Synchrotron-Light Monitors

Alan Fisher

PEP-II Machine Advisory Committee

2006-10-26

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Topics

Turn-by-turn measurements with the rotating mirror Beam-abort movies LER x-ray monitor IP beam size HER M1 inspection Streak camera Bunch-by-bunch x-ray profiles

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Turn-by-Turn Transverse Imaging

As you saw last time, this new diagnostic can: 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; Injection, as charge is added to the bunch; Instability, perhaps leading to a beam abort.

LER setup was first. Now available for both rings. Some new images…

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Sweep Images across Gated Camera

Project the transverse image onto one axis at a time. Reshape the beam ellipse with cylindrical lenses.

Different horizontal and vertical magnifications. Lenses turn the ellipse into a thin vertical stripe.

Split the light into two paths, rotate one, form two images. One projection along the beam’s x axis, another along y. Image these projections onto camera at different heights.

Retrigger the fast gated camera on one bunch over many consecutive turns (or every nth turn).

Rapidly rotating mirror sweeps images across camera. One camera readout then captures many turns.

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Bolt-Holt Grid, X (inches)

Bo

lt-H

ole

Gri

d,

Y (

inch

es)

Major Path

Minor Path

Major-Path Image

Minor-Path Image

Bunch Train

Table Corners

Bolt Hole Grid

Optical Table Layout for LER Hutch

Incoming light

Polarizer

Color filter

Beamsplitter

Gated-camera locations

Turn-by-turn imagingBunch-by-bunch

imaging (also for streak camera)

Scanning mirror

Out-of-plane periscope (rotates 90)

In-plane periscope(no rotation)

Cylindrical lenses

Flip-up mirror(bunch-by-bunch

imagingor streak camera)

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1st and 2nd Injections into a LER Bucket

80 turns = 587 s

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Minor-axis projection showing injected charge arriving on the 13th of 80 consecutive turns.

(a) Bucket is initially empty.

(b) 2nd injection merges with previously stored bunch.

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1st and 2nd Injections, 800 Turns

800 turns = 5.87 ms

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Again 80 images, but now taken on every 10th turn.

(a) Initially empty bucket.

(b) 2nd injection.

Injected charge oscillates about stored orbit and begins to damp, although time interval is an order of magnitude below the damping time.

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2005-12-13: LER Transverse Instability

73 ms0

LER oscillating following a HER abort.

x axis of beam

y axis of beam

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Measuring Beam Aborts

Triggering to view milliseconds before an abort: Image one bunch every nth turn over a longer time.

Image every 80th turn (587 s) 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 camera’s readout and transfer to computer. Chance of capturing any one abort is then 73/500 = 15%.

Disable camera’s trigger on an abort to freeze final image. You saw images of “fast LER dI/dt” aborts at

January’s review. HER had similar fast blow-up aborts later in the run.

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2006-07-03: HER y blow up in 14 ms

73 ms0

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Beam-Abort Movies: Instant Replays

Using frame grabbers in SLM computers SLMs use multiplexed 4-channel frame grabbers (beam

image, interferometer, alignment,…). Software now records a 1.5-s (45-frame) movie every time

it switches to the beam-image camera. Disadvantage: Not always recording this channel during the abort.

Previous movie is overwritten, except after an abort. Then the movie is saved to disk.

Using a digital video recorder in Main Control Artem Kulikov got a 4-input DVR with 24-hour capacity.

Records 3 SLM images and screen of transverse-feedback scope. Advantage: Always available after the abort. Disadvantage: Some resolution loss from cable-TV system and

from compression of 4 signals onto one recorded video track.

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LER Synchrotron X-Ray Monitor

At the last review, I showed images from the SXM. Dipole radiation from Arc 7, via a modified photon stop. Pinhole, beryllium window, scintillator, video camera.

In May, a pinhole leak developed in the Be window. Valve automatically closed to protect LER vacuum. Pressure on SXM side of valve rose to 500 nTorr.

No access to repair the leak until after the run. New window is now in place.

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Transporting Beam Sizes to the IP

Transfer the sizes at the three SLMs to the IP. Based on the measured model for each ring. Code added to the LabView program for SLM analysis.

Updates the IP values with every measurement.

For LER Using the SLM or SXM alone gave unphysical answers. An overconstrained solution using SVD to combine data

from both has given promising numbers. Devised after the SXM leak. Haven’t yet run this on-line.

For HER x size at SLM (and so at IP) is double what is expected. Primary mirror suspected, now being examined.

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IP Beam Sizes: LabView Display

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Primary Mirror of HER SLM

Using downtime to inspect HER’s first mirror (M1). To understand large x size observed with SLM.

M1 is in a dipole chamber, almost flush with vertical wall for grazing incidence.

Borescope inspected the mirror in situ. Protruding too far into the chamber. Upbeam edge not masked by cutout in copper wall. Synchrotron radiation on leading edge at normal incidence. Thermal distortion is possible.

Observed changes in image as ring was filled. Mirror position can be adjusted. Tools are in preparation.

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Interferometric Inspection of HER M1

Converted optical path into a Michelson interferometer.

Expanded HeNe laser on table. Split beam near CCD camera. Reference arm on table. Second arm (10 m) hits M1,

reflects from temporary mirror.

M1 has a midplane slot to bypass the hot x-ray fan.

Observed fringe pattern on either side.

Fringes show curvature, but not substantial. No damage.

Doesn’t show any curvature from thermal distortion.

Above slot

Below slot

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Streak Camera

At times we have borrowed a streak camera from Berkeley (ALS), plus the synchroscan plug-in from Argonne (APS). Images using the ALS camera were shown last January.

We have just purchased a similar camera for PEP. Hamamatsu C5680 with dual axis and synchroscan. Digital CCD for better resolution and triggering.

Arrived in September, too late for the PEP run. SPEAR is now turning on after their shutdown.

Now commissioning the new streak camera on their diagnostic beamline.

Will move the camera to the LER when PEP turns on.

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What’s Next?

Add bunch-by-bunch capability to the x-ray monitor. Described at the last MAC review. Will scan an x-ray mask through the image plane.

Gives projection of the beam onto one axis.

Fast digitizer and electronics (FPGA) will sort photomultiplier pulses from each bunch. Get profiles from each bunch for fitting. Will use a new multipurpose bunch-by-bunch processor.

Developed by Dmitry Teytelman. Now being fabricated by Jeff Olsen. Initial application will be upgrading the bunch-current monitor. Other uses: feedbacks, bunch-by-bunch luminosity.

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Bunch-by-Bunch Transverse Profiles

Modeled on a wire scanner. A rotating x-ray mask based

on modified optical chopper wheel.

100-m-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.

Slots scanning across a 5σ beam ellipse