1. LHC Crab Cavity Conceptual Design at SLAC
Liling Xiao, Zenghai Li
Advanced Computations Department, SLAC
Graeme reported the waveguide coupler design for the LHC crab cavity.
We had proposed another coupler schemes we named it the coax-to-coax coupler and coax-to-waveguide coupler. They utilize a coax coupler or waveguide coupler feeding into a short coaxial beampipe. They should give strong damping in a compact structure. Today I will update the progress on the coax-to-coax coupler design for the LHC crab cavity.
Presented at LARP-CM11, FNAL, Oct , 2008

2. Outline
Cell Shape Optimization
Optimum Squash Ratio
LOM/SOM Coupler Design
HOM Coupler Design
Cavity Configuration Considerations
Summary

3. Cell Shape Optimization
800MHz crab cavity design
Maximum kick gradient limited by Bpeak
Optimize disk parameters for optimal Epeak and Bpeak
Cell_l=187.5mm, R_disk=70mm
TM110-pi
400MHz Cavity Design, Courtesy Rama Calaga's
Without Squashed, Courtesy Rama Calaga

4. Optimum Squash Ratio
LOM
HOM(TE111)
SOM FM
Squash ratio is chosen to optimize mode separation.
Max Dx is limited by available horizontal space Dy Dx

5. 800MHz Crab Cavity RF Parameters
TM110-pi mode Ep Bp
E-field
B-field
24.72MV/m
Epeak
6.67MV/m
Deflecting Gradient Ekick
82.75mT
Bpeak
2.5MV
Deflecting Voltage VT
89MHz
Mode separation (Opt.-SOM)
117ohm/cavity
(R/Q)_T
800MHz
Frequency
TESLA TDR cavity peak fields for comparison
(Eacc: 37-47MV/m)
Epeak: 70-90MV/m
Bpeak: mT

6. Damping Requirements Qext for LOM<200FM
SOM
Courtesy F. Zimmermann, R. Calaga
High R/Q LOM and SOM modes need to be strongly damped.

7. LOM/SOM Damping Schemes
Symmetric Waveguide
Shorted Waveguide
Coax
Coax-to-coax
Coax-to-waveguide
Hook type coax
Other coupler ideas (BNL, JLAB, LBNL, FNAL, ANL, UK …)

8. LOM/SOM Coax-to-Coax Coupler Design
R_disk=70mm
R_beampipe=60mm
gap d
Give strong damping on unwanted modes in a compact structure because no cutoff for TEM mode in the coax
Is more flexible to achieve the required damping by adjusting the location of the beampipe tube and the intrusion of the coupler probe
Gap=10mm
There are two additional LOM modes due to the coupling of the cavity modes to the shorted coaxial beampipe TEM modes.

9. LOM/SOM damping results meet the requirements! Qext<100

10. HOM Coupler Design HOM notch filter transmission curve
HOM coupler: damp the dipole modes in the horizontal plane
reject the operating TM110 mode at 800MHz.
HOM notch filter transmission curve
A two-stub antenna instead of the coupling loop is proposed.
This can simplify the model and remove the narrow gap between the loop and the outer cylinder which is prone to multipacting.
Next we will work on the HOM coupler with filter. It is under progress.
There are many possible options on the arrangement about the FPC/HOM/LOM/SOM couplers. Here is one concept design for the whole LHC crab cavity.
Once the impedance budget and other constraints are confirmed, we can chose one of them.
So far the max transverse dimension in the 800 MHz cavity is about 237mm that is larger than the beam-to-beam separation of 200mm. One possible option which was suggested before is to turn the cavity to use the high frequency TM110-pi mode as operating mode.
df/dgap=0.4MHz/micron

11. Cavity Configuration Considerations
LOM/SOM couplers use the electric node to reject operating mode, and symmetry is very important in cavity-coupler integration.
FPC
HOM
Option 1: Significant coupling of input power to LOM/SOM couplers
LOM/SOM
Next we will work on the HOM coupler with filter. It is under progress.
There are many possible options on the arrangement about the FPC/HOM/LOM/SOM couplers. Here is one concept design for the whole LHC crab cavity.
Once the impedance budget and other constraints are confirmed, we can chose one of them.
So far the max transverse dimension in the 800 MHz cavity is about 237mm that is larger than the beam-to-beam separation of 200mm. One possible option which was suggested before is to turn the cavity to use the high frequency TM110-pi mode as operating mode.
LOM/SOM
HOM
FPC
Option 2: LOM mode may shift to one side due to asymmetry, may result in ineffective damping

12. Option 3: Power coupler and HOM couplers in between of the two cells
Maintain field symmetry in LOM/SOM coupler regions
Avoid cross coupling between power coupler and LOM/SOM couplers
FPC
cell_l
Modes
R/Q& (R/Q)T (ohm/cavity)
Qext
LOM: TM010-pi (~600MHz) 94 80
LOM: TM010-0 (~600MHz)
130
Operating Mode TM110-0 (800MHz)
110
FPC
SOM: TM110-0 (890MHz) 50
Further HOM coupler optimization is needed. Waveguide coupler can also be considered as HOM coupler.

13. Summary
Significant progresses have been made for the 800MHz crab cavity design for LHC
Strong damping achieved using coaxial-beampipe to coax LOM/SOM couplers, Qext of high R/Q modes below 100
HOM coupler optimization is underway
Multipacting simulations in cavity/couplers are in progress
Further simulations planed to analyze RF/thermal/mechanical effects
Thanks all the members of LHC Crab Cavity Collaboration for valuable discussions.