TeV Orbit Calculations: Beam Smoothing Tools

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TeV Orbit CalculationsBeam Smoothing Tools

• Cusper
• Smoother

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Cusper

• Fitter to detect distortions in the magnetic
  field based on deviations from the expected form
  of the beam position measurements
• Includes heuristics for rejecting BPM
  measurements as suspected bad readings
• Identifies positions and strengths of apparent
  distortions

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Smoother

• Tool to suggest optimum DFG settings to correct
  for distortions
• Optimization includes criteria which existing
  tools do not
• Recognition of DFG current range restrictions
• Penalty for larger DFG currents
• Penalty for large inter-step slew

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Identifying Maverick Readings

• Technique
• Local sine-curve fits to BPM data (corrected for
  b function)
• Fit with and without a key point (or pair of
  points) to be evaluated
• Bayesian approach Assume some a priori
  likelihood of a given measurement being bad
• Which is more likely?
• the (worse c2) probability of the fit including
  the key point
• the (better) probability of the fit excluding the
  key point, times the assumed likelihood of a bad
  measurement
• Slide the fits so that a single cusp does not
  spoil the identification of mavericks

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Subtleties in Identifying Mavericks

• There are various ways for the data to fool a
  heuristic for identifying mavericks
• The presence of a maverick, coupled with unlucky
  normal fluctuations, to cause
• incorrect rejection of a good reading
• incorrect acceptance of a bad reading
• Unit-testing the code exposed a number of such
  headaches

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Subtleties in Identifying Mavericks

• The presence of a true maverick here distorts the
  fit such that an acceptable measurement is
  rejected
• Modifications to the heuristic to correctly
  handle most of these subtleties have been made.
• Some situations still fool the filter, but these
  now involve very rare combinations of
  circumstances

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Identifying Cusps

• Fit with varying window sizes and positions
• Find three best fit curves
• across the interval in question
• just to the right of the interval
• just to the left of the interval
• Again a Bayesian approach is used
• Some a priori likelihood for distortion in any
  given interval
• Which is more likely?
• the (worse c2) probability of the fit without a
  distortion
• the (better) probability of the fit excluding the
  key point, times the assumed likelihood of a
  distortion
• Last-pass overall optimization of cusp positions
  and magnitudes

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Cusper Status

• Can read BPM and Twiss parameter info from a
  couple of formats and use it to try to do the fit
• Display capability via Root
• Rejection filter implemented and unit-tested and
  tuned
• 99 of the effort, as usual, went into the
  boundary and pathological cases
• Cusp fitter implemented but as yet not at
  tested/tuned
• Major issue once the linux-based cusper exists
• How (and whether) to integrate this with the
  control-room accessible tools
• Effort has been on back burner for a while

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Smoother

• Given the BPM readings and the desired target
  orbit readings, find the best DFG settings to
  correct the orbit.
• Global optimization across time steps in the
  ramp and squeeze, to minimize large slews
• Avoidance of suggested currents which are too
  large for the DFG to handle
• Consideration to avoiding large total currents
• Two DFGs should not fight each other at large
  currents
• The overall optimization can be computationally
  intense
• This has been on the back burner for quite a
  while
• When the tool exists, integration with
  accelerator controls will also be an issue

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Beta Tool

• Once
• cusper is implemented
• new BPMs are in place
• it becomes possible to evaluate, for a given
  frame, the corrections to the b function which
  would explain BPM fluctuations not explained by
  identified cusps.
• A single such measurement is not very meaningful.
• An ensemble of these corrections, over time,
  might be helpful in determining the b function to
  better accuracy.
• Is this something people would be interested in?