1. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 1 The RF Power Coupler Development Program at LAL-Orsay and DESY-Hamburg for TESLA and the European X-FEL T. Garvey Labratoire de l’Accélérateur Linéaire – Orsay, W.D.-Möller, DESY-Hamburg

2. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 2 Function of the power coupler Transmit the radio-frequency waves from the power source (klystron) to the accelerating cavity.

3. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 3 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.

4. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 4 Coupler power requirements TTFTESLA 500 / TESLA 800X-FEL Peak power 208 kW230 kW / 925 kW200 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 kW2.7 kW Coupling (Qext) adjustable ( 10 6 - 10 7 ) fixed 3x10 6 / 2.5x10 6 adjustable ( 10 6 - 10 7 ) Operating frequency: 1300 MHz

5. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 5 The solution…. the TTF-III coupler

6. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 6 Co-axial is easy for: - variable coupling - fabrication - assembly two windows for: -clean assembly -safe operation cold coax: -at 70 Ohm, 40 mm diameter warm coax: -at 50 Ohm, 62 mm diameter D.C. bias:-suppress multipacting flexibility: -bellows in the warm and cold coax ceramics: -Al 2 O 3 with TiN coating copper plating: -20 μm -high thermal conductivity -high purity, hydrogen free Design of the TTF-III Couplers

7. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 7 Multipacting – resonant discharge when mixing vacuum and RF with surfaces having SEE > 1. Apply thin coating (~ 10 nm) of TiN to reduce SEE of Al 2 O 3 ceramic surface. Power levels scale as ~ Z.  diameter) 4

8. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 8 Warm assembly Warm ceramic protection cap

9. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 9 Cold assembly

10. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 10 Wave-guide assembly HV input coil

11. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 11 RF Coupler in Module module 70K shield 4K shield cavity cold windowwarm window

12. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 12 Coupling to the VUV-FEL LINAC Cavities Power towards cavity Coupler

13. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 13 Coupler development RF design –Electromagnetic, multipactor, mechanical, thermal. Fabrication Reception, preparation, assembly High power tests

14. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 14 RF design of couplers Extensive use of electro-magnetic simulation codes -coupler well matched; VSWR < 1.2 -- minimise E-fields in critical areas.

15. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 15 TTF-III coupler production requires many technologies. Copper coated bellows E-beam welding Ceramic-metal brazing Laser weld of bellows TiN coating

16. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 16 Coupler reception, preparation and mounting Inspection Cleaning with ultra-pure water High temperature vacuum bake-out Assembly in Class 10 clean room

17. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 17 The RF Power Source

18. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 18 High power test configuration Pair of couplers under test at Orsay.

19. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 19 RF “conditioning” RF conditioning (or processing) is the procedure whereby the coupler power is gradually increased from zero to its nominal operating level. Corresponds to ‘controlled’ desorption of gases from RF surfaces ⃕ multipacting Surfaces “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.

20. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 20 Diagnostics used as interlocks Photo-multiplier Vacuum port Electron pick-ups

21. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 21 Control and Data Acquisition

22. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 22 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!

23. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 23 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.

24. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 24 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.

25. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 25 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.

26. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 26 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

27. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 27 Measures towards cheaper coupler production Industrial study of coupler “mass” production for X-FEL.

28. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 28 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.

29. SPIE Conference - Photonics Applications in Industry and Research, Warsaw, 30 August, 2005 29 TW60 Coupler Proto-type Radically different from TTF-III coupler, uses “thin” planar ceramic windows. Easy to braze, low dielectric loss. Insensitive to multipactor Warm 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.

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