The Hall D Photon Beam Overview Richard Jones, University of Connecticut Hall D Tagged Photon Beam 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 Tagged Photon Beam Review, Nov , 2008, Newport News2 Outline Photon beam requirements Coherent bremsstrahlung source Photon beam collimation Diamond crystal requirements Beam monitoring and instrumentation

Hall D Tagged Photon Beam 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 8  /s 0.5% EE E solenoidal spectrometer meson/baryon resonance separation m X =2.8GeV/c 2 lineshape fidelity up to m X =2.8GeV/c 2 adequate for distinguishing reactions opposite parity exchanges involving opposite parity exchanges PWA provides sufficient statistics for PWA on reactions down to 10nb in 2 years better than resolution of the GlueX calorimeters and tracking system

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News4 I. 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 10 MeV r.m.s. absolute 1% r.m.s. absolute < 1% of tagging rate crystal alignment, spectrum monitoring endpoint tagging, spectrum monitoring

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News5 II. Coherent Bremsstrahlung Source 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

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News6 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 II. 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 Tagged Photon Beam Review, Nov , 2008, Newport News7 tagging interval Rates based on: 12 GeV endpoint 20  m diamond crystal 2.2 nA 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%. II. Coherent Bremsstrahlung Source – Spectrum

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News8 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. II. Coherent Bremsstrahlung Source – Flexibility

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News9 III. Photon Beam Collimation radiator collimator D nominal beam axis electron beam dump C c v r  :beam emittance (rms) C :characteristic bremsstralung angle e :electron beam divergence angle (1)  = v e (2) r = D e (3) c = D C / 2 v << c  << r C / 2  << 3 x m.r

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News10 III. Photon Beam Collimation 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

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News11 As collimator aperture is reduced:  polarization grows  tagging efficiency  tagging efficiency drops off III. Photon Beam Collimation – size

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News12 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 III. Photon Beam Collimation – size

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News13 III. Electron Beam Requirements beam energy and energy spread range of deliverable beam currents beam emittance beam position controls upper limits on beam halo (to be discussed) energy12 GeV r.m.s. energy spread< 10 MeV transverse x emittance< 10 mm µr transverse y emittance< 2.5 mm µr minimum current100 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 Summary of key results:

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News14 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 3 nA total absorption counter lower bound of 0.3 nA is required to permit accurate measurement of the tagging efficiency using a in-beam total absorption counter during special low-current runs. III. Electron Beam Requirements: current

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News15 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. 2,

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News16 IV. Diamond crystal requirements orientation requirements mosaic spread requirement thickness requirements radiation damage lifetime mount and heat relief

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News17 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 IV. Diamond crystal requirements: orientation

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News18 rocking curve from X-ray scattering natural fwhm (Element Six) reliable source of high- quality synthetics from industry (Element Six) established procedure in place for selection and assessment using X-rays Work is ongoing towards the reliable thinning and mounting of 20  m crystals IV. Diamond crystal requirements: mosaic

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News19 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… The choice of 20  m is a trade-off between MS and radiation damage IV. Diamond crystal requirements: thickness

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News20 conservative estimate (SLAC) for useful lifetime (before significant degradation): running at 10 8  /s this gives 100 hrs of running before a spot move a “good” crystal accommodates 5-10 spot moves 3-6 crystals / year conservative estimate: 3-6 crystals / year SLAC – crystals can be reused following annealing in an oven, over several cycles SLAC – crystals can be reused following annealing in an oven, over several cycles 0.25 C / mm 2 IV. Diamond crystal requirements: lifetime

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News21 temperature profile of crystal at full intensity, radiation only oCoC IV. Diamond crystal requirements: mounting

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News22 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 ~100 m upstream controlled by steering magnets ~100 m upstream V. Beam Monitoring – Electron Beam Position 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.

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News23 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: 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 ~  -4 log Intensity

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News24 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 “active collimation” 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 Tagged Photon Beam Review, Nov , 2008, Newport News25 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 Tagged Photon Beam Review, Nov , 2008, Newport News26 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)  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

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News27 Active Collimator Simulation 12 cm5 cm beam

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News28 12 cm x (mm) y (mm) current asymmetry vs. beam offset 20% 40% 60% Active Collimator Simulation

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News29 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

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News30 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

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News31 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.

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News32 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  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.

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News33 Other Photon Beam Instrumentation visual photon beam monitors total absorption counter safety systems

Hall D Tagged Photon Beam Review, Nov , 2008, Newport News34 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: