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The Hall D Photon Beam

The Hall D Photon Beam

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The Hall D Photon Beam

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  1. Hall D Tagger-Photon Beamline Review Jan. 23-24, 2005, Newport News The Hall D Photon Beam presented by Richard Jones, University of Connecticut GlueX Tagged Beam Working Group University of Glasgow University of Connecticut Catholic University of America

  2. Presentation Overview • Photon beam properties • Competing factors and optimization • Electron beam requirements • Beam monitoring and instrumentation • Diamond crystal requirements

  3. 107g/s dE 10-3 E I. Photon Beam Properties 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=2.8GeV/c2 9 GeV 40 % adequate for distinguishing reactions involving opposite parity exchanges provides sufficient statistics for PWA on key channels in initial three years matches resolution of the GlueX spectrometer tracking system

  4. 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.

  5. incoherent (black) and coherent (red) kinematics Kinematics of Coherent Bremsstrahlung effects of collimation: to enhance high-energy flux and increase polarization effects of collimation at 80 m distance from radiator

  6. Polarization from Coherent Bremsstrahlung Linear polarization arises from the two-body nature of the CB kinematics • linear polarization • determined by crystal orientation • vanishes at end-point • not affected by 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.

  7. 4 nominal tagging interval Photon Beam Intensity Spectrum • Rates based on: • 12 GeV endpoint • 20mm diamond crystal • 100nA electron beam • Leads to 107g/s on target • (after the collimator) Design goal is to build an experiment with ultimate rate capability as high as 108g/s on target.

  8. II. Optimization Understanding competing factors is necessary to optimize the design • photon energy vs. polarization • crystal radiation damage vs. multiple scattering • collimation enhancement vs. tagging efficiency

  9. Optimization: chosing a photon energy • A minimum useful energy for GlueX is 8 GeV;9-10 GeV is better for several reasons, • for a fixed endpoint 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 but

  10. Optimization: choice of diamond thickness • Design calls for a diamond thickness of20mmwhich is approximately10-4 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 The choice of 20mm is a trade-off between MS and radiation damage.

  11. Optimization: scheme for collimation The argument for why a new experimental hall is required for GlueX • the short answer: because of beam emittance • a key concept: the virtual electron spot on the collimator face. It must be much smaller than the real photon spot size for collimation to be effective but the convergence angle a must remain small to preserve a sharp coherent peak. Putting in the numbers…

  12. Optimization: radiator – collimator distance • < 20 mr s0 < 1/3 c c/d = 1/2 (m/E) • With decreased collimator angle: • polarization grows • tagging efficiencydrops off d > 70 m

  13. Optimization: varying the collimator diameter 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 linear polarization

  14. Results: summary of photon 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 1 1 2,3 • Rates reflect a beam current of 3mA which corresponds to 108g/s in the coherent peak, which is the maximum current foreseen to be used in Hall D. Normal GlueX running is planned to be at a factor of 10 lower intensity, at least during the initial running period. • 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.

  15. III. Electron Beam Requirements Specification of what electron beam properties are consistent with this design • beam energy and energy spread • range of deliverable beam currents • beam emittance • beam position controls • upper limits on beam halo

  16. Requirement: >12 GeV Electron Beam Energy effects of endpoint energy on figure of merit: rate (8-9 GeV) * p2 @ fixed hadronic rate • The polarization figure of merit for GlueX is very sensitive to the electron beam energy. • Decreasing the upgrade energy by only 500 MeV would have a substantial impact on GlueX.

  17. Electron Beam Energy Resolution • beam energy spread dE/E requirement: 0.1 % r.m.s. • compares favorably with best estimate: 0.06 % Typical channel where one of the particles might escape detection • tied to the energy resolution requirement for the tagger • derived from optimizing the ability to reject events with a missing final-state particle. gp K+K-p+p- p [p0]

  18. Range of Required Beam Currents • 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 at a nominal rate of 107g/s : I =300 nA • lower bound of 0.1 nA is required to permit accurate measurement of the tagging efficiency using a in-beam total absorption counter during special low-current runs.

  19. Electron Beam Emittance This is a key issue for achieving the requirements for the GlueX Photon Beam • requirement : <10-8 m•r • emittances are r.m.s. values • derivation : • virtual spot size: 500 mm • radiator-collimator: 76 m • crystal dimensions: 5 mm • In reality, one dimension (y) is much better than the other (x 2.5) Optics study: goal is achievable, but close to the limits according to 12 GeV machine models

  20. Hall D Optics Conceptual Design Study Summary of key results: energy 12 GeV r.m.s. energy spread 7 MeV transverse x emittance 10 mm µr transverse y emittance 2.5 mm µr minimum current 100 pA maximum current 5 µA x spot size at radiator1.6 mm r.m.s. y spot size at radiator 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

  21. Electron Beam Position Controls • Must satisfy two criteria: • The virtual electron spot must be centered on the collimator. • A significant fraction of the real electron beam must pass through the diamond crystal. • criteria for “centering”: dx < s / 2  200 mm • controlled by steering magnets ~100 m upstream Using upstream BPM’s and a known tune, operators can “find the collimator”. Once it is approximately centered ( 5 mm ) an active collimator must provide feedback.

  22. Integrated tail current is less than of the total beam current. 10-6 Electron Beam Halo • two important consequences of beam halo: • distortion of the active collimator response matrix • backgrounds in the tagging counters • Beam halo model: • central Gaussian • power-law tails • Requirement: • Further study is underway central Gaussian power-law tail central + tail log Intensity ~q-4 r / s 5 2 3 4 1

  23. 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 “active collimation” • position sensitivity out to 30 mm from beam axis • maximum sensitivity of 200 mm r.m.s. within 2 mm

  24. 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

  25. Active Collimator Design primary collimator (tungsten) • Tungsten pin-cushion detector • used on SLAC coherent bremsstrahlung beam line since 1970’s • SLAC team developed the technology through several iterations • reference: Miller and Walz, NIM 117 (1974) 33-37 • SLAC experiment E-160 (ca. 2002, Bosted latest users, built new ones • performance is known active device incident photon beam

  26. Active Collimator Simulation beam 12 cm 5 cm

  27. Active Collimator Simulation current asymmetry vs. beam offset y (mm) 20% 40% 60% x (mm) 12 cm

  28. 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

  29. Active Collimator Position Sensitivity using inner ring only for fine-centering ±200 mm of motion of beam centroid on photon detector corresponds to ±5% change in the left/right current balance in the inner ring

  30. Photon Beam Quality Monitoring • tagger broad-band focal plane counter array • necessary for crystal alignment during setup • provides a continuous monitor of beam/crystal stability • electron pair spectrometer • located downstream of the collimation area • sees post-collimated photon beam directly after cleanup • 10-3 radiator located upstream of pair spectrometer • pairs swept from beamline by spectrometer field and detected in a coarse-grained hodoscope • energy resolution in PS not critical, only left+right timing • coincidences with the tagger provide a continuous monitor of the post-collimator photon beam spectrum.

  31. Other Photon Beam Instrumentation • visual photon beam monitors • total absorption counter • safety systems

  32. V. Diamond crystal requirements • orientation requirements • limitations from mosaic spread • radiation damage assessment

  33. Diamond Orientation • 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 method described in a later talk (F. Klein) alignment zone operating zone microscope fixed hodoscope

  34. Diamond Crystal Quality rocking curve from X-ray scattering • reliable source of high-quality synthetics from industry (Univ. of Glasgow contact) • established procedure in place for selection and assessment using X-rays • R&D is ongoing towards reliable operation of one 20mm crystal (Hall B) natural fwhm

  35. 0.25 C / mm2 Diamond Crystal Lifetime • conservative estimate (SLAC) for useful lifetime (before significant degradation): • during initial running at 107g/s this gives 600 hrs of running before a spot move • a “good” crystal accommodates 5 spot moves • R&D is planned that will improve the precision of this estimate.

  36. 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 properties of the photon beam were generated and successfully simulated using the nominal parameters of the 12 GeV electron beam. • The design parameters have been carefully optimized. • The design includes sufficient beam line instrumentation to insure stable operation.

  37. Diamond crystal: goniometer mount temperature profile of crystal at full operating intensity oC