Liling Xiao, Zenghai Li

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Liling Xiao, Zenghai Li Advanced Computations
Department, SLAC
Presented at LARP-CM12, April 9, 2009
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Outline

• Baseline Design (CM11)
• Cell Shape Optimization (R/Q)_T,
  Bpeaklt100mT for TM110
• LOM/SOM/HOM Coupler Design Qextlt200 for
  LOM/SOM
• Alternative Cavity Design
• Multipacting Study
• Summary

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Baseline Design (CM11)
Cell Shape Optimization

• Cell_l 187.5 mm, R_disk 70mm
• VT gt 2.5MV, Maximum kick gradient limited by
  Bpeak
• Optimize disk parameters for optimal Epeak and
  Bpeak

Squash ratio0.8
Squash or racetrack
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Crab Cavity RF Parameters
TM110-pi mode
B-field
E-field
TESLA TDR cavity peak fields for comparison
(Eacc 25-30MV/m) Ep50-60MV/m, Bp107-128mT
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LOM/SOM Coax-to-Coax Design
d

• Strongly damps unwanted modes in a compact
  structure because no cutoff for TEM mode in the
  coax
• Node at the electric field rejects operating mode
  in the vertical plane

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Qext vs. Coax Position
Gap10mm
There are two additional LOM modes due to the
coupling of the cavity modes to the coaxial
beampipe TEM modes. Coupler can be optimized to
achieve Qext less than 100 for LOM/SOM with a
smaller gap.
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HOM Coupler Design

• Damp the dipole modes at the horizontal plane
  and reject the operating TM110 mode at 800MHz.
• A two-stub antenna instead of the coupling loop
  is proposed.

LOM
Opt.
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Drawback in Baseline Design
There is significant coupling of input power to
the LOM/SOM couplers in this baseline design
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Alternative Design

• Input coupler is between of the two cells to
  avoid the cross coupling
• Coax TM110-pi coupler rejects operating mode
  using field symmetry.
• LOM/SOM couplers reject operating mode using
  electric field node.
• HOM coupler rejects operating mode using filter.

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Rejection of Operating Mode
LOM/SOM couplers reject operating mode using
electric field node. Field symmetry is very
important.
The center conductors of LOM/SOM couplers will be
slightly bent so that the rotating flange can
adjust the tips of the center conductors to lie
exactly along the E-field node of the operating
mode field.
shift
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Rejection of Operating Mode
TM110-p coupler rejects operating mode using
field symmetry.
TM110-0 (R/Q)_T100O/cavity
TM110-p (R/Q)_T0.3O/cavity
The misalignment of p-mode coupler should be less
than 1mm to avoid the cross coupling between the
FPC and pi-mode coupler.
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Monopole Vertical Dipole Modes
Damping requirements for the LOM and SOM modes
Qext lt 200.
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Horizontal Dipole Modes
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Track3P - Multipacting Studies
MP in the Cell -
Resonant particles with impact energy between
502000eV
TM110-0 mode VT0.11.25MV
E-BC
y
eV
2mm
z
x
Avoid E 200ev600ev
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Multipacting at the Disk
2-points MP, 0.5,1, 1.5 order
VT2.3MV
In the disk area at the short axis of the KEKB
509MHz crab cavity, MP were found but it can be
processed through.
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MP vs. Disk Profile
R_bp70mm
r
r13.75mm, Bs124mT
Smaller disk radius can suppress MP but is at
risk of higher Bpeak. Larger disk radius is hard
to push the MP barrier beyond VT2.5MV.
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MP vs Local Geometry
E-field
Radius2mm Radius4mm
B-field
Adding grooves along the short axis without
changing the operating modes RF parameters can
suppress MP activities.
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MP in LOM Coupler Coax BP
There is no MP in the coaxial beampipe. With
fully rounded tip of LOM center conductor, MP can
be suppressed.
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MP in HOM Coupler
There are MP events in the gap of HOM notch
filter.
eV
y
eV
x
VT
z
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MP in Input p Mode Couplers
Assuming FPC coupling Qext1e6 VT2.5MV,
(R/Q)_T100O/cavity Pin63KW
There is no MP in FPC and p mode couplers below
Pin100KW.
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Summary

• Baseline 800MHz crab cavity design has been
  proposed.
• With optimum cell shape, Bpeak83mT _at_ VT2.5MV
  (Critical Bpeak100mT)
• Using coax-to-coax couplers, Qext _at_ LOM/SOM lt
  150. (Requirement Qextlt200)
• Strong MP in disk area at the short axis of
  normal cavity shape.
• Adding a small groove along the cavity surface
  at the short axis can suppress MP
• No MP barriers found in LOM/SOM/FPC couplers and
  coaxial beampipe.
• If needed, further optimization can be done to
  fully suppress MP in HOM coupler.
• Cavity model put on wiki page for cross check
  and cryostat study.
• https//twiki.cern.ch/twiki/bin/view/Main/CrabC
  avityRF
• Search for eigenmodes below 2GHz in progress.
  
• Further simulations planned to analyze
  RF/thermal/mechanical effects.

Thanks to members of LHC Crab Cavity
Collaboration for valuable discussions.