Subject: Notes on GlueX Tagger Review Dry Run From: Elton Smith Date: Wed, 16 Nov 2005 07:40:50 -0500 (EST) To: halld-jlab@jlab.org Notes on GlueX Tagger Review Dry Run ------------------------------------ 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=108 photons/s in coherent peak, Ne = 2.6*108/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