1. The GlueX Photon Beam Richard Jones, University of Connecticut GlueX Photon Beamline-Tagger ReviewJan. 23-25, 2005, Newport News presented by GlueX Tagged Beam Working Group University of Glasgow University of Connecticut Catholic University of America

2. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 2 Presentation Overview Photon beam properties Competing factors and optimization Electron beam requirements Beam monitoring and instrumentation Diamond crystal requirements

3. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 3 I. Photon Beam Properties Direct connections with the physics goals of the GlueX experiment: Energy Polarization Intensity Resolution 9 GeV 40 % 10 7  /s 10 -3 EE E solenoidal spectrometer meson/baryon resonance separation 2.8GeV/c 2 forward m X production up to 2.8GeV/c 2 adequate for resolving resonances opposite dominant exchanges with opposite dominant exchanges PWA provides sufficent statistics for PWA on major channels with ~1yr running all-charged modes matches resolution of the GlueX spectrometer in all-charged modes

4. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 4 No other solution was found that could meet all of these requirements at an existing or planned nuclear physics facility. Coherent Bremsstrahlung with Collimation A laser backscatter facility would need to wait for new construction of a new multi-G$ 20GeV+ storage ring (XFEL?). With the closure of SLAC, Jefferson Lab is the unique place where high-energy polarized photons can be found. The continuous beams from CEBAF are ideal for an experiment seeking to detect high-multiplicity final states. By upgrading CEBAF to 12 GeV, a 9 GeV polarized photon beam can be produced with high polarization and intensity. Unique:

5. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 5 Kinematics of Coherent Bremsstrahlung effects of collimation at 80 m distance from radiator incoherent (black) and coherent (red) kinematics effects of collimation: to enhance high-energy flux and increase polarization

6. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 6 circular polarization  transfer from electron beam  reaches 100% at end-point linear polarization  determined by crystal orientation  vanishes at end-point Polarization from Coherent Bremsstrahlung Linear polarization arises from the two-body nature of the CB kinematics Linear polarization has unique advantages for GlueX physics: a requirement not a requirement Circular polarization is potentially useful as well, but not a requirement to achieve the physics goals of GlueX.

7. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 7 Photon Beam Intensity Spectrum 4 nominal tagging interval Rates based on: 12 GeV endpoint 20  m diamond crystal 100nA electron beam Leads to 10 7  /s on target (after the collimator) Design goal is to build an experiment with ultimate rate capability as high as 10 8  /s on target.

8. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 8 II. Optimization photon energy vs. polarization crystal radiation damage vs. multiple scattering collimation enhancement vs. tagging efficiency Understanding competing factors is necessary to optimize the design

9. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 9 Optimization: chosing a photon energy 8 GeV10 GeV A minimum useful energy for GlueX is 8 GeV, 10 GeV would be better for several reasons, steep function of peak energy for a fixed endpoint of 12 GeV, the coherent gain factor is a steep function of peak energy. CB polarization is a key factor in the choice of a peak energy of 9 GeV for GlueX but

10. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 10 Optimization: choice of diamond thickness 20  m 10 -4 rad.len Design calls for a diamond thickness of 20  m which is approximately 10 -4 rad.len. thinning Requires thinning: special fabrication steps and $$. Impact from multiple- scattering is significant. up to a point… Loss of rate is recovered by increasing beam current, up to a point… The choice of 20  m is a trade-off between MS and radiation damage.

11. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 11 Optimization: scheme for collimation The argument for why a new experimental hall is required for GlueX the short answer: because of beam emittance virtual electron spot 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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 12 Optimization: radiator – collimator distance a < 20 mr s 0 < 1/3 c d > 70 m With increased collimator distance:  polarization grows  low-energy backgrounds shrink  tagging efficiency  tagging efficiency drops off

13. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 13 Optimization: varying the collimator diameter linear polarization effects of collimation on polarization spectrum collimator distance = 80 m 5 figure of merit: effects of collimation on figure of merit: rate (8-9 GeV) * p 2 @ fixed hadronic rate

14. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 14 peak energy 8 GeV 9 GeV 10 GeV 11 GeV N  in peak 185 M/s 100 M/s 45 M/s 15 M/s peak polarization 0.54 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 collimator 5.3 W 4.7 W 4.2 W 3.8 W power on H 2 target 810 mW 690 mW 600 mW 540 mW total hadronic rate 385 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) Results: summary of photon beam properties the maximum current a factor of 10 lower 1.Rates reflect a beam current of 3  A which corresponds to 10 8  /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. 2.Total hadronic rate is dominated by the nucleon resonance region. 3.For a given electron beam and collimator, background is almost independent of coherent peak energy, comes mostly from incoherent part. 2,3 1 1 1

15. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 15 III. Electron Beam Requirements beam energy and energy spread range of deliverable beam currents beam emittance beam position controls upper limits on beam halo Specification of what electron beam properties are consistent with this design

16. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 16 Electron Beam Energy effects of endpoint energy on figure of merit: rate (8-9 GeV) * p 2 @ fixed hadronic rate  The polarization figure of merit for GlueX is very sensitive to the electron beam energy. Requirement: >12 GeV  Decreasing the upgrade energy by only 500 MeV would have a substantial impact on GlueX.

17. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 17 Electron Beam Energy Resolution 0.01 % r.m.s. beam energy spread  E/E requirement: < 0.01 % r.m.s. 0.06 % compares favorably with best estimate: 0.06 % absolute energy scale determination to 0.1% via known meson masses in the GlueX detector: eg.  (1020)  p  +  +  - n 1.tied to the energy resolution requirement for the tagger 2.derived from optimizing the missing- mass resolution in the channels with a missing final-state particle. Typical channel analyzed using the missing mass technique:

18. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 18 3  A upper bound of 3  A projected for GlueX at high intensity corresponding to 10 8  /s on the GlueX target. 5  A with safety factor, translates to 5  A for the maximum current to be delivered to the Hall D electron beam dump I =300 nA during running at a nominal rate of 10 7  /s : I = 300 nA 100 pA total absorption counter lower bound of 100 pA is required to permit accurate measurement of the tagging efficiency using a in-beam total absorption counter during special low-current runs. similar to low-current operation in Hall B Range of Required Beam Currents

19. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 19 Electron Beam Emittance <10 -8 m r requirement : < 10 -8 m r all expressions are r.m.s. values derivation :  virtual spot size: 500  m  radiator-collimator: 76 m  crystal dimensions: 5 mm In reality, one dimension (y) is much better than the other ( x 2.5) This is a key issue for achieving the requirements for the GlueX Photon Beam Optics study: goal is achievable, but close to the limits according to 12 GeV machine models

20. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 20 Hall D Optics Conceptual Design Study energy12 GeV r.m.s. energy spread7 MeV transverse x emittance10 mm µr transverse y emittance2.5 mm µr minimum current100 pA maximum current5 µA x spot size at radiator1.6 mm r.m.s. y spot size at radiator0.6 mm r.m.s. x spot size at collimator0.5 mm r.m.s. y spot size at collimator0.5 mm r.m.s. position stability±200 µm Summary of key results:

21. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 21 Must satisfy two criteria: 1.The virtual electron spot must be centered on the collimator. 2.A significant fraction of the real electron beam must pass through the diamond crystal.  x <  m criteria for “centering”:  x <  m ~100m upstream controlled by steering magnets ~100m upstream Electron Beam Position Controls 1.Using upstream BPM’s and a known tune, operators can “find the collimator”. 2.Once it is approximately centered (  5 mm ) an active collimator must provide feedback.

22. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 22 Electron Beam Halo two important consequences of beam halo: 1.distortion of the active collimator response matrix 2.backgrounds in the tagging counters Beam halo model:  central Gaussian  power-law tails Requirement: r /  central Gaussian power-law tail central + tail 1 234 5 Integrated tail current is less than of the total beam current. 10 -5 ~  -4

23. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 23 Photon Beam Position Controls conventional BPM’s provide coarse centering 100  m r.m.s.  position resolution 100  m r.m.s. 4mm r.m.s. at the collimator  a pair separated by 2m : 4mm r.m.s. at the collimator can find the collimator  matches the collimator aperture: can find the collimator primary beam collimator is instrumented  provides “active collimation” 30mm  position sensitivity out to 30mm from beam axis 200  m r.m.s.  maximum sensitivity of 200  m r.m.s. within 2mm

24. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 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. 1.1 mm 3.5 mm 1  contour of electron beam at radiator

25. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 25 Active Collimator Design 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 et.al.) latest users, built new ones  performance is known active device primary collimator (tungsten) incident photon beam

26. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 26 Active Collimator Simulation 12 cm5 cm

27. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 27 12 cm x (mm) y (mm) current asymmetry vs. beam offset 20% 40% 60% Active Collimator Simulation

28. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 28 Detector response from simulation inner ring of pin-cushion plates outer ring of pin-cushion plates beam centered at 0,0 10 -4 radiator I e = 1  A

29. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 29 Active Collimator Position Sensitivity using inner ring only for fine-centering ±200  m of motion of beam centroil on photon detector corresponds to ±5% change in the left/right current balance in the inner ring

30. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 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. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 31 Other Photon Beam Instrumentation visual photon beam monitors total absorption counter safety systems

32. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 32 V. Diamond crystal requirements how much to say here? should probably be brief

33. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 33 Diamond Crystal Properties limits on thickness from multiple-scatteringrocking curve from X-ray scattering natural fwhm

34. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 34 Diamond crystal: goniometer mount temperature profile of crystal at full operating intensity oCoC

35. Richard Jones, GlueX Beamline-Tagger Review, Newport News, Jan 23-25, 2006 35 Summary review highlights