Cavity BPM Simulations

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Cavity BPM Simulations

• A. Liapine

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Analysis of the Existing BPMs
BINP
KEK
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Analysis of the Existing BPMs
BPM BPM R/Q, Ohm at 1mm ß (coupling factor) Qext t, ns (decay time)
BINP Simulation 0.80 0.31 26350 304
BINP NA 0.35 25000 317
BINP Diode 0.87 9400 216
KEK Simulation 0.90 0.17 47020 338
KEK NA 0.1 81800 361
KEK Diode 0.25 32120 317
ATF2 plans ATF2 plans 0.6 13600 248
Q08600 used for calculation, elsewhere Q08180
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KEK BPM Issues

• Usual way of simulation cavity and wg-to-coax
  adapter are simulated separately to reduce the
  total computation time and memory consumption
• Suggestion (M.Ross) simulate the whole geometry
  as well for a better understanding
• Results are different! Coax adapter geometry is
  important

Coupling coefficient Qext Decay time
waveguide only separate adapter whole structure 0.48 0.34 0.17 16990 24270 47020 268 298 338
diode measurement NA measurement 0.25 0.1 32120 81800 317 361
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KEK BPM Issues

• Structure is not symmetric, some monopole leakage
  is possible
• Qext is huge, therefore estimating the amplitudes
  for a unit of the stored energy, then normalizing
  with shunt impedances

A0105.65 V/m A1101250 V/m R/Q010114
Ohm R/Q1101.26 Ohm
Introducing asymmetries Slot wg feedthrough
shifted by 0.5 mm 73 µm Slot shifted by 0.5 mm
73 µm Feedthrough shifted by 0.5 mm 63 µm
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ATF2 Design

• Successor of previous designs
• Longitudinal design
• 4-coupler symmetrical structure
• Beam pipe diameter increased to 20 mm to meet
  the ATF2 beam optics requirements
• Increased coupling for a higher sensitivity
  Qext13000
• Dipole mode frequency 6426 MHz

Monopole leakage Total waveguide length 20 mm
500 nm 30 mm or more leakage is less
than computation error Introducing asymmetries
(Slot wg feedthrough shifted by 0.5 mm, 0.25
mm meshstep) Total waveguide length 30 mm 40
µm 40 mm 20 µm 50 mm 12 µm Check with 0.2 mm
meshstep 50 mm 10 µm
Incline component estimation 7 µm/mrad
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ATF2 Design
Bandwidth 1500 MHz
Dimension Refl. growth Accuracy
ff position lt 0.05/100 µm 50µm
pin length 0.05/100 µm 20µm
ins. radius 0.05/100 µm 20µm
ins. height lt 0.05/100 µm 50µm