Titanium Nitride Coating by Reactive DC Magnetron Sputtering as a Multipactor Suppressor on Coupler

RF Ceramic Windows

Walid KAABI

Anti e- Cloud Coatings- AEC’09

Laboratoire de l’Accélérateur Linéaire

CERN- Octobre 12th 2009 2

Power coupler for superconductive cavities

Power coupler have 3 functions to fulfil: Electromagnetic role Vacuum barrier Thermal interface

Principle and Functions

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Power coupler for superconductive cavities

Coupler Design: example of TTF-III Model

RF Source

Cavities

Cryogenic module

TTF-III is the baseline solution for the future XFEL linac in DESY. In this framework, our laboratory is responsible of the RF conditioning task of 800 TTF-III couplers.

CERN- Octobre 12th 2009 4

Power coupler for superconductive cavities

Coupler Ceramic WindowsCold windows

(Ø = 47 mm, h= 48 mm)

Warm windows (Ø = 75 mm, h= 57 mm)

Windows made of Alumina ceramic (97.6% Al2O3):

Mechanical and dielectric strength

High thermal stability

Low out-gazing rate

High Secondary Electron Emission Yield (SEY)

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Multipactor phenomenon

RF Field +

Best conditions for Multipactor phenomenon occurring:

Electron-avalanche discharge

+ High vacuum

Conditions of occurring

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Multipactor phenomenon

Multipactor phenomenon may cause damages in ceramic windows:

Creation of damaging arcs,

Surface overheating that may lead to surface evaporation,

Load mismatch causing dangerous power reflection to the RF source.

Necessity of suppression, or at least limitation of this effect

Solution: decrease Alumina’s SEY by surface coating with multipactor suppressor thin layer: Titanium Nitride

Damages caused in ceramic windows

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Multipactor suppressor thin film

TiN is characterised by a low SEY that remain stable on RF operational conditions

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Multipactor suppressor thin film

Ionisation gas inlet (Ar)

Reactive gas inlet (N2)

Magnetron

Substrate

Ar +

- Target

Pumping

Vaccum chamber

Elec

tric

al

pola

risat

ion N2

DC Reactive Magnetron Sputtering of TiN

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TiN sputtering system

Titanium target

Rotating magnet pack

Shield

Rotating magnet shape

=Plasma shape Rotating sample holder

Sputtering machine overview

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TiN sputtering system

Sample holder motion

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Practical guidances

Ceramic window exposure to RF field induce some additional constraints:

Stoichiometry control: the layer can get the multipactor suppressor property (XRD analyses ),

Thickness control: deposit should be thick enough to reduce multipactor, but not so much to prevent increasing RF power reflection in ceramic surface (X-Ray reflectivity Analyses, in addition to Quartz Crystal Microbalance monitoring).

Deposits made on 10x10mm quartz substrates,

Use of multi-sample holder.