the cavity beam position monitor (bpm) massimo dal forno paolo craievich, raffaele de monte, thomas...
TRANSCRIPT
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The Cavity Beam Position Monitor (BPM)
Massimo Dal FornoPaolo Craievich, Raffaele De Monte, Thomas Borden,
Andrea Borga, Mauro Predonzani, Mario Ferianis, Roberto Vescovo
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Contents
Introduction: The Cavity BPM HFSS Simulations CST Simulations The new electronic system The in-tunnel test Outlook of the future work
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Introduction: The Cavity BPM
Devices able to determine the X and Y position of the electron beam in the beam pipe
Based on a resonant cavity
Good resolution (~1μ target for FERMI@Elettra), High signal level in single shot
WaveguidesResonant Cavity
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The dipole mode: TM110
It is the position sensing mode Its intensity is proportional to the beam offset There are two different polarizations: vertical and
horizontal
The separation of the monopole and of the two polarizations is achieved with the cavity-waveguide coupling
Working Frequency: ~6.5 GHz
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Separation of the two dipole polarizations
The magnetic coupling is described by “HR” (radial component of H) Allows the separation of the two polarizations
X polarization port 1, 3 Y polarization port 2, 4
Propagation
HR
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The signal of port 1, 3 is proportional to the X position The signal of port 2, 4 is proportional to the Y position
An additional signal is used as “reference signal”, to: Obtain a bipolar output signal (for +X, +Y), Separate the offset from the tilt component
Signal processing
1
2
3
4
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Beam offset and tilt effects on the output signals
Only offset
Only tilt
The electronics must even separate the offset from the tilt component in quadrature (IQ demodulation or our
approach)
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jkzzoffsetacc
2
11 ,
2cos
22sin
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,
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ak
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Purely immaginary
Purely real
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HFSS Simulations
Aim: Simulating the RF parameters of the cavities with 90°, 180° and no symmetry planes:
Aim: Estimating the output signal levels with 1 nC of bunch charge, the voltage is given by the following relation:
Reference cavityBPM
cavity
qkQ
ZVEXT
OUT 01002
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HFSS Simulations results
Workbench measured frequencies:
The simulation result is 19 MHz different from the measured
value
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CST Simulations
Aim: Simulating the output signal levels with 1 nC of bunch charge
Summary of the sigal levels:
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The new electronic system
Aim: designing a new electronic system that avoids the IQ demodulation
First type of circuit
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)sin()cos()(
)sin()cos()(
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The tilt component must be negligible with respect to the offset
(for 1μm, the tilt must be < 0.1 mrad)
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The new electronic system
Advantages: Beam in the centre High output signal level (∑=Δ) Calibration system
R
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Calibration
Matrix
A∑ cal
AΔ cal
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The new electronic system
Second type of circuit
The tilt component is rejected:
ABA
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d
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BR
4
4 //22
Anologous result to the coherent demodulation
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The in-tunnel test
The prototype has been installed in tunnel during the last commissioning
Aim: determining the output voltage with 1 nC of bunch charge
Signal levels: Reference cavity: 2.52 V Cavity BPM, X offset: 0.33 V/mm Cavity BPM, Y offset: 0.30 V/mm
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The in-tunnel test
Spectrum (FFT) of the BPM output signal
Dipole modef = 6.476 GHz
Quadrupole modef= 9.046 GHz
Rectangular waveguidef= 7.7 GHz
Dipole of the referencef = 8.47 GHz
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10 cavity BPMs have been installed in the undulator hall Each one has a mover (Encoder resolution: 1 μm)
Testing the RF frontendMeasurements of the resolution
Outlook of the future work
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Thank you for your attention
Questions?
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14th ESLS RF Workshop
ELETTRA / Trieste, Italy / 2010 September 29 30
The resonant modes of the cavity
(Frequencies of the FERMI@Elettra BPM)
TM010 TM110 TM210 TM020
4.63 GHz 6.5 GHz 9.04 GHz 10.5 GHz
The electron beam excites the resonant modes of the cavity The first four resonant modes are the following:
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14th ESLS RF Workshop
ELETTRA / Trieste, Italy / 2010 September 29 30
The monopole mode: TM010
It is an unwanted mode Its signal voltage is only proportional to the beam
intensity and does not depend on the beam position.
Rejection achieved with: Cut-off frequency of the rectangular waveguide Cavity-Waveguide Coupling Band pass filter centred on the dipole frequency
Working Frequency: 4.63 GHz
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14th ESLS RF Workshop
ELETTRA / Trieste, Italy / 2010 September 29 30
Rejection of the TM010 mode:Cut-off frequency of the waveguide
Waveguides behave as high-pass filter Cut-off frequency for the fundamental mode
(TE10):
The monopole, at 4.63 GHz is under cut-off
GHza
cfL 5
2
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14th ESLS RF Workshop
ELETTRA / Trieste, Italy / 2010 September 29 30
Rejection of the TM010 mode: Cavity-Waveguide Coupling
Magnetic coupling: only the magnetic field (Hr) of the dipole will couple with the waveguide
Radial component of H (Hr)
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14th ESLS RF Workshop
ELETTRA / Trieste, Italy / 2010 September 29 30
Rejection of the TM010 mode: Cavity-Waveguide Coupling
The monopole does not couple with the waveguide
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14th ESLS RF Workshop
ELETTRA / Trieste, Italy / 2010 September 29 30
Cavity-Waveguide Coupling: Separation of the two dipole polarizations
However, due to the mechanical tolerances, the two polarizations are not perfectly orthogonal
The orthogonal ports are not isolated between them This phenomena is called “Cross-Talking”
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14th ESLS RF Workshop
ELETTRA / Trieste, Italy / 2010 September 29 30
Rejection of the TM010 mode: Cavity-Waveguide Coupling
Consequences of the cavity-waveguide coupling
The monopole does not couple with the waveguide
It separates the vertical and the horizontal polarizations
An additional band-pass filter is placed to have only the dipole signal and to reject the higher modes
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14th ESLS RF Workshop
ELETTRA / Trieste, Italy / 2010 September 29 30
Vout
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Energy
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22
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14th ESLS RF Workshop
ELETTRA / Trieste, Italy / 2010 September 29 30
Vacc, Energy, Bessel, Linearity
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14th ESLS RF Workshop
ELETTRA / Trieste, Italy / 2010 September 29 30
Offset, Tilt