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

Hall D Tagger and Beamline Review, Nov , 2008, Newport News2 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 , 2008, Newport News3 I. Photon Beam Requirements Direct connections with the physics goals of the GlueX experiment: Energy Polarization Intensity Resolution GeV 40 % 10 7  /s 0.5% EE E solenoidal spectrometer meson/baryon resonance separation m X = 2.5 GeV/c 2 lineshape fidelity up to m X = 2.5 GeV/c 2 adequate for distinguishing reactions opposite parity exchanges involving opposite parity exchanges PWA provides sufficient statistics for PWA on reactions down to 100nb in 5 years † better than resolution of the GlueX calorimeters and tracking system † Assumes 10 7 events and 20% acceptance. Design goal is 10 8  /s – factor 10 higher luminosity.

Hall D Tagger and Beamline Review, Nov , 2008, Newport News4 Photon Beam Requirements, continued Tagger coverage – 3 ranges Tagging efficiency † Energy calibration Polarization measurement Tagger backgrounds tagging within the coherent peak i.8.3 – 9.1 GeV ii.3.0 – 9.0 GeV iii.9.0 – 11.7 GeV 70% in coherent peak < 60 MeV r.m.s. absolute < 3% r.m.s. absolute < 1% of tagging rate crystal alignment, spectrum monitoring endpoint tagging, spectrum monitoring † Defined as the ratio of tagged photons on target to tagged electrons in the tagger focal plane

Hall D Tagger and Beamline Review, Nov , 2008, Newport News5 Top View 75 m Tagger Area Experimental Hall D Electron beam / dump Coherent Bremsstrahlung photon beam Solenoid- Based detector Collimator Photon Beam dump Counting House Radiator PairSpectrometer Collimator Cave II. Coherent Bremsstrahlung Beam Line 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 , 2008, Newport News6 Effects of Collimation effects of collimation at 80 m distance from radiator Purpose: to enhance high-energy flux and increase polarization diameter incoherent (black) and coherent (red) kinematics

Hall D Tagger and Beamline Review, Nov , 2008, Newport News7 Photon Beam Collimation Geometry 1.Determine constraints from beam emittance, radiator size, and radiator quality on collimator geometry. 2.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 , 2008, Newport News8 v Photon Beam Collimation Geometry radiator D nominal beam axis electron beam dump C c r  :beam emittance (rms) e :electron beam divergence angle C :characteristic bremsstralung angle (1)  = v e (2) r = D e (3) c = D C / 2 v << c  << r C / 2  << 3 x m.r e collimator (vertical scale is expanded ~10 5 )

Hall D Tagger and Beamline Review, Nov , 2008, Newport News9 Photon Beam Collimation Geometry radiator collimator D nominal beam axis electron beam dump C c v r (1)  = v e (2) r = D e (3) c = D C / 2 Length scale for D: e convoluted with crystal mosaic spread m sets scale for smearing of coherent edge. m ~ 20 µr e = 20 µr r = 1.5 mmD = 75 m and thus e (vertical scale is expanded ~10 5 )

Hall D Tagger and Beamline Review, Nov , 2008, Newport News10 As collimator aperture is reduced:  polarization grows  tagging efficiency  tagging efficiency drops off Photon Beam Collimation Angle diameter m = mass of electron E = electron beam energy m/E = characteristic bremsstrahlung angle

Hall D Tagger and Beamline Review, Nov , 2008, Newport News11 linear polarization effects of collimation on polarization spectrum collimator distance = 80 m figure of merit: effects of collimation on figure of merit: rate (8-9 GeV) * p fixed hadronic rate Polarization and Tagging Efficiency Limits collimator diameter curves end where tagging efficiency  < 30%

Hall D Tagger and Beamline Review, Nov , 2008, Newport News12 tagging interval Rates based on: 12 GeV endpoint 20  m diamond crystal 2.2  A electron beam Leads to 10 8  /s on target (after the collimator) Design goal is to build a photon source with 10 8  /s in the range 8.4 – 9.0 GeV and peak linear polarization 40%. III. Beam Rates and Polarization

Hall D Tagger and Beamline Review, Nov , 2008, Newport News13 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 (f.w.h.m.) (1140 MeV)(900 MeV)(600 MeV)(240 MeV) peak tagging eff (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) Summary of Collimated Beam Properties 1.Rates reflect a beam current of 2.2  A which corresponds to 10 8  /s in the coherent peak. 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. 4. Does not include 30% improvement obtained by selecting one fiber row in the microscope. 2,

Hall D Tagger and Beamline Review, Nov , 2008, Newport News14 IV. Electron Beam Requirements beam energy and energy spread range of deliverable beam currents beam emittance beam position controls upper limits on beam halo energy12 GeV r.m.s. energy spread< 60 MeV transverse x emittance< 10 mm µr transverse y emittance< 2.5 mm µr minimum current700 pA maximum current5 µA x spot size at radiator0.8–1.6 mm r.m.s. y spot size at radiator0.3–0.6 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 5mm Summary of key results:

Hall D Tagger and Beamline Review, Nov , 2008, Newport News15 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 during running with 20 micron crystal at 10 8  /s : I =2.2  A I = 2.2  A 7 nA total absorption counter 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. Electron Beam Requirements: current

Hall D Tagger and Beamline Review, Nov , 2008, Newport News16 Electron Beam Requirements: halo two important consequences of beam halo: 1.impact active collimator accuracy 2.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. Integrated tail current is less than of the total beam current r /  central Gaussian power-law tail central + tail log Intensity

Hall D Tagger and Beamline Review, Nov , 2008, Newport News17 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 , 2008, Newport News18 3 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 fixed hodoscope microscope Diamond crystal requirements: orientation (mr) translation step:200 μm horizontal 25 μm target ladder (fine tuning) rotational step:1.5 μrad pitch and yaw 3.0 μrad azimuthal rotation

Hall D Tagger and Beamline Review, Nov , 2008, Newport News19 Diamond crystal requirements: mosaic rms angular deviation = “mosaic spread” mosaic of quasi-perfect domains  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 , 2008, Newport News20 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 = 2 d sin(  )   d

Hall D Tagger and Beamline Review, Nov , 2008, Newport News21 rocking curve from X-ray scattering natural width (fwhm) Example rocking curve Example rocking curve (Element Six) 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 Diamond crystal requirements: mosaic intensity

Hall D Tagger and Beamline Review, Nov , 2008, Newport News22 20  m rad.len Design calls for a diamond thickness of 20  m which is approx. 1.7 x 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… Choice of thickness is a trade-off between MS and radiation damage Diamond crystal requirements: thickness

Hall D Tagger and Beamline Review, Nov , 2008, Newport News23 conservative estimate (SLAC) for useful lifetime (before significant degradation): 3-6 crystals / year conservative estimate: 3-6 crystals / year of full-intensity running More details provided in a later talk. More details provided in a later talk C / mm 2 Diamond crystal requirements: lifetime

Hall D Tagger and Beamline Review, Nov , 2008, Newport News24 temperature profile of crystal at full intensity, radiation only oCoC Diamond crystal requirements: mounting diamond-graphite transition sets in ~800 o C Heat dissipation specification for the mount is not required. x (mm) 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

Hall D Tagger and Beamline Review, Nov , 2008, Newport News25 1.The virtual electron spot must be centered on the collimator. 2.Tolerance set by effect of offset on collimated intensity spectrum ~100 m upstream Photon beam position is controlled by steering magnets ~100 m upstream Feedback from active collimator to electron beam position stabilization system is planned. Feedback from active collimator to electron beam position stabilization system is planned. VI. Beam Monitoring – Photon Beam Position  x < 200  m Specification for the “active collimator” photon beam position monitor

Hall D Tagger and Beamline Review, Nov , 2008, Newport News26 Active Collimator Design Tungsten pin-cushion detector  reference: Miller and Walz, NIM 117 (1974)  measures current due to knock-ons in EM showers  performance is known active device primary collimator (tungsten) incident photon beam

Hall D Tagger and Beamline Review, Nov , 2008, Newport News27 12 cm x (mm) y (mm) current asymmetry vs. beam offset 20% 40% 60% Active Collimator Simulation beam tungsten plates tungsten pins

Hall D Tagger and Beamline Review, Nov , 2008, Newport News28 Active Collimator Position Sensitivity using inner ring only for fine-centering ±200  m of motion of beam centroid on photon detector corresponds to ±5% change in the left/right current balance in the inner ring Monte Carlo simulation

Hall D Tagger and Beamline Review, Nov , 2008, Newport News29 coherent bremsstrahlung beam coherent bremsstrahlung beam end-point energy 5.05 GeV 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. Active Collimator Prototype Beam Tests inner wedges, raw data inner cable outer Beam test in Hall B during G11 run, April Intensity in good agreement with Monte Carlo simulations.

Hall D Tagger and Beamline Review, Nov , 2008, Newport News30 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  radiator located upstream of pair spectrometer enables continuous monitoring during normal running determination of the beam polarization  essential for determination of the beam polarization

Hall D Tagger and Beamline Review, Nov , 2008, Newport News31 Photon Beam Polarimetry Coherent Bremsstrahlung Spectrum Analysis 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 Comparison between CBSA polarization spectrum and measurement with pair polarimeter at Yerevan Synchrotron (NIMA 579 (2007) p.973–978) CBSA prediction direct measurement spectrum measured in pair polarimeter data points model fit curve 5% stat. 2-3% syst.

Hall D Tagger and Beamline Review, Nov , 2008, Newport News32 Other Photon Beam Instrumentation visual photon beam monitors total absorption counter safety systems

Hall D Tagger and Beamline Review, Nov , 2008, Newport News33 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 , 2008, Newport News34 backup slides

Hall D Tagger and Beamline Review, Nov , 2008, Newport News35 peak polarizationcoherent gain factorsteep functions of peak energy 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. Coherent Bremsstrahlung Source – Flexibility

Hall D Tagger and Beamline Review, Nov , 2008, Newport News36 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?). Even with a future for high-energy beams at SLAC, the low duty factor < 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. Unique:

Hall D Tagger and Beamline Review, Nov , 2008, Newport News37 circular polarization  transfer from electron beam  reaches 100% at end-point linear polarization  determined by crystal orientation  vanishes at end-point  independent of electron polarization Coherent Bremsstrahlung Source Polarization Linear polarization arises from the two-body nature of the CB kinematics Linear polarization has unique advantages for GlueX physics: a requirement Changes the azimuthal  coordinate from a uniform random variable to carrying physically rich information.

Hall D Tagger and Beamline Review, Nov , 2008, Newport News38 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

Hall D Tagger and Beamline Review, Nov , 2008, Newport News39 Photon Beam Position Controls electron Beam Position Monitors provide coarse centering 100  m r.m.s.  position resolution 100  m r.m.s. ~ mm r.m.s. at the collimator  a pair separated by 10 m : ~1 mm r.m.s. at the collimator can find the collimator  matches the collimator aperture: can find the collimator primary beam collimator is instrumented  provides photon beam position measurement 30 mm  position sensitivity out to 30 mm from beam axis 200  m r.m.s.  maximum sensitivity of 200  m r.m.s. within 2 mm

Hall D Tagger and Beamline Review, Nov , 2008, Newport News40 Active Collimator Simulation 12 cm5 cm beam

Hall D Tagger and Beamline Review, Nov , 2008, Newport News41 Detector response from simulation inner ring of pin-cushion plates outer ring of pin-cushion plates beam centered at 0, radiator I e = 1  A