High resolution beam position
monitoring for linear accelerators
R. Apsimon
University of Oxford
Contents
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What is a BPM?
How does a BPM work?
Signal processing
Calibration
Resolution
Current status
Future development
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High resolution beam position
monitoring for linear accelerators
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What is a BPM?
• Beam Position Monitor
– 4 metal striplines placed along beam pipe to measure the
position of beam in horizontal and vertical direction
• Different types of BPM
– Stripline: ~10cm long, 4 strips along beam pipe
• Quick response, moderate resolution
– Button: Like striplines, but shorter
• Quick response, poorer resolution
– Cavity: Uses antennae rather than strips as pickups
• Slow response, good resolution
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High resolution beam position
monitoring for linear accelerators
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• Feedback On Nanosecond Timescales
– Minimise bunch-bunch jitter
• Need good BPM resolution
– Time between bunches: 154ns
• Need fast BPM response
• Stripline BPMs used as a compromise
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High resolution beam position
monitoring for linear accelerators
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How does a BPM work?
• Electron beam is ultra-relativistic
• Electric field Lorentz contracted longitudinally
• E-field forms a disc perpendicular to direction of motion
• E-field now varies as r -1 NOT r -2 as it does classically
• Signal induced in pickups by electric field
• Signal amplitude proportional to bunch charge and angular width of
pickup as seen from electron bunch (Gauss’ law).
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High resolution beam position
monitoring for linear accelerators
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Cross section of a BPM
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monitoring for linear accelerators
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The technical bit…
x, y  r

 a
a

  1  y 
1  2 tan
 2 r  y 2  x 2  r



 a
a
1 
  1  y 
 3  2 tan
 2 r  y 2  x 2  r


1
Vi  Q i
2a
Q
r
2a
 y  V1  V3 
Qy
r

 y
Similar result in x

 y  V1  V3 
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High resolution beam position
monitoring for linear accelerators
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Signal processing
• BPM signals need to be digitised
• BPM output ~1GHz, too fast for digital system
• Need to “slow” signals down to ~100MHz
• Process raw signals before digitisation
• 2 main processing options:
– Low-pass filter:
• Easy to operate, robust
• Cuts most of the signal so poor resolution
– Local oscillator (LO) to mix down frequency
• Keeps more signal so better resolution
• More complicated, LO needs to be correctly phased
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High resolution beam position
monitoring for linear accelerators
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LO scheme
Raw stripline
signals
LO
(714MHz)
Phase shifter
180o hybrid
Band-pass
filter
Mixer
Low-pass
filter
Subtracts one
signal from
the other
Cuts
frequency
components
at 714MHz ±
100MHz
Effectively
multiplies two
input signals
together
Cuts high
frequency
components
Output
Similar for the sum signal, except the hybrid adds the signals together,
rather than subtracts.
LO scheme used for optimal resolution.
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High resolution beam position
monitoring for linear accelerators
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Mixer
1
cos  At  cos Bt   cos  A  B t  cos  A  B t 
2 High frequency
Low frequency
The mixer produces a high and low frequency term, the low-pass filter then
removes the high frequency term.
Mixer is semiconductor device, only linear for ~±50mV for our processors
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High resolution beam position
monitoring for linear accelerators
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After processing
• Processed sum and diff signals
– Low enough frequency to be passed through ADCs
– Beam position calculated from signals digitally
– Digital system is clocked at 357MHz with 14 bit resolution
• 357MHz clock generated from 714MHz LO
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High resolution beam position
monitoring for linear accelerators
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Calibration

 Cy

• C is the calibration constant of the BPM
• 2 ways to calibrate the BPMs
– Move the beam using an upstream dipole
– Keep beam steady, move BPMs with movers
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High resolution beam position
monitoring for linear accelerators
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Calibration methods
• Moving the beam
– Need well calibrated dipole magnet
– Need to know where the beam moves to in BPMs
• Transfer matrix from simulation, potentially unreliable
• Moving the BPMs
– Need to calibrate mover
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High resolution beam position
monitoring for linear accelerators
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Resolution
• Not a trivial problem
– Beam jitter bunch to bunch
• Correlated jitter
– BPM resolution
• Uncorrelated jitter
• Have 3 BPMs
– Use 2 to calculate the beam position in third
• Use this to remove correlated jitter to leave uncorrelated jitter
• Calculate resolution from residual
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monitoring for linear accelerators
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A bit more maths…
y
  Vd
C   Vs 
δV represents the noise on the processor signals
 y  2  y  2  y
       
  
  
 Vd
2
2
 y2  
Correlated jitter
terms
2
2
 2
 y  2
  Vd  
  Vs

 Vs 
Uncorrelated jitter terms
Vd  Vs  V
 V
uncorr
y

1 C 2 y2
C
Σ is charge dependent
Resolution is charge dependent and position dependent
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High resolution beam position
monitoring for linear accelerators
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Current status
• Calibration stable
– LO phase stable
• Were suffering phase jumps generating the 357MHz clock
• Phase drift with the LO so needed to recalibrate every hour
• Both problems solved now
– Processors working
• Had several component faults with the processors, now fixed
– This also affects resolution
• Typical stripline BPM resolution ~ 10μm
– Currently we can achieve ~ 2μm
– Hope to achieve < 1μm with future improvements
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High resolution beam position
monitoring for linear accelerators
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Future developments
• Calibrate with movers rather than beam
– Also allows beam position to be zeroed in all BPMs
• Minimises resolution
• Amplify signals
– Use low noise amplifiers on output of processors
• Improves signal to noise ratio, improving resolution
• Develop new BPM processors
– Use higher power mixers
• Allows larger signals, better signal to noise ratio, better resolution
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High resolution beam position
monitoring for linear accelerators
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Questions?
• Please feel free to ask any questions now
or later, and thank you for listening
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monitoring for linear accelerators
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