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Beam Position Measurements in TTF Cavities using
Dipole Higher Order Modes
G. Devanz, O. Napoly, CEA, Gif-sur-Yvette A.
Gössel, S. Schreiber, M. Wendt, DESY, Hamburg
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Motivations

• Beat the 300-500 µm pre-alignement of
  cavities-modules in the Linac
• de ? dycav2
• Use well-measured dipole modes TE111 and TM110 to
  monitor the beam position w.r.t. cavity
  electrical axis. This is the NLC method, but we
  dont know a priori the polarisation of the
  modes.
• No cavity movers, but one steerer / module and
  the knowledge of the real orbit is of great help
  for tuning bumps.
• How refined signal acquisition needs to be ?
• It is a bunch train measurement

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TTF 9-cell cavity HOMs

• Modes Below cutoff
• ? no propagation
• R/Q easy to compute
• in one cavity.

D5
Frequency GHz R/Q O/cm2
TE111_6 1.705 11.1
TE111_7 1.730 15.6
TM110_4 1.865 6.4
TM110_5 1.875 9.0
D3
D2
D1
R. Wanzenberg
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HOM couplers as BPMs
Output power P (beam loading) from a dipole mode
in a cavity
1st polarization P1 C1 r 2 cos2(F) 2nd
polarization P2 C2 r 2 sin2(F)
? r 2 P1 / C1 P2 / C2
C1 and C2 are calculated from fi , R/Q , Qi
of the HOM 1st power measurement ? r2
x2y2 2nd power measurement with known beam
offset dy ? y and x
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Data Acquisition
Module II
Module III
ON
OFF
Beam
HOM 2
q 3.5 nC fb 2.25 MHz Tp 780 ms
HOM 1
Agilent E8563E spectrum analyser
Spectrum analyser

• used as a parametric bandpass filter
• central frequency
• resolution bandwidth
• signals in time domain

GPIB
Att 10 dB
zero span
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Cable calibration
Cables inside cryostat reflection measurement
with network analyser
Cables outside cryostat attenuation
measurement with power-meter
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Check on monopole HOMs
GOAL

• check calculations
• check cable attenuation

Choose HOM

• well known HOM ? non propagating bands
• high R/Q to get strong signal

TM011_8
TM011_9
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Dipole mode measurements
2 positions computed using 2 modes with the same
beam
High gradient in cavities ( 20 MV/m) ? orbit is
expected to cross ACC1 module axis
if entering at an offset
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Conclusion

• First experiment HOM parameters not all
  accurate
• Data acquisition not optimum
• Some selected encouraging results
• Program for a PhD student co-financed by
  Saclay-DESY (selection committee in June)
• Need 1) accurate information on TE111 and
  TM110 mode parameters
• 2) HOM signals transported out of the tunnel,
  with well calibrated cables, for at least one
  module.
• Most of the measurements can be done in
  parasitic mode