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

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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, I. Photon Beam Properties Direct connections with the physics goals of the GlueX experiment: Energy Polarization Intensity Resolution 9 GeV 40 % 10 7  /s 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 key channels in initial three years matches resolution of the GlueX spectrometer tracking system

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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:

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, circular polarization  transfer from electron beam  reaches 100% at end-point linear polarization  determined by crystal orientation  vanishes at end-point  not affected by electron polarization 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 Changes the azimuthal  coordinate from a uniform random variable to carrying physically rich information.

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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.

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, Optimization: chosing a photon energy 8 GeV;9-10 GeV A minimum useful energy for GlueX is 8 GeV; 9-10 GeV is better for several reasons, peak polarization coherent gain factor steep functions of peak energy 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 GeV for GlueX but

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, Optimization: choice of diamond thickness 20  m rad.len Design calls for a diamond thickness of 20  m which is approximately 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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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…

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, d > 70 m Optimization: radiator – collimator distance With decreased collimator angle:  polarization grows  tagging efficiency  tagging efficiency drops off  < 20  r  0 < 1/3 c c/d = 1/2 (m/E)

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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 fixed hadronic rate

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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) 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,

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, Electron Beam Energy effects of endpoint energy on figure of merit: rate (8-9 GeV) * p 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.

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, Electron Beam Energy Resolution 0.1 % r.m.s. beam energy spread  E/E requirement: 0.1 % r.m.s % compares favorably with best estimate: 0.06 %  p K + K -  +  - p [  0 ] 1.tied to the energy resolution requirement for the tagger 2.derived from optimizing the ability to reject events with a missing final-state particle. Typical channel where one of the particles might escape detection

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25,  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 1 nA total absorption counter lower bound of 1 nA is required to permit accurate measurement of the tagging efficiency using a in-beam total absorption counter during special low-current runs. Range of Required Beam Currents total 1nA (E min = 1 MeV) = 2 MHz

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, Electron Beam Emittance <10 -8 m r requirement : < m r emittances 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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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:

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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 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.

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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: Further study is underway r /  central Gaussian power-law tail central + tail Integrated tail current is less than of the total beam current ~  -4 log Intensity

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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)  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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, Active Collimator Simulation 12 cm5 cm beam

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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 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.

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, Other Photon Beam Instrumentation visual photon beam monitors total absorption counter safety systems

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, V. Diamond crystal requirements orientation requirements limitations from mosaic spread radiation damage assessment

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, Diamond Orientation 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 method described in a later talk (F. Klein) alignment zone operating zone fixed hodoscope microscope

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, Diamond Crystal Quality rocking curve from X-ray scattering natural fwhm (Univ. of Glasgow contact) reliable source of high-quality synthetics from industry (Univ. of Glasgow contact) established procedure in place for selection and assessment using X-rays (Hall B) R&D is ongoing towards reliable operation of one 20  m crystal (Hall B)

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, conservative estimate (SLAC) for useful lifetime (before significant degradation: during initial running at 10 7  /s this gives 600 hrs of running before a spot move a “good” crystal accomodates 5 spot moves R&D is planned that will improve the precision of this estimate. Diamond Crystal Lifetime 0.25 C / mm 2

Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, Summary A design has been put forward by the GlueX collaboration for a polarized photon beam line that meets the requirements for that experiment and matches the capabilities of 12GeV. The design parameters have been carefully optimized. The design includes sufficient beam line instrumentation to insure stable operation.

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