spie conference - photonics applications in industry and research, warsaw, 30 august, 2005 1 the rf...

SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005

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The RF Power Coupler Development Program at LAL-Orsay and DESY-Hamburg for TESLA and the

European X-FEL

T. GarveyLabratoire de l’Accélérateur Linéaire – Orsay,

W.D.-Möller,

DESY-Hamburg


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Function of the power coupler

• Transmit the radio-frequency waves from the power source (klystron) to the accelerating cavity.


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Additional coupler functions• Minimise forward loss of power from klystron to cavity.

• Isolate the evacuated cavity from the air filled wave-guide

• Protect the cavity against ‘contamination’.

• Minimise the thermal losses between the wave-guide (room temperature) and the cavity (2 K).

• Allow flexibility for thermal cycling of cryomodule.

• Allow variable “coupling” to the cavity.


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Coupler power requirements

TTF TESLA 500 / TESLA 800 X-FEL

Peak power208 kW 230 kW / 925 kW 200 kW

Repetition rate /

Pulse length

10 Hz

1.33 ms

5 Hz / 4 Hz

1.37 ms / 1.33 ms

10 Hz

1.33 ms

Average power 2.8 kW 1.6 kW / 4.9 kW 2.7 kW

Coupling (Qext)adjustable( 106 - 107 )

fixed3x106 / 2.5x106

adjustable( 106 - 107 )

Operating frequency: 1300 MHz


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The solution…. the TTF-III coupler


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Co-axial is easy for: - variable coupling - fabrication

- assemblytwo windows for: -clean assembly

-safe operationcold coax: -at 70 Ohm, 40 mm diameter

warm coax: -at 50 Ohm, 62 mm diameter

D.C. bias: -suppress multipactingflexibility: -bellows in the warm and cold

coaxceramics: -Al2O3 with TiN coating

copper plating: -20 μm-high thermal conductivity -high purity, hydrogen free

Design of the TTF-III Couplers


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Multipacting – resonant discharge when mixing vacuum and RFwith surfaces having SEE > 1.

Apply thin coating (~ 10 nm) of TiN to reduce SEEof Al2O3 ceramic surface.

Power levels scale as ~ Z.diameter)4


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Warm assemblyWarm ceramicprotection cap


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Cold assembly


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Wave-guide assembly

HV inputcoil


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RF Coupler in Module

module 70K shield 4K shield

cavitycold windowwarm window


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Coupling to the VUV-FEL LINAC Cavities

Power towardscavity

Coupler


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Coupler development

• RF design– Electromagnetic, multipactor, mechanical,

thermal.

• Fabrication

• Reception, preparation, assembly

• High power tests


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RF design of couplersExtensive use of electro-magnetic simulation codes-coupler well matched; VSWR < 1.2-- minimise E-fields in critical areas.


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TTF-III coupler production requires many technologies.

Copper coated bellows

E-beam welding

Ceramic-metal brazing

Laser weld of bellows

TiN coating


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Coupler reception, preparation and mounting

Inspection Cleaning with ultra-pure water

High temperature vacuum bake-out Assembly in Class 10 clean room


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The RF Power Source


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High power test configuration

Pair of couplers under test at Orsay.


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RF “conditioning” RF conditioning (or processing) is the procedure whereby thecoupler power is gradually increased from zero to its nominal operating level.

Corresponds to ‘controlled’ desorption of gases from RF surfaces ⃕ multipactingSurfaces “cleaned-up” as result of conditioning.Cold surfaces can re-collect gas following periods of operation.

Interlocks required to protect coupler during processing ⃗ monitor vacuum levels, electron activity, light signals reflected and transmitted power, window temperature.


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Diagnostics used as interlocks

Photo-multiplier

Vacuum port

Electron pick-ups


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Control and Data Acquisition


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Conditioning procedure

• Pulse length 20 s, ramp power to 1.0 MW in 0.2 dB steps• Pulse length 50 s, ramp power to 1.0 MW in 0.2 dB steps• Pulse length 100 s, ramp power to 1.0 MW in 0.2 dB steps• Pulse length 200 s, ramp power to 1.0 MW in 0.2 dB steps• Pulse length 400 s, ramp power to 1.0 MW in 0.2 dB steps• Pulse length 800 s, ramp power to 0.5 MW in 0.2 dB steps• Pulse length 1300 s, ramp power to 0.5 MW in 0.2 dB steps• Sweep power @ 1.3 ms, 50 to 500 kW.

• Interlocks• e- pick-ups, photo-multiplier, vacuum, ceramic temperature• Wave-guide sparks – stop operation!


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RF Coupler Performance @ DESY

Total of 40 RF power couplers connected to the superconducting cavities in the VUV FEL linac, eight per module.

All couplers in the VUV FEL linac have been processed and operated up to the limit of the cavity performance.

RF power couplers have been tested up to 1 MW of pulsed power at 1.3 ms pulse length, 2 Hz on the test stand.

RF power couplers operated ~270,000 coupler*hours.

TTF-III couplers have been tested together with cavities at gradients of 35 MV/m (600 kW) 5 Hz without degradation of cavity or coupler.


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Coupler processing data

RF power as a function of time for different pulse widths.

Electron activity as a function of RF power and pulse width.


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Conditioning times for couplers

Why are the conditioning times different ? - real differences between couplers ?-differences in preparation / installation ?-Answer is object of our present studies.


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R&D to reduce conditioning time

• Investigate new threshold levels for safe operation

• Apply titanium-nitride on all RF surfaces

• Use glow-discharge treatment of RF surfaces

• DC bias sweep to provoke multipactor

• Alternative anti-multipactor coatings


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Measures towards cheaper coupler production

Industrial study of coupler “mass” production for X-FEL.


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TTF-V Proto-type

• Essentially = TTF-III “warm” part + 62 mm “cold” part.• Larger diameter – to transmit more power and push multipactor levels to higher power• Candidate coupler for a two x 9 cell TESLA/ILC cavity• Engineering drawings and technical specifications completed.• Four such couplers will be built in 2005 for high power tests in early 2006.• Will be subsequently used for studies on RF conditioning times.


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TW60 Coupler Proto-type

Radically different from TTF-III coupler, uses “thin” planar ceramic windows.Easy to braze, low dielectric loss.Insensitive to multipactorWarm transition is matched with reduced-height wave-guide standing wave.“Line of sight to cavity”Cold window matched with reactive impedance elements on inner co-ax. Low power measurements already performed. High power tests planned for 2006 on four proto-types to be built by ACCEL.


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Summary

The TTF-III coupler performance is adequate for the X-FEL project

The TTF-III coupler may also be acceptable for ILC Alternative ILC designs are being studied

R&D is in progress to reduce processing times

Industrial studies will lead to reduced costs

Thank you for your attention

I thank all my DESY and Orsay colleagues for their contributions to this work

spie conference - photonics applications in industry and research, warsaw, 30 august, 2005 1 the rf...

Documents

cavity coupler slide

cold assembly slide

desyhamburg slide

rf coupler

ms average power

accelerating cavity

evacuated cavity

power source klystron