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Cold Cavity BPM R&D for the ILC. Manfred Wendt Fermilab. The International Linear Collider. ILC Beam Parameters (nominal):. ILC Beam Instrumentation. ~ 2000 Button/stripline BPM’s ~ 1800 Cavity BPM’s (warm) 770 Cavity BPM’s (cold, part of the cryostat) 21 LASER Wirescanners

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cold cavity bpm r d for the ilc

Cold Cavity BPM R&Dfor the ILC

Manfred Wendt


Global Design Effort

the international linear collider
The International Linear Collider

ILC Beam Parameters (nominal):

Global Design Effort

ilc beam instrumentation
ILC Beam Instrumentation
  • ~ 2000 Button/stripline BPM’s
  • ~ 1800 Cavity BPM’s (warm)
  • 770 Cavity BPM’s (cold, part of the cryostat)
  • 21 LASER Wirescanners
  • 20 Wirescanners (traditional)
  • 15 Deflecting Mode Cavities (bunchlenght)
  • ~ 1600 BLM’s
  • Many other beam monitors, including toroids, beam phase monitors, wall current monitors, faraday cups, OTR & other screen monitors, sync light monitors, streak cameras, feedback systems, etc.

Global Design Effort

cold bpm requirements
Cold BPM Requirements
  • BPM location in the cryostat, at the SC-quad
  • Every 3rd cryostat is equipped with a BPM/quad: 650x cold BPM’s total.
    • Real estate: ~ 170 mm length, 78 mm beam pipe diameter (???).
    • Cryogenic environment (~ 4 K)
    • Cleanroom class 100 certification (SC-cavities nearby!)
    • UHV certification
  • < 1 µm single bunch resolution, i.e. measurement (integration) time < 300 ns.
  • < 200 µm error between electrical BPM center and magnetic center of the quad.
  • Related issues:
    • RF signal feedthroughs.
    • Cabling in the cryostat
    • Read-out System

Global Design Effort

possible cold bpm solutions
Possible Cold BPM Solutions
  • Dedicated, high resolution BPM (baseline design):

Cavity BPM, based on the characterization of beam excited dipole eigenmodes, also requires the measurement of the monopole modes for normalization and evt. sign of the beam displacement.

  • Combination of dedicated, lower resolution BPM’s and HOM coupler signal BPM’s (alternative design):
    • Simple, button style BPM’s (~ 50 µm resolution) for machine tune-up and single bunch orbit measurements.
    • HOM coupler BPM signal processor as high resolution BPM

Global Design Effort

cavity bpm principle
Cavity BPM Principle

Problems with simple

“Pill-Box” Cavity BPM’s

  • TM010 monopole common mode (CM)
  • Cross-talk (xy-axes, polarization)
  • Transient response (single-bunch measurements)
  • Wake-potential (heat-load, BBU)
  • Cryogenic and cleanroom requirements

Global Design Effort

cm free cavity bpm
CM-free Cavity BPM
  • uses waveguide ports to suppress the monopole mode (no hybrid-junction required)
  • very high resolution potential (~ 20 nm)!
  • complicated mechanics, i.e. cleanroom and cryogenic issues

Global Design Effort

kek atf nanobpm collaboration
KEK ATF nanoBPM Collaboration

BINP cavity BPM:

  • C-Band (6426 MHz)
  • 20 mm aperture
  • Selective dipole-mode waveguide couplers
  • 3 BPM’s in a LLBL hexapod spaceframe (6 degrees of freedom for alignment)
  • Dual-downconversion electronics (476 & 25 MHz)
  • 14-bit, 100 MSPS digitizer

Global Design Effort

cavity bpm resolution at atf
Cavity BPM Resolution at ATF
  • 10 minute run
  • 800 samples
  • σ ≈ 24 nm

Move BPM in 1 µm steps

Global Design Effort

slac cavity bpm
  • S-Band design for 35 mm beam-pipe aperture
  • Waveguide cut to beam-pipe (better cleaning)
  • Successful beam measurements at SLAC-ESA (~ 0.8 µm resolution)
  • No cryogenic temperature tests so far.
  • No clean-room certification
  • Needs a reference cavity or signal
  • Reduced beam-pipe aperture (nominal: 78 mm)

Global Design Effort

cold l band cavity bpm design
Cold L-Band Cavity BPM Design
  • Waveguide-loaded pillbox with slot coupling.
  • Dimensioning for f010 and f110 symmetric to fRF, fRF = 1.3 GHz, f010 ≈ 1.1 GHz, f110 ≈ 1.5 GHz.
  • Dipole- and monopole ports, no reference cavity for intensity signal normalization and signal phase (sign).
  • Qload ≈ 600 (~ 10 % cross-talk at 300 ns bunch-to-bunch spacing).
  • Minimization of the X-Y cross-talk (dimple tuning).
  • Simple (cleanable) mechanics.
  • Iteration of EM-simulations for optimizing all dimensions.
  • Vacuum/cryo tests of the ceramic slot window.
  • Copper model for bench measurements.

Global Design Effort

scaling of the slac cavity bpm
Scaling of the SLAC Cavity BPM

Discrete port (current)

x=10 mm

y=30 mm

Excitation signal


General view

Global Design Effort

slac bpm scaled eigen modes
SLAC BPM (scaled): Eigen Modes
  • Mode Frequency
  • 1.017 – Parasitic E11-like
  • 1.023 – Parasitic E21-like
  • 1.121 – Monopole E01
  • 1.198 - Waveguide
  • 1.465 - Dipole E11
  • 1.627


Parasitic mode. Coupling through

horizontal slots is clearly seen

Parasitic mode

Ez distribution

Global Design Effort

pillbox with wg slot coupling
Pillbox with WG Slot Coupling

Global Design Effort

optimization of the slot dimensions
Optimization of the Slot Dimensions
  • EM: Eigen-mode solver
  • FD: Frequency-domain solver
  • Slot-L = 55 mm & Slot-W = 5 mm Qload = 678

Global Design Effort

ceramic windows in the coupling slots
Ceramic Windows in the Coupling Slots

Window –

Ceramic brick of alumina 96%

er≈ 9.4

Size: the same as slot

N type receptacle,

50 Ohm,

D=9.75 mm

d=3.05 mm

Global Design Effort

matched wg to coaxial transition
Matched WG-to-Coaxial Transition

11.13 mm

8.9 mm



Diam. 4.46 mm

Global Design Effort

dipole mode sensitivity resolution
Dipole Mode Sensitivity (Resolution)



Global Design Effort

monopole mode investigation
Monopole-Mode Investigation

Monopole mode damping

using simple pin-antennas

Global Design Effort

unmatched transmission line combiner
Unmatched Transmission-line Combiner
  • 180 degrees for dipole-mode. Standing wave with some frequency detuning.
  • lTL~ 200 mm to avoid resonances around 1.46 GHz (SW eigenmodes for lTL~ 200 mm at: f3 ~1.1 GHz, f5 ~1.9 GHz)

In-phase signal combining for the monopole-mode signal

Global Design Effort

combiner induced frequency shift
Combiner-induced Frequency-shift

Appropriate length of combiner – reasonable length and non-resonant

Interaction with dipole mode

Global Design Effort

l band cavity assembly
L-Band Cavity Assembly

Global Design Effort

next steps
Next Steps…
  • N2 temperature cycles with the test model.
  • Drafting of the complete assembly.
  • EM modeling and fine tuning of the dimensions.
  • Investigation of the tolerances.
  • Prototype manufacturing.
  • RF measurements and characterization.

Thanks for your patience!

Global Design Effort