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

2. TaCo - Tuner adjustable waveguide Copler Rolf Wegner
outline
why adjustable couplers for standing wave cavities
principles of couplers and their adjustment ‘nozzle’ coupler, RLWG coupler, T-type coupler, TaCo
TaCos for Linac4/SPL
TaCo prototype
Advantages and potential applications of TaCo
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

3. Unpredictable Q-value variations
tuning elements
dQ/Q ~ 1..2%
surface roughness
dQ/Q ~ 3..5%
EB welding
shrinkage 0.2 to 0.3 mm dQ/Q ~ 1%
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

4. Example: surface roughness
Example 350 MHz cavities:
Ra demanded 0.8 µm
δs ~ 3.5 µm
ρ/δs ~ Ra/δs = 0.227
add. Losses ~ 7.5%
machining:
Ra ~ 0.7 µm
Ra/δs = 0.199
add. Losses ~ 5%
=> unpredictable dQ ~ 2.5%
Ralph Lending: "New criteria for microwave component surface".
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

5. Why adjustable couplers
Critical coupling of cavities (with beam) to minimise reflected power
coupling factor
(critical coupling: βd = 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)
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

6. TaCo - Tuner adjustable waveguide Copler Rolf Wegner
‘nozzle’ coupler
‘nozzle’ coupler
adaptation
Δβ / β0
 courtesy of Jim Stovall & Craig Deibele
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

7. ‘nozzle’ coupler, equiv. circuit
f0: resonant frequency of cavity
f: frequency of drive
Q0: cavity Q-value, B: cavity bandwidth
ZL: waveguide impedance (f)
β0: coupling factor
adjustment of coupling factor:
mechanical adaptation
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

8. ‘nozzle’ coupler, remarks
1. Cavity operated at resonance
2. Coupling factor can (only) be increased
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

9. ‘nozzle’ coupler, types
Iris shape: circular, elliptical, racetrack or rectangular
Waveguide shape: straight (not tapered)
LAMPF SCL, Fermilab SCS, J-PARC ACS, SLAC LCLS RF-gun
tapered (one or both directions)
SNS SCS, LANL OCS
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

10. Ridged-Loaded Wave-Guide (RLWG) coupler
‘dog-bone’ iris
+ small opening, small perturbation, small frequency shift
- sensitivity, difficulties seen: multipacting, arcing, overheating
used for nearly all RFQs and for cavities requiring small freq. shift
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

11. TaCo - Tuner adjustable waveguide Copler Rolf Wegner
T-type coupler
used for RF guns, proton accelerators
adjustment of coupling factor: LS
principle: compensation of reactance by stub line
2 knobs ?
amplitude and phase?
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

12. T-type coupler, equiv. circuit
coupling factor:
1. Cavity (slightly) detuned
2. Coupling factor can (only) be decreased
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

13. T-type coupler, adaptation
1. movable short circuit
=> LS
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

14. T-type coupler, adaptation
2. machining of appropriate short circuit (vacuum)
(+ copper plating)
==> delay
4. installation
(helicoflex joint, bolts M12)
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

15. TaCo - Tuner-adjustable w.g. Coupler
plunger tuner
LS fixed
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

16. TaCo - Tuner adjustable waveguide Copler Rolf Wegner
TaCo, remarks
1. Cavity (slightly) detuned to compensate stub line reactance
2. Coupling factor can be decreased as well as increased
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

17. TaCo - Tuner adjustable waveguide Copler Rolf Wegner
TaCo for Linac4/SPL
xt = 350 mm
zt = 0 mm
LS = 311 mm
øt = 83 mm
RRt = 10 mm
vacuum section
Pin=1 MW, cavity: ES~33 MV/m
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

18. TaCo, increase of coupling factor
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

19. TaCo - Tuner adjustable waveguide Copler Rolf Wegner
TaCo prototype
xt = 292 mm
zt = -345 mm
LS = 420 mm
øt = 43 mm
RRt = 2.0 mm
Lt = 113 mm
Pin=1 MW
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

20. TaCo - Tuner adjustable waveguide Copler Rolf Wegner
TaCo prototype
high power test:
Pin= 1.1 MW,
Es,m= 8 MV/m
TP= 0.8 ms,
frep= 2 Hz,
ttest= 72 h
no breakdowns
no multipactor
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

21. TaCo - Tuner adjustable waveguide Copler Rolf Wegner
Advantages of TaCo
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
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner

22. TaCo’s (potential) applications
Linac4 / SPL 26 TaCos (20 x decrease β, 5 x increase β, 1 x variable β)
Replacement of nearly all T-type couplers (! Emax) (TERA / ADAM side coupled structures, RF guns, ESS Bilbao)
Replacement of ‘nozzle’ couplers 1. no waveguide taper => direct replacement 2. waveguide taper => no taper + bigger coupling slot (e.g. bridge couplers of π/2 mode structures [=many structures]) 3. waveguide taper => plunger tuner placed in taper
TaCo principle could also be applied to RLWG couplers
19-Oct-2011
TaCo - Tuner adjustable waveguide Copler Rolf Wegner