Transcript Slide 1
Cold L-Band Cavity BPM: Design Status July 2006
Gennady Romanov Linda Valerio Manfred Wendt
Fermilab July 21, 2006 1
Backup: Cold BPM Requirements and Issues
• BPM location in the cryostat, at the SC-quad • Every 3 rd 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 2
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 3
Cold Cavity BPM Development
• • • • • Problems with simple “Pill-Box” Cavity BPM’s TM 010 monopole common mode (CM) Cross-talk (xy-axes, polarization) Transient response (single-bunch measurements) Wake-potential (heat load, BBU) Cryogenic and cleanroom requirements 4
• Waveguide-loaded pillbox with slot coupling.
• Dimensioning for
f
010
f
RF = 1.3 GHz,
f
010 and
f
110 symmetric to
f
RF , ≈ 1.1 GHz,
f
110 ≈ 1.5 GHz.
• Dipole- and monopole ports, no reference cavity for intensity signal normalization and signal phase (sign).
• Q load ≈ 600 (~ 10 % cross-talk at 300 ns bunch-to-bunch spacing).
• Minimization of the X-Y cross-talk (isolation).
• Simple (cleanable) mechanics.
• Iteration of EM-simulations for optimizing all dimensions.
• Vacuum/cryo tests of the ceramic slot window.
• Copper model for bench measurements.
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General view
Cavity-BPM: SLAC style
Discrete port (current) x=10 mm y=30 mm Excitation signal Ports 6
Eigen modes
4 5 6 1 2 3
Mode Frequency
1.017
– Parasitic E 11 -like 1.023 – Parasitic E 21 -like 1.121 – Monopole E 01 1.198 - Waveguide 1.465 - Dipole E 11 1.627
Dipole Parasitic mode. Coupling through horizontal slots is clearly seen Parasitic mode E z distribution 7
Transient solution. Probe magnitude
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Cavity-BPM: Pillbox with WG slot coupling
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Optimization of slot dimensions 9000 8000 7000 6000 5000 4000 3000 2000 1000 0 30
Q external and Q loaded vs slot length Qload (EM) vs Slot_W (Slot_L=55)
Q ext EM Q load EM Q load FD 1400 1200 1000 800 600 400 200 0 1 40 50 60
Slot length, mm
70 80 2 3 4 5
Width, mm
6 EM - Eigen mode solver. FD – frequency domain solver. Slot_L=55 mm and Slot_W = 5 mm ->
Q load = 678
7 8 10 9
Ceramic windows in coupling slots Window – Ceramic brick of alumina 96% e r ≈ 9.4
Size: the same as slot Frequency, GHz Loaded Q Beam pipe radius, mm Cell radius, mm Cell gap, mm Waveguide, mm Coupling slot, mm 1.46
~ 600 39 114 10 122x110x25 47x5x3 N type receptacle, 50 Ohm, D=9.75 mm d=3.05 mm 11
Pick-up dimensions 47.03.mm
11.13 mm 8.9 mm Diam. 4.46 mm 2 1 12
Dipole Mode Sensitivity (Resolution)
V
110 (
x
)
x
f
110
Z
0 1
Q
1
Q
0
R sh Q
x
110
q x V
110 (
x
)
x
4 .
145 10 3
V
/
nC V
110 4
mV
/
nC
m
with:
R sh Q f
110 (
x
) 1 .
46
Z
0 50
Q
x Q
0 1
mm
110
q
600 2000 14 1
nC GHz V ThermalNoi se
Z
0
k T BW
0 .
7
V
with:
Z
0 50
T k
1 .
38 10 23
J
300
K
/
K BW
f
110
Q
110 2 .
4
MHz
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Monopole mode damping using simple pin-antennas 14
Damping with antennas: Transmission-line Combiner.
180 degrees for dipole. Standing wave with some frequency detuning.
l TL ~ 200 mm to avoid resonances around 1.46 GHz (SW eigenmodes for l TL ~ mm at: f 3 ~1.1 GHz, f 5 ~1.9 GHz) 200 In phase for monopole 15
BPM spectrum vs length of combiner (one leg) 3 2.5
2 1.5
1 0.5
0 0 Quadrupole Dipole Monopole 50 100 150 mm 200 250 300 350
Appropriate length of combiner – reasonable length and non-resonant Interaction with dipole mode 16
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