1. 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 Orsay Cedex France
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

2. 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….
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

3. 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
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

4. How does it looks like? Example: TTFIII Pulsed Cylindrical window
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

5. 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
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

6. HIGH POWER & COUPLERS Before production……
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 ( ), mechanical adjustements (bellows), thermal transitions ( 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 .
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

7. 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
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

8. 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
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

9. Reduce the dynamic and static losses (optimisation)
Thermal design (extremely important in CW)
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
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

10. 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)
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

11. 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 mbar. Titanium =>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 (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
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

12. After Production VERY IMPORTANT => Integration in a SC cavity : the coupler does not have to hinder the SC cavity performance
Class 10 clean room
Ultra-pure water production
A detailed and complex process:
-Reception
-Cleaning
Rinsing
Drying
Diagnostics
Baking
Assembling
Mounting
Conditioning
Vacuum furnace
Vacuum: No hydrocarbons, no contaminants, Mounting on laminar flow, in-situ baking with silicon Strips/UV lamps/baking jacket.
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

13. 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, 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 degrees)
Assembling
Baking on the conditioning test bench
RF for conditioning
Storing warm part under nitrogen and cold parts under vacuum
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

14. Results : 15/11/2018 Thanks to W.D Moeller & D.Kostin
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

15. 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
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

16. $ 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
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

17. Other examples KEK – Capacitive coupling Cornell ERL – Modified TTFIII
1 cylindrical – 1 planar
Cornell ERL – Modified TTFIII
for CW mode
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

18. TEST - Coupler (NEG, Surface analysis etc)
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
Adhesion (RF sticking)
Oxidation
Quality of coating
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

19. 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
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

20. 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 !!!!!!!
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

21. ACKNOWLEDGEMENTS
I would like to acknowledge the conference board for inviting me Special thanks to all the colleagues providing me suggestions and information.
Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018

22. 15/11/2018 Alessandro Variola, LAL-ORSAY, IN2P3-CNRS
2006 Thin Films Workshop. Legnaro, Italy
15/11/2018