1. TaCo Tuner-adjustable waveguide Coupler A new generation of waveguide couplers for standing wave cavities Publication: CERN-ATS-Note-2011-085 Frank Gerigk, Jean-Michel Giguet, Rolf Wegner, Patricia Ugena

2. why adjustable couplers for standing wave cavities principles of couplers and their adjustment ‘nozzle’ coupler, T-type coupler, TaCo TaCos for Linac4/SPL TaCo prototype Advantages and potential applications of TaCo 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 2

3. 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 3 EB welding shrinkage 0.2 to 0.3 mm dQ/Q ~ 1% tuning elements dQ/Q ~ 1..2% surface roughness dQ/Q ~ 3..5%

4. Critical coupling of cavities (with beam) to minimise reflected power 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 4 coupling factor (critical coupling: β d = 1) small adaptation of coupling factor desired to compensate for unpredictable variation => safety margin (often adjustment in one direction only; proton Linacs: cavities often have different dimensions)

5. 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 5 ‘nozzle’ coupler adaptation Δβ / β 0 courtesy of Jim Stovall & Craig Deibele

6. 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 6 adjustment of coupling factor: L S principle: compensation of reactance by stub line

7. 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 7 1 st Step: movable short circuit L S 1 st Step: movable short circuit L S

8. 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 8 2. machining of appropriate short circuit (vacuum) (+ copper plating)  delay 3. installation (helicoflex joint, 30 bolts M12)

9. 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 9 plunger tuner 1. Cavity (slightly) detuned to compensate stub line reactance 2. Coupling factor can be decreased as well as increased

10. 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 10 x t = 350 mm z t = 0 mm L S = 311 mm ø t = 83 mm R Rt = 10 mm vacuum section P in =1 MW, cavity: E S ~33 MV/m

11. 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 11 increase of coupling factor

12. 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 12 x t = 292 mm z t = -345 mm L S = 420 mm ø t = 43 mm R Rt = 2.0 mm L t = 113 mm P in =1 MW

13. TaCo // Tuner-adjustable waveguide Coupler TP= 0.8 ms, frep= 2 Hz, ttest= 72 h high power test: P in = 1.1 MW, E s,m = 8 MV/m 09/12/1113 no breakdowns no multipactor no breakdowns no multipactor

14. Coupling factor can be increased as well as decreased => cavity design for desired coupling => safety margins included in coupler design TaCo machined in one process (T-type: short circuit machined after RF measurements -> delay) RF matching simple and quick (final RF contact between coupler and cavity) Change of beam current => exchange of plunger tuner Movable tuner => variable coupling during operation TaCo applications: Linac4 / SPL 26 TaCos (20 x decrease β, 5 x increase β, 1 x variable β) 09/12/11 TaCo // Tuner-adjustable waveguide Coupler 14