high power couplers and… pushing the limits of sc cavities

04/22/23 Alessandro Variola, LAL-ORSAY, IN2P3-CNRS 2006 Thin Films Workshop. Legnaro, Italy 1

High Power Couplers and… pushing the limits of SC

cavitiesAlessandro VariolaLaboratoire de l’Accélérateur Linéaire, IN2P3 -CNRS and the Université de Paris-Sud 11, Centre Scientifique d’Orsay, B.P. 34 F-91898 Orsay Cedex [email protected]


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:

• 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

•


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 ( 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 & COUPLERSBefore production……


Design : Two Basic Options• Coaxial (SC cavities)

Aperture & sizes (integration)Thermal heat leak

Matching & Flexibility

• Waveguide (RF-GUNS, Sources, Injectors)

Simplicity of the designLower Surface E-fieldCooling systemPumping efficiencyNo transition WG / Coax


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)


ProductionThins films : ceramic window coating

Alumina windows & TiN coating

Alumina windows = critical component

Thermal, Vacuum, Multipacting (High SEY)

1) High purity alumina2) TiN coating (evaporation,

sputtering)


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

Vacuum furnace

Class 10 clean roomUltra-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• 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 :

Thanks to W.D Moeller & D.Kostin


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 $

TTFIII XFEL industrialisationFor 1000 couplers.

PROCESS : Coupler design re-analysis in term of-performance-material-tolerances-assembly (brazes…)-diagnostics-interfacesTaking into account : Mechanical, Vacuum, Thermal, Electrical, Geometry, Assembly and Cleanliness constraints

It is necessary to share withIndustries the expertise on :Welding, Brazing. Cu Plating, TiN coating&Surface Analysis, Handling&Clean Room Procedures, Conditionning


KEK – Capacitive coupling1 cylindrical – 1 planar

Cornell ERL – Modified TTFIIIfor 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 testdifferent solutions and to have surfacequality 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.

high power couplers and… pushing the limits of sc cavities

Documents

rf power

coupler design

cavities limits

cavity world

higher power performances

impedance matching design

vacuum transition

strong cavity constraints