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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.

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The Hall D Photon Beam Overview

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  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

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