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
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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Function of the power coupler
• Transmit the radio-frequency waves from the power source (klystron) to the accelerating cavity.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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The solution…. the TTF-III coupler
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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Warm assemblyWarm ceramicprotection cap
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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Cold assembly
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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Wave-guide assembly
HV inputcoil
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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RF Coupler in Module
module 70K shield 4K shield
cavitycold windowwarm window
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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Coupling to the VUV-FEL LINAC Cavities
Power towardscavity
Coupler
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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Coupler development
• RF design– Electromagnetic, multipactor, mechanical,
thermal.
• Fabrication
• Reception, preparation, assembly
• High power tests
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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The RF Power Source
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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High power test configuration
Pair of couplers under test at Orsay.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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Diagnostics used as interlocks
Photo-multiplier
Vacuum port
Electron pick-ups
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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Control and Data Acquisition
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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!
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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Measures towards cheaper coupler production
Industrial study of coupler “mass” production for X-FEL.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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.
SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005
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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