1 / 41

The Hall D Photon Beam Overview

Hall D Tagger and Beamline Review Nov. 19-20, 2008, Newport News. The Hall D Photon Beam Overview. presented by. Richard Jones, University of Connecticut. GlueX Tagged Beam Working Group. Jefferson Laboratory University of Connecticut Catholic University of America University of Glasgow.

Download Presentation

The Hall D Photon Beam Overview

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Hall D Tagger and Beamline Review Nov. 19-20, 2008, Newport News The Hall D Photon Beam Overview presented by Richard Jones, University of Connecticut GlueX Tagged Beam Working Group Jefferson Laboratory University of Connecticut Catholic University of America University of Glasgow

  2. Outline • Photon beam requirements • Photon beam collimation • Beam rates and polarization • Electron beam requirements • Diamond crystal requirements • Beam monitoring and instrumentation Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  3. 107g/s dE 0.5% E I. Photon Beam Requirements Direct connections with the physics goals of the GlueX experiment: • Energy • Polarization • Intensity • Resolution solenoidal spectrometer meson/baryon resonance separation lineshape fidelity up to mX= 2.5 GeV/c2 8.4-9.0 GeV 40 % adequate for distinguishing reactions involving opposite parity exchanges provides sufficient statistics for PWA on reactions down to 100nb in 5 years† better than resolution of the GlueX calorimeters and tracking system † Assumes 107 events and 20% acceptance. Design goal is 108g/s – factor 10 higher luminosity. Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  4. Photon Beam Requirements, continued • Tagger coverage – 3 ranges • Tagging efficiency† • Energy calibration • Polarization measurement • Tagger backgrounds tagging within the coherent peak • 8.3 – 9.1 GeV • 3.0 – 9.0 GeV • 9.0 – 11.7 GeV crystal alignment, spectrum monitoring endpoint tagging, spectrum monitoring 70% in coherent peak < 60 MeV r.m.s. absolute < 3% r.m.s. absolute < 1% of tagging rate †Defined as the ratio of tagged photons on target to tagged electrons in the tagger focal plane Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  5. II. Coherent Bremsstrahlung Beam Line needs a better figure • Coherent bremsstrahlung beam contains both coherent and incoherent components. • Only the coherent component is polarized. • Incoherent component is suppressed by narrow collimation. Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  6. incoherent (black) and coherent (red) kinematics Effects of Collimation Purpose: to enhance high-energy flux and increase polarization effects of collimation at 80 m distance from radiator bremsstrahlung angle diameter Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  7. Photon Beam Collimation Geometry • Determine constraints from beam emittance, radiator size, and radiator quality on collimator geometry. • Optimize collimation angle as a compromise between high beam polarization and high tagging efficiency. Steps taken to fix the collimator geometry: Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  8. Photon Beam Collimation Geometry (vertical scale is expanded ~105) • : beam emittance (rms) e : electron beam divergence angle C: characteristic bremsstralung angle D r v c nominal beam axis e C (1)  = v e (2) r = D e (3) c = D C / 2  << r C / 2 v << c electron beam dump collimator radiator  << 3 x 10-8 m.r Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  9. Photon Beam Collimation Geometry (vertical scale is expanded ~105) (1)  = v e D (2) r = D e (3) c = D C / 2 r v c nominal beam axis Length scale for D: e convoluted with crystal mosaic spread m sets scale for smearing of coherent edge. e C m ~ 20 µr e = 20 µr electron beam dump collimator radiator and thus r = 1.5 mm D = 75 m Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  10. Photon Beam Collimation Angle • As collimator aperture is reduced: • polarization grows • tagging efficiencydrops off m = mass of electron E = electron beam energy m/E = characteristic bremsstrahlung angle diameter Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  11. Polarization and Tagging Efficiency Limits effects of collimation on polarization spectrum collimator distance = 80 m effects of collimation on figure of merit: rate (8-9 GeV) * p2 @ fixed hadronic rate collimator diameter linear polarization curves end where tagging efficiency e < 30% Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  12. III. Beam Rates and Polarization • Rates based on: • 12 GeV endpoint • 20 mm diamond crystal • 2.2 mA electron beam • Leads to 108g/s on target • (after the collimator) tagging interval Design goal is to build a photon source with 108g/s in the range 8.4 – 9.0 GeV and peak linear polarization 40%. Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  13. Summary of Collimated Beam Properties peak energy8 GeV 9 GeV 10 GeV 11 GeV N in peak185 M/s 100 M/s 45 M/s 15 M/s peak polarization0.54 0.41 0.27 0.11 (f.w.h.m.)(1140 MeV) (900 MeV) (600 MeV) (240 MeV) peak tagging eff.0.55 0.50 0.45 0.29 (f.w.h.m.)(720 MeV) (600 MeV) (420 MeV) (300 MeV) power on collimator5.3 W 4.7 W 4.2 W 3.8 W power on H2 target 810 mW 690 mW 600 mW 540 mW total hadronic rate385 K/s 365 K/s 350 K/s 345 K/s (in tagged peak) (26 K/s) (14 K/s) (6.3 K/s) (2.1 K/s) 1 4 1 1 2,3 • Rates reflect a beam current of 2.2 mA which corresponds to 108g/s in the coherent peak. • Total hadronic rate is dominated by the nucleon resonance region. • For a given electron beam and collimator, background is almost • independent of coherent peak energy, comes mostly from incoherent part. • 4. Does not include 30% improvement obtained by selecting one fiber row in the microscope. Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  14. IV. Electron Beam Requirements Summary of key results: energy 12 GeV r.m.s. energy spread < 60 MeV transverse x emittance < 10 mm µr transverse y emittance < 2.5 mm µr minimum current 700 pA maximum current 5 µA x spot size at radiator0.8–1.6 mm r.m.s. y spot size at radiator 0.3–0.6 mm r.m.s. x spot size at collimator < 0.5 mm r.m.s. y spot size at collimator < 0.5 mm r.m.s. position stability ±200 µm beam halo<10-5 @ r>5mm • beam energy and energy spread • range of deliverable beam currents • beam emittance • beam position controls • upper limits on beam halo Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  15. Electron Beam Requirements: current • upper bound of 3 mA projected for GlueX at high intensity corresponding to 108g/s on the GlueX target. • with safety factor, translates to 5 mAfor the maximum current to be delivered to the Hall D electron beam dump • during running with 20 micron crystal at 108g/s : I =2.2 A • lower bound of 0.7 nA is required to permit accurate measurement of the tagging efficiency using a in-beam total absorption counter during special low-current runs. Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  16. Integrated tail current is less than of the total beam current. 10-5 Electron Beam Requirements: halo • two important consequences of beam halo: • impact active collimator accuracy • backgrounds in the tagging counters • Beam halo model: • central Gaussian • power-law tails • Requirement: • Definition: “tails” are whatever extends outside r = 5 mm from the beam axis. central Gaussian power-law tail central + tail log Intensity r / s 5 2 3 4 1 Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  17. V. Diamond crystal requirements • orientation requirements • mosaic spread requirement • thickness requirements • radiation damage lifetime • mount and heat relief Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  18. Diamond crystal requirements: orientation (mr) • orientation angle is relatively large at 9 GeV: 3 mr • initial setup takes place at near-normal incidence • goniometer precision requirements for stable operation at 9 GeV are not severe. alignment zone operating zone microscope translation step: 200 μm horizontal 25 μm target ladder (fine tuning) rotational step: 1.5 μrad pitch and yaw 3.0 μrad azimuthal rotation fixed hodoscope Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  19. rms angular deviation = “mosaic spread” mosaic of quasi-perfect domains Diamond crystal requirements: mosaic • Actually includes other kinds of effects • distributed strain • plastic deformation • Measured directly by width of X-ray diffraction peaks: “rocking curves” Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  20. Diamond crystal requirements: mosaic • X-ray diffraction of crystals • but peaks have width • natural width: quantum mechanical zero-point motion, thermal • mosaic spread: must be measured • contributions add in quadrature l = 2 d sin(q) q q d Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  21. Diamond crystal requirements: mosaic rocking curve from X-ray scattering • Example rocking curve • Actual measurement of a high-quality synthetic diamond from industry (Element Six) • X-ray rocking curve measurements require a synchrotron light source • Daresbury, UK (SRS) – now phased out • Cornell, NY (CHESS) – present facility of choice intensity natural width (fwhm) Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  22. Diamond crystal requirements: thickness Choice of thickness is a trade-off between MS and radiation damage. • Design calls for a diamond thickness of20 mmwhich is approx.1.7 x 10-4 rad.len. • Requires thinning: special fabrication steps and $$. • Impact from multiple-scattering is significant. • Loss of rate is recovered by increasing beam current, up to a point… -4 -3 Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  23. 0.25 C / mm2 Diamond crystal requirements: lifetime • conservative estimate (SLAC) for useful lifetime (before significant degradation): • conservative estimate: 3-6 crystals / year of full-intensity running • More details provided in a later talk. Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  24. Diamond crystal requirements: mounting temperature profile of crystal at full intensity, radiation only Heat dissipation specification for the mount is not required. oC y (mm) translation step: 200 μm horizontal 25 μm target ladder (fine tuning) rotational step: 1.5 μrad pitch and yaw 3.0 μrad azimuthal rotation x (mm) diamond-graphite transition sets in ~800oC Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  25. VI. Beam Monitoring – Photon Beam Position Specification for the “active collimator” photon beam position monitor • The virtual electron spot must be centered on the collimator. • Tolerance set by effect of offset on collimated intensity spectrum • Photon beam position is controlled by steering magnets ~100 m upstream • Feedback from active collimator to electron beam position stabilization system is planned. dx < 200 mm Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  26. Active Collimator Design primary collimator (tungsten) • Tungsten pin-cushion detector • reference: Miller and Walz, NIM 117 (1974) 33-37 • measures current due to knock-ons in EM showers • performance is known active device incident photon beam Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  27. Active Collimator Simulation tungsten plates current asymmetry vs. beam offset tungsten pins y (mm) 20% 40% 60% beam x (mm) 12 cm Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  28. Active Collimator Position Sensitivity using inner ring only for fine-centering Monte Carlo simulation ±200 mm of motion of beam centroid on photon detector corresponds to ±5% change in the left/right current balance in the inner ring Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  29. Active Collimator Prototype Beam Tests Beam test in Hall B during G11 run, April 2007 • coherent bremsstrahlung beam • end-point energy 5.05 GeV • two opposing inner sectors instrumented in prototype • collimator was swept across the beam in steps of 0.5 mm • beam intensity ~ 1% of full intensity in Hall D. inner wedges, raw data inner cable outer Intensity in good agreement with Monte Carlo simulations. 0 Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  30. Photon Beam Spectrum Monitoring • tagger broad-band counter array • necessary for crystal alignment during setup • provides a continuous monitor of beam/crystal stability • electron pair spectrometer • measures post-collimated photon beam spectrum • 10-3 radiator located upstream of pair spectrometer enables continuous monitoring during normal running • essential for determination of the beam polarization Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  31. Photon Beam Polarimetry Comparison between CBSA polarization spectrum and measurement with pair polarimeter at Yerevan Synchrotron (NIMA 579 (2007) p.973–978) • Method: CBSA – Coherent Bremsstrahlung Spectrum Analysis • Measure both the pre-collimated and post-collimated beam spectra. • Fit primary peak region in both spectra to a model of the source + collimation system. • Model gives polarization spectrum 5% stat. 2-3% syst. direct measurement spectrum measured in pair polarimeter CBSA prediction data points model fit curve Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  32. Other Photon Beam Instrumentation • visual photon beam monitors • total absorption counter • safety systems Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  33. Summary • A design has been put forward for a polarized photon beam line that meets the requirements for the experimental program in Hall D. • The design parameters have been carefully optimized for operation with 40% polarization at 9 GeV. • The implications of the photon source design for the 12 GeV electron beam have been worked out and shown to be compatible with the 12 GeV accelerator design. • Quality assurance procedures for selection and procurement and of thin diamond crystals have been developed that can ensure a supply of radiators with the required properties. • The design includes sufficient beam line instrumentation to insure stable operation, with polarization uncertainty < 3%. Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  34. backup slides Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  35. Coherent Bremsstrahlung Source – Flexibility • For a fixed electron beam energy of 12 GeV, the peak polarization and the coherent gain factor are both steep functions of peak energy. • CB polarization is a key factor in the choice of a energy range of 8.4 – 9.0 GeV for GlueX. • Higher polarization can be obtained by running at lower peak energies to concentrate on a reduced mass range. Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  36. Coherent Bremsstrahlung with Collimation No other solution was found that could meet all of these requirements at an existing or planned nuclear physics facility. Unique: • A laser backscatter facility would need to wait for new construction of a new multi-G$ 20GeV+ storage ring (XFEL?). • Even with a future for high-energy beams at SLAC, the low duty factor <10-4 essentially eliminates photon tagging there. • The continuous beams from CEBAF are essential for tagging and well-suited to detecting multi-particle final states. • By upgrading CEBAF to 12 GeV, a 9 GeV polarized photon beam can be produced with high polarization and intensity. Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  37. Coherent Bremsstrahlung Source Polarization Linear polarization arises from the two-body nature of the CB kinematics • linear polarization • determined by crystal orientation • vanishes at end-point • independent of electron polarization • circular polarization • transfer from electron beam • reaches 100% at end-point Linear polarization has unique advantages for GlueX physics: a requirement Changes the azimuthal F coordinate from a uniform random variable to carrying physically rich information. Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  38. Overview of Photon Beam Stabilization • Monitor alignment of both beams • BPM’s monitor electron beam position to control the spot on the radiator and point at the collimator • BPM precision in x is affected by the large beam size along this axis at the radiator • independent monitor of photon spot on the face of the collimator guarantees good alignment • photon monitor also provides a check of the focal properties of the electron beam that are not measured with BPMs. 3.5 mm 1s contour of electron beam at radiator 1.1 mm Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  39. Photon Beam Position Controls • electron Beam Position Monitors provide coarse centering • position resolution 100 mm r.m.s. • a pair separated by 10 m : ~1 mm r.m.s. at the collimator • matches the collimator aperture: can find the collimator • primary beam collimator is instrumented • provides photon beam position measurement • position sensitivity out to 30 mm from beam axis • maximum sensitivity of 200 mm r.m.s. within 2 mm Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  40. Active Collimator Simulation beam 12 cm 5 cm Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

  41. Detector response from simulation beam centered at 0,0 10-4 radiator Ie = 1mA inner ring of pin-cushion plates outer ring of pin-cushion plates Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News

More Related