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Notes on GlueX Tagger Review Dry Run
Notes on GlueX Tagger Review Dry Run
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Comments compiled from notes from Bernhard, Arne, Paul and Elton.
(Disclaimer: any transcription errors are mine. Elton)
1. Introduction
- If cost and schedule are to be reviewed, this must be explicitly stated
in charge.
2. Overview of Tagger/Beamline (Jim Kellie)
- Need to put down a table of requirements for the electron and photon
beams. This includes location and capabilities of all diagnostic devices.
Work with accelerator to flush this out.
- Keep electron current/tagger rate/pair spectrometer rates within 1-2%:
Justify and add to specification list.
- ebeam diagnostics: cavity BPMs, but need detailed specification
- Jay: To achieve stability need active stabilization. Review experience
of position stability for g8.
Photon polarimetry:
- Determine polarisation by comparing calculated to measured
bremsstrahlung spectra (collimated and uncollimated), estimated accuracy
5%.
- Need specification on uncertainly of photon beam polarization.
- Si strip + pair spectrometer at entrance to Hall D (possible concern
about spray from collimator box)
- g8 changed polarization from horizontal to vertical polarization every
20 min to adjustments to diamond.
3. Tagger vacuum chamber design (Jim Kellie)
Hodoscope:
- Focal plane is about 9 m long It would be useful to superimpose region
of coherent peak (8.5 - 9.0) GeV onto focal plane for visual impression.
-141 fixed scintillators (0.5% resolution). Present idea is to cover only
a small fraction via sampling
-121 movable microscope (0.1% resolution). May want to cover top part of
bremsstrahlung spectrum continuously to veto feeddown from high-energy
photons. Dan Sober: under discussion, rates tolerable - need to give
rates.
- Venitian Blinds for sampling/full coverage?
- What is the spot size of the electrons due to multiple scattering and
natural width relative to tagging counters?
Vacuum Chamber:
- 12.5 m long, 0.8 m wide
- uses pole as support and rubber O-ring seals (compressed), 70 tons of
force
- back rods needed?
- Length of vacuum chamber/ background from flanges? Kashy: Options for
beam exiting vacuum box into He?
- Kashy: stress in magnet unrealistic for single solid model. Stress on
bolts can be much higher. If conclusions are drawn from comparisons
between Mainz measured/calculated rations this should be explained.
4. Coherent Bremsstrahlung (Richard Jones)
- Need to concentrate on nominal design.
- 40% polarization, Ntag=10^8 photons/s in coherent peak, Ne = 2.6*10^8/s
- circular polarization may be useful: if this is a requirement from
GlueX, it needs to be discussed with accelerator team.
- Position stability of photon beam at (virtual) focus is +/-200microns.
- Pin cussion photon beam position monitor in front of first collimator
(SLAC design relies on asymmetry of e+/e- production from delta-rays in
the shower)
- Background in tagger?
- How to specify tails? Currently using tails derived from Hall A Compton?
- Support for radiator should be conducting to drain charge.
- Vertical segmentation of counters is 70x5=350? Nominal count is 120 in
the detector summaries
- Pin-cushion design: specify the dynamic range. (If the readout is 1 nA
at 1micro beam current, to go down to 5 nA beam current we are reading out
5 pA current out of the electronics. Is this ok?)
- Do we need remote positioning of photon collimation/monitoring devices?
- Polarimetry: Use
comparison of bremsstrahlung spectrum with calculated
shape (accuracy estimated at 5%) and azimuthal distribution of e+e- pairs
using polarimeter.
Notes on beamline from Arne:
- Some of the mentioned diagnostic elements were not defined
[requirements/device...] at all [like the photon beam profile/position
monitor.
- Need to define requirements for all diagnostic elements, especially for
the electron beam to communicate information to accelerator.
Suggestions for organization of beamline talk:
1. Tagger magnet and detector
1. define the requirements for the incoming electron beam, to match
the tagger acceptance
2. properties of coherent bremsstrahlung
1. define the requirements of the incident electron beam
2. define the requirements/properties of the photon beam
3. beamline diagnostics, mostly photon, but a few electron beam
diagnostics to define the incoming beam.
1. list instrumentation that will determine that the electron beam
meets the requirements.
2. list instrumentation that will determine if the photon beam meets
the requirements.
4. summary
5. Magnetic field calculations (Guangliang Yang)
- OPERA magnetic field calculations + optics
- Show some detector sizes on slides that show trajectories. This might
help convey issues/concepts to audience.
- Start with single straight focal plane during presentation.
6. Tagger magnet assembly proceedure (Jim Kellie)
- Comment on figure inserts: very light and not readable
- Add discussion on magnet alignments at beginning of talk. Note that no
adjustments are possible after vacuum box is added to structure, but also
that mm-size tolerances are allowed by the O-rings.
- Put both magnets and 'rooster tail' vacuum box on a common
strong-back support. This will allow for settlement of building.
- Note that beam pipe is needed for photon beam (not shown on vacuum box).
Hole (or slot) also needed to be drilled through the back yoke.
- Don't need template to align pole faces (optical alignment will
suffice).
Additional comments on assembly (Paul):
Please revise all drawings to show a single weldment support for both
magnets and the vacuum box extension.
Testing is lacking at several critical points in the assembly sequence
Step 6 Hipoit test lower coil
step 10 leak test vacuum chamber
step 11 hipot upper and lower coils
step 13 hipot both coils
step 14 final vacuum leak test
Suggest you drop the "bent" focal plane results and the transport results.
It only confuses. Keep as backup if there are questions.
In general you should present your solution , leave out all the
alternatives that are not as good.
Elton Smith
Jefferson Lab
elton@jlab.org
(757) 269-7625