A Transverse Deflecting Cavity
for Longitudinal Beam
Measurements on VELA
Louise Cowie
Contents
•
•
•
•
•
VELA
The transverse deflecting cavity and how it works
Design choices
Results
Conclusion
VELA
TDC
• S-band cavity operating at 2.9985 GHz and providing a 5
MV kick
• The cavity gives each electron in the bunch a transverse
kick that is proportional to its position
• Converts longitudinal to transverse distribution
Transverse force
• The force on an electron is 𝑭 = 𝑒(𝑬 + 𝒗 × 𝑩)
• To give an electron travelling in the z direction a
transverse kick in the y direction requires either
-An electric field in the y direction
-A magnetic field in the x direction
• Use what is known as a dipole-like mode: TM1np,
TE1np
TM110
Magnetic field
force
Magnetic field in the x direction
Beam pipe
E field
Electric field
Electric field in the y direction
Number of cells
• Chose a 9 cell 2.9985 GHz cavity as a compromise
between HOM separation and shunt impedance.
1 𝑉˔ 2
𝑅˔ =
2 𝑃𝑐
• The cumulative transverse voltage should be around 5 MV
• Cell length = one half wavelength = 50 mm
• Required gradient = 5 MV / 450mm = 11.11 MV/m
Cell Shape
• Chose a well understood
multi-cell pillbox shape
• As used by Tsinghau
University1 and SPARC2
End cells
• The end cells were shortened to compensate for the
effect of the beampipes. Dipole magnets will also be
employed.
Mode separation
• Coupler in middle cell does not
excite even modes
• Coupling holes separate the
SOM
• Iris radii chosen to maximise
mode separation in the pass
band
• Nearest mode is 6.5 MHz away =
5x cavity bandwidth
Design Timeline
Begin design
September 2011
Design finished
December 2011
3-cell prototype
ordered
March 2012
Begin
re-optimisation
September 2012
Field
measurements
July/August 2012
Design finished
and 9-cell ordered
December 2012
Simulation Results
• Frequency of cavity is 2.9985 GHz
• R/Q of operating mode is 292.48 Ohms
• Q0 = 16046 QE=16140
Field flatness
Conclusions
• A 2.9985 GHz cavity giving a 5MV transverse kick
was required for VELA
• The cavity was designed to meet the requirements
and the mechanical designs sent to RI for
manufacture
Acknowledgements
Thanks to Philippe Goudket and Graeme Burt.
References
1. Jiaru Shi et al. Bunch Length Measurement with RF
Deflecting Cavity at Tsinhua Thomson-Scattering X-Ray
Source. TH5PFP094 Proc. PAC09, Vancouver, Ca.
2. D. Alesini et al., “An RF Deflector Design for 6D Phase
Space Characterization of the SPARC Beam”, EPAC’04,
Lucerne, July 2004, p. 2616.