour in low pressure ammonia (99,98%). Pre vacuum 5 10-6mbar. Titanium wire @1000o =>Ceramic pre-heating ~150o.Sublimation T ~ 1500o, than nitrification with ammonia pressure up to 300 mbar x 10h. Sputtering [KEK – multipactor on couplers, SNS – e cloud in damping rings] on SS chambers. KEK (couplers coaxial parts) : DC sputtering, Pre vacuum ~10-6, gas Ar:N2=1:2 (0.24 mbar). Sputtering has also been used in KEK to coat alumina windows but very few information about (next talk) SNS (vacuum chambers) :DC sputtering, Pre vacuum 5 10-7 (0.1T), Pre sputtering in Argon, Ar:N2, chamber T ~ 250o Other Coatings : Can NEG be a solution? Controversial exp. Needs a clear answer especially for the sticking under high power HF

After Production VERY IMPORTANT => Integration in a SC cavity : the coupler does not have to hinder the SC cavity performance: 

After Production VERY IMPORTANT => Integration in a SC cavity : the coupler does not have to hinder the SC cavity performance Vacuum furnace Class 10 clean room Ultra-pure water production A detailed and complex process: -Reception -Cleaning Rinsing Drying Diagnostics Baking Assembling Mounting Conditioning Vacuum: No hydrocarbons, no contaminants, Mounting on laminar flow, in-situ baking with silicon Strips/UV lamps/baking jacket.

An example : the TTF3 procedure: 

An example : the TTF3 procedure Reception (endoscope analysis). Data-base for mechanical defects Ultrasound bath with detergent (5 l per 160 l of water), 25 or 40 Hz frequency, 4000 W transmitter power (ultrasonic power), 15 minutes in the bath with ultrasonic power, heating of water at 50 °C Cleaning – ultra-pure water - 18 MOhm cm – ion, molecules and durapore (particles with 0.22 microns diam.) filter - with resistivity check – ionized nitrogen drying (3bar) with particle counting check Pre baking in vacuum oven (10-6@150 degrees) Assembling Baking on the conditioning test bench RF for conditioning Storing warm part under nitrogen and cold parts under vacuum

Results :: 

Results : Thanks to W.D Moeller & D.Kostin

Pushing the limits…: 

Pushing the limits… What are the possible improvements? - Design => Multipactor and thermal Brazing and welding techniques (reliability). Diagnostics and surface analysis. What we can learn from thin film community? Cleaning : what we can transfer from cavity experience, Argon discharge Design : bellows, coupling (capacitive) Materials : alumina windows, plating Antenna bias Coating : All deposited couplers (TiN, NEG…??). Windows coating techniques

$ Mass Production. Industrial studies $: 

$ Mass Production. Industrial studies $ TTFIII XFEL industrialisation For 1000 couplers. PROCESS : Coupler design re-analysis in term of -performance -material -tolerances -assembly (brazes…) -diagnostics -interfaces Taking into account : Mechanical, Vacuum, Thermal, Electrical, Geometry, Assembly and Cleanliness constraints It is necessary to share with Industries the expertise on : Welding, Brazing. Cu Plating, TiN coating&Surface Analysis, Handling&Clean Room Procedures, Conditionning


KEK – Capacitive coupling 1 cylindrical – 1 planar Cornell ERL – Modified TTFIII for CW mode Other examples


TEST - Coupler (NEG, Surface analysis etc) Adhesion (RF sticking) Oxidation Quality of coating Couplers are very expensive devices : It is important to have some reliable, not expensive and accessible model to test different solutions and to have surface quality measurements


Conclusions The power coupler is a integrate part of the SC cavity….. …so pushing the limit of cavities means to develop in parallel the appropriate coupler. Improvements : Design (e.m. multipactor and thermal - bellows), preparation and integration, materials (alumina, conductors). Thin Films can play a determinant role also for power couplers. Coatings are determinant : Ti / TiN on windows and coaxial parts. Cleanliness !!! It is important to dispose of “test couplers” in which different materials and techniques (thin films…) could be tested (also if expensive…). Costs must be reduced


Remember: For ILC power couplers are cost divers as the cavities If thin films for SC cavities will work cavities will cost less…….. At this point it will be mandatory to work on thin films for couplers !!!!!!!


ACKNOWLEDGEMENTS I would like to acknowledge the conference board for inviting me Special thanks to all the colleagues providing me suggestions and information.

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