Tapered coaxial coupler
Yoshiyuki Morita
Crab Cavity
KEKB

Design Changes
Previous design
Straight coaxial coupler
HOM/LOM damping of the cavity : OKMode damping in the coupler: insufficient
Dangerous parasitic modes in the coupler.
The most dangerous mode is 1/4λ mode which has high Q-factor.

Current design
Three modifications appliedzTapered coaxial coupler
zTip cutting design
zTE-mode stop band splitting of notch filter
Large HOM damper

HOM damping scheme
Monopole modes (including LOM) can
propagate along the coaxial coupler.
Dipole modes above 600 MHz can propagate.
Crab mode (509 MHz) can not propagate.
Attenuation factor of this mode : 60 dB/m
If the coupler deflects, crab mode canpropagate as TEM mode in the coupler.
Notch filter push this mode back to the cavity.

Parasitic mode in the coaxial coupler
Tip cutting

TE-1/4λ modef = 595 MHz
R/Q = 5.3 Ω
Q = 106¼ wave length

Coaxial coupler has parasitic modes.
The most dangerous mode is the TE-1/4λ mode.
High R/Q (5.3Ω) and lower f than cut-off.
Can not propagate to HOM damper.
This mode remains with high Q factor and gives high impedance.
Three modifications were applied to extract this mode.
1. Tapered coupler design for lower cut-off.
2. Tip cutting design for raising f of TE-1/4λ mode.
3. TE stop band splitting of notch filter.

TaperedStop band splittingTapered coaxial coupler design
Coaxial coupler has several tapered sections.
Decrease cut-off frequency (fc) for TE-mode propagation.

R94/60fc=620 MHz

R94/65
fc=600 MHz

R94/74
fc=568 MHz

Stub support

R106/74
fc=530 MHz

Notch filter

R120/90
fc=454 MHz

HOM damper

Tip cutting design

To raise the frequency of TE-1/4λ mode, outer
diameter of the coupler is adjusted to 120 mm,
which gives the cut-off of 620 MHz.
120 mm
Tip cutting design makes TE-1/4λ mode above
600 MHz and lower R/Q.
200 mmEffect of tip cutting design∆f, ∆f/∆L, Qext and R/Q
Frequency shift of the crab mode
Tuning parameter ∆f/∆L
External Q factor of the TM110 mode (LOM)
R/Q of TE-1/4λ mode
The coupler has to be inserted 5 mm
to compensate frequency shift of crab mode.

∆f(crab)No cutting
Tip cutting-145kHz∆f/∆L

Qext(LOM)

R/Q(TE-1/4l)

30 kHz/mm

695.2 Ω

30 kHz/mm

58

2.1 Ω

Effect of tip cutting design
Electro-magnetic property
Peak fields at the tip of coupler were calculatedusing MAFIA.
Tip cutting design has lower peak electric field,
although the tip was inserted 5mm.

Electric field

Magnetic field

Peak electric and magnetic fields.
Operating kick voltage: 1.44 MV
Tip cutting

5.5 MV/m

420 G

Tip cutting+10mm insertion

6.5 MV/m

447 G

No cutting

6.7 MV/m420 G

Stop band splitting design of the notch filterPrevious design has a stop band at 630 MHz forTE mode. This band is close to the TE-1/4λmode.
Stop band splitting notch filter has partitions in
mid-plane to separate stop bands for horizontally
polarized TE mode (650 MHz) and verticallypolarized one (570 MHz).

Partitions
Stop band(H-V splitting)
TEM

509 MHzTE(H)650 MHzTE(V)570 MHz

Frequency (MHz)Frequency (MHz)HOM damper
S
HOM damper
Ferrite RF absorber
Size: 240φ x 120 mm
Exactly the same size of LBP damper
200Q

150

Position of the damper was optimized togive minimum Q factor of LOM.

10050080

100120140s (mm)160180200

Q factor of the TM110 mode (LOM) as a function ofdamper position.

Comparison of TE-1/4λ mode damping

Straight type + previous notch filter

Tapered type + stop band splitting notch

Tip cutting
Typef

R/Q(MWS)

Q(HFSS)

Straight

595 MHz

5.7 Ω

106

Tapered602 MHz

5.2 Ω

5800

Tapered + tip cutting

607 MHz

2.1 Ω

2700

Q and R/Q of LOM/HOM
Modef (MHz)R/Q (Ω)

Q

Calculation results

TM110406 /406*

80 /80*48 /25*

TM310

684

2047

TM1118941160f, Q : calculated by HFSS
R/Q : calculated by MWS* tip cutting

TM510

980

0.16

3200

l/4λ

602 /607*

5.2 /2.1*

5800 /2700*

TE011

707

3.1

143

TM410

834

6.8

107

TE211

927

2.1

83

l/4λ

597 /602*

5.0 /2.8*

1700 /1000*

TE101

641

10181

TM210

661

4.9

47

TM320909

3.0185

Monopole

Dipole (H)Dipole (V)summary
Coaxial coupler has dangerous parasitic modes with high Q factor.
Three modifications were applied.
z
Tapered coaxial coupler designz
Tip cuttingz
TE stop band splitting of notch filter
without disturbing mode-damping of the cavity.
Mode damping of the coaxial coupler was drastically improved.
Tapered coaxial coupler with stop band splitting notch filter is being assembled
for low current operation.
Tip cutting design can be used for high current operation
Should be tested before installation.