HEBT, Ring, and RTBT BPM System Beam-based Alignment

1
HEBT, Ring, and RTBT BPM SystemBeam-based
Alignment

• Peter Cameron
• July 23, 2002

2
Outline

• Techniques and Principles
• Measurement Time and Accuracy
• Trim Quad Switching
• Differences between Ring and HEBT/RTBT
• A Challenge to AP?
• Conclusions/Action Items

3
A Variety of Techniques

• Ballistic Alignment - CLIC
• Dispersion Free Steering - some processing
• Two-Beam Alignment
• SLC Injector Consecutive Quad Pairs
• NLC Main Linac - individual quad modulation
• Fast K modulation
• Slow K modulation

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Principle of BBA

• Beam off-center thru quad imparts kick, which can
  be measured by all BPMS of proper phase
• Use dipole corrector to sweep beam across quad,
  record all BPMs
• Increment quad trim and repeat

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Basic Method
Icorr 0
Icorr 10

• Icorr -10

offset
ya
ya/ Iquad
-10 0 10
0 20 0
20 0 20
Iquad Iquad
Iquad
y0
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Measurement Time

• Characteristic time of solid core quads is a few
  seconds (measurement in progress), settling time
  10 seconds
• Characteristic time of solid core correction
  dipoles is a few tenths of a second (measurement
  planned), settling time 1 second
• If a measurement consists of 5 quad settings for
  each corrector setting, and 5 corrector settings,
  then measurement time per quad plane 5 minutes,
  or an 8 hour shift to do the ring
• The order in which you take the data doesnt
  matter!
• If a measurement consists of 5 corrector setting
  for each quad setting, and 5 quad settings, then
  measurement time per quad plane lt 1 minute, or an
  hour to do the ring

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Kick Amplitude

• Beam offset in quad gets kicked. Magnitude of
  kick is
• q k . L . y
• Where
• k quad strength .2998 g/p 0.3/m2
• g quad gradient 10 T/m
• p beam momentum 1 GeV/c
• L quad length 0.4 m
• y beam offset 10 mm
• So that q 1.2 milliradians and bkicker
  bpickup 15 m,
• bq 180 mm
• With a quad modulation of dk 210-2, the
  position modulation is
• dbq 3.6 mm

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Processing Simulation Comparison 15ma Beam 1mm
displacement 210mm diam BPM
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Alignment Accuracy

• Position modulation due to quad modulation is of
  the order of a few mm, can be measured at all
  available pickups.
• For HEBT electronics, resolution is 100 microns
  for a single minipulse
• For Ring, we might expect to do BBA with 10 times
  the 15mA minimum current shown on the previous
  transparency shown on previous transparency,
  expected resolutions will also be in the 100
  micron range
• Can average over many pulses/turns, and over many
  BPMs
• Reasonable expectation is that accuracy will be
  considerably greater than the 1mm AP spec.

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Trim Quad Switching

• For the Ring, a total of 52 trim windings are
  powered by 16 power supplies (typically 4
  trims/supply) max 40A at 75V
• Supplies are two 20 modules in parallel, spares
  may be available for BBA supply
• Trim windings come back to an intermediate
  junction location, where switching will be done
• Make-before-Break and Sequence carefully to
  equalize coil energy before disconnecting
• Consider coil inductance and resistance to assure
  clean switching transient

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Trim Quad Switching Diagram

• Close switch A to shunt one trim winding - a
  little ripple
• Use switch B to connect the BBA power supply, and
  modulate
• Bring BBA power supply current to zero, close
  switch C, switch B
• Either
• Ramp trim power supply to zero (preferred choice)
  or
• Have transient absorbers across each magnet
• Open switch A, adjust trim current as required

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Required Hardware

• 150 switches, 40A 75V
• 20 power supply (spare may be available)
• Transient absorbers (if required)
• Controller
• Rack space

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HEBT and RTBT

• Trim windings are present on all quads in HEBT
  and RTBT, but there are no trim power supplies.
  Trims are available for Diagnostics as single
  user.
• Only half of RTBT quads have BPMs
• Half of HEBT and RTBT quads have their own
  individual power supplies. The balance are in six
  strings of four, two strings of three, and one
  string of two.

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Complications

• Absence of downstream BPMs at end of HEBT
• Use Ring 400MHz system?
• Effect of stripper?
• Absence of quad trims on 36cm quads at end of
  RTBT - but only one quad per power supply
• Absence of downstream BPMs at end of RTBT will
  position measurement be available on target?

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A Challenge to AP?

• Those guys get to have all the fun!
• Do we need independent BBA quad trim modulation?
• For a string of four quads modulated, with full
  complement of ring BPMs and corrections dipoles,
  is it not possible to extract relative offsets of
  the BPMs on those four quads using matrix methods?

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Conclusions/Action Items

• Refine details of switching scheme, improve cost
  estimate
• Refine accuracy calculation
• Examine techniques beyond simple sweep the
  dipole, change the quad, sweep the dipole
• Explore BBA in the background of machine
  operations - continuous BPM calibration