SynchrotronLight Monitors

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Synchrotron-Light Monitors

• Alan Fisher
• PEP-II Machine Advisory Committee
• 2006-01-19

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Topics

• Commissioning the LERs x-ray monitor
• Turn-by-turn measurements with a rotating mirror
• Beam-abort diagnostics
• Video abort buffers
• Rotating mirror
• Streak camera
• Plans
• IP beam size
• Bunch-by-bunch profiles

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LER Synchrotron X-Ray Monitor

• At the last MAC meeting, I presented the design
  of the SXM, which was installed and waiting for
  beam.
• Dipole radiation from Arc 7, via a modified
  photon stop
• Pinhole, scintillator, video camera
• Commissioned with first beam last April.

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Typical SXM Image

• Images have expected calibration.
• Same size with all 4 pinholes.
• Size is above resolution limit.
• Ellipse is strongly tilted.
• Tilt increases from 10? with LER only at low
  current, to over 50? in collision.
• Tilt agrees with Yiton Yans MIA code for
  single-beam at low current.
• A coupling maximum near the x-ray source point.

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Beam-Abort Movies

• Frame grabbers for synchrotron-light monitors
  have 4 channels (beam image, interferometer,
  alignment,).
• Software saves a 9-frame movie every time it
  switches to the beam-image camera.
• Spans 300 ms with 30-Hz frames.
• Previous movie is overwritten, unless the beam
  has aborted.
• Then the movie is transferred to a buffer.
• User in Main Control can view and save the movie.

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Frame -8 Stable Beam
SXM, as instability is about to start.
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Frame -7
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Frame -6
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Frame -5
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Frame -4
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Frame -3
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Frame -2
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Frame -1
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Frame 0 Movie Ends, Abort Follows
Final frame recorded. BaBar radiation triggered
an abort afterward.
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Turn-by-Turn Transverse Imaging

• Concept
• Image the transverse profile of a single bunch
• Over many turns
• In a full ring
• See changes over 100 or 1000 turns
• During steady running
• During injection, as charge is added to the bunch
• Investigate injection backgrounds
• Instability preceding a beam abort

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Gated Camera

• Gives transverse (x and y) images of the beam.
• Gate can be as short as 2 ns.
• Can capture single-bunch images in a full ring.
• Can be gated repeatedly on successive turns.
• Image intensifier (microchannel plate) gives
  sufficient brightness.
• Butimage readout is too slow.
• Repetition rate is a few Hz.
• Cannot resolve x or y evolution over consecutive
  turns.

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Sweep Image across Gated Camera

• Project the transverse image onto one axis.
• Measure only one transverse dimension at a time.
• Turn the beam ellipse into a thin vertical
  stripe.
• Two cylindrical lenses image the synchrotron
  light with different magnifications in x and y.
• Fire the cameras gate on one bunch over many
  consecutive turns (or every nth turn).
• Use a rapidly rotating mirror to sweep this
  projection across the camera.
• One camera readout then captures many turns.

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July 2004 Dissected Bar-Code Scanner
Quick proof-of-principle test in LER at the end
of the 2004 run
Aperture of gated camera
Cylindrical lens for horizontal demagnification
DC motor with servo
Rotating octagonal mirror, 3 mm high, 13 mm across
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2004 July 30 64 Turns
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What Needed Improving?

• Cannot be triggered to take data after injection.
• Motor is free running, not synchronized with PEP.
• Scanner issues a timing pulse on each turn of the
  polygon.
• Used to trigger the camera.
• Motor starts camera, rather than a PEP timing
  signal.
• Only measures one axis of the beam.
• The polygon is too small 3 mm high, while
  cameras photocathode is 12.4 mm high.
• Needs demagnification vertically, making it
  harder to do even more horizontally.
• Dissected scanner cant be mounted rigidly.

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Scanning Galvanometer

• Mirror on a servo motor
• Used for laser scanners, often in pairs for
  raster scans.
• Built for stable mounting.
• Can get a bigger mirror
• 14.2 mm high ? 8.7 mm wide
• Mounts on VM500 motor (smallest one in photo).
• Driven by a ramp waveform
• Triggered by PEP timing.

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Measuring x and y Simultaneously

• Split the light, then rotate one beam by 90?.
• Image one path onto the top half of the camera,
  the other onto the bottom half.
• Two sets of stripes swept out across the camera.
• Different magnifications on each path.
• Major-axis projection of beam ellipse viewed on
  top half of video image, minor axis on bottom.
• Paths must be equal within 2 ns to catch the same
  bunch in the cameras gate.
• New optics installed in summer and fall of 2005
  (after PEP resumed operation) for both rings.

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Optical Table Layout for LER Hutch
Scanning mirror
Cylindrical lenses
Gated-camera locations Turn-by-turn
imaging Bunch-by-bunch imaging (also for streak
camera)
In-plane periscope (no rotation)
Color filter
Polarizer
Out-of-plane periscope (rotates 90?)
Flip-up mirror (bunch-by-bunch imaging or streak
camera)
Beamsplitter
Incoming light
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2005 Oct 8 Stable Beams, 100 Turns
Major axis
Minor axis
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Injecting into an Empty LER Bucket

• 2005 August 5
• 44 turns, starting near first turn
• Hard to get enough light to see a small injected
  bunch
• Viewed y axis only
• Summed 3 images
• Triggered on PEP injection fiducial
• Images dont show any specific bucket, but
  whichever one is getting injection.

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Adding a Second Injection
New charge oscillating around previously injected
bunch
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Now the Third Injection
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Measuring Beam Aborts

• The problem of fast LER dI/dt aborts, not
  injection backgrounds, became my objective in
  November.
• Triggering to view milliseconds before an abort
• Image one bunch every nth turn over a longer
  time.
• Image every 80th turn (587 ms) 125 times 10,000
  turns (73 ms).
• Trigger on the ring-turn clock, not the injection
  fiducial.
• Repeat as fast as possible.
• 2 Hz due to cameras readout and transfer to
  computer.
• Chance of capturing any one abort is then 73/500
  15.
• Disable cameras trigger on an abort.
• Abort logic signal was available near LER hutch.
• HER hutch is remote, needed tunnel access to pull
  a 200-m cable.
• Abort logic should be completed this week.

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2005 Dec 7 Blow-up in x and y in lt5ms

• Correlation with BPM and transverse-feedback
  abort data shows
• Centroid motion in BPMs for 1 ms.
• Seen here in last image before blow-up.
• Current drops for 3 ms while beam blows up.
• Abort triggered by LER current loss (dI/dt).

75 ms
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2005 Dec 7 Transverse Instability
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Longitudinal Axis Streak Camera

• Abort spectra showed 2ns Quadrupole motion?
• Borrowed streak camera from Berkeley (ALS) at
  Christmas.
• Set-up
• On LER table. A small focus, no cylindrical
  imaging.
• 500-ps fast time sweep vertically.
• Dual axis 20-ms sweep horizontally.
• Image one bunch every 250 ms.
• 80 images across the screen, with overlap, making
  a stripe.
• Images separated by 133 ms (eight 60-Hz periods).
• 15 chance of capturing any one abort.
• But aborts since Christmas caused mostly by IP
  vacuum, not dI/dt.
• Have not yet caught a dI/dt abort, but have taken
  interesting images.

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Dual-Axis Streak of Stable Beam
500 ps
60-Hz camera blanking
Bowed envelope 60-Hz subharmonic on trigger?
20 ms
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Radiation Abort No Longitudinal Motion
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Adding Charge to a Bucket during a Fill
After first injection. Dim image? 0.1 mA in
bunch.
Next image, 133 ms later. Oscillation has damped.
Second injection enters, a bit late in
phase. Image brightens. Injected charge
oscillates, starts to damp.
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Longitudinal Growth and Recovery
9 ms
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Whats Next?

• Install HER abort monitoring.
• Cable has been run interface board is almost
  ready.
• On-line beam sizes at IP using machine model.
• I have started to add this code to my SLM
  software.
• Add bunch-by-bunch capability to x-ray monitor.
• Presented to the MAC last year, as an upgrade
  option.
• Scan an x-ray mask through the image plane.
• A joint project with Caltech (if their grant is
  funded), with application to the ILC damping
  rings as well.

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Bunch-by-Bunch Transverse Profiles
Slots scanning across a 5s beam ellipse

• Modeled on a wire scanner.
• A rotating x-ray mask based on modified optical
  chopper wheel.
• 100-mm-thick tungsten.
• 3 moving slots on image plane.
• Form projections on x, y, and u (45?) axes as
  slots move.
• Followed by a 1-ns scintillator and PMT.
• Fast digitizer and electronics (FPGA) sort pulses
  by bunch.
• Get profiles for fitting.
• Will use the IGP board being built as a
  longitudinal feedback upgrade.