The Hall D Photon Beam

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The Hall D Photon Beam
Hall D Tagger-Photon Beamline Review Jan. 23-24,
2005, Newport News
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
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 mX2.8GeV/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
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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 lt10-4 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.

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Kinematics of Coherent Bremsstrahlung
effects of collimation to enhance high-energy
flux and increase polarization
effects of collimation at 80 m distance from
radiator
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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.
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Photon Beam Intensity Spectrum

• Rates based on
• 12 GeV endpoint
• 20mm diamond crystal
• 100nA electron beam
• Leads to 107 g/s on target
• (after the collimator)

Design goal is to build an experiment with
ultimate rate capability as high as 108 g/s on
target.
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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

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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
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Optimization choice of diamond thickness

• Design calls for a diamond thickness of 20mm
  which is approximately 10-4 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.

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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
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Optimization radiator collimator distance

• lt 20 mr
• s0 lt 1/3 c
• c/d 1/2 (m/E)

• With decreased collimator angle
• polarization grows
• tagging efficiency drops off

d gt 70 m
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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 _at_ fixed hadronic rate
linear polarization
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Results summary of photon beam properties
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 H2 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)
1
1
1
2,3

• Rates reflect a beam current of 3mA which
  corresponds to 108 g/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.

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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

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Electron Beam Energy
effects of endpoint energy on figure of
merit rate (8-9 GeV) p2 _at_ 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.

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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 KK-p p- p p0
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Range of Required Beam Currents

• upper bound of 3 mA projected for GlueX at high
  intensity corresponding to 108 g/s on the GlueX
  target.
• with safety factor, translates to 5 mA for the
  maximum current to be delivered to the Hall D
  electron beam dump
• during running at a nominal rate of 107 g/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.

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Electron Beam Emittance
This is a key issue for achieving the
requirements for the GlueX Photon Beam

• requirement lt 10-8 mr
• 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
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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
radiator 1.6 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
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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 lt s / 2 ? 200 mm
• controlled by steering magnets 100 m upstream

1. Using upstream BPMs and a known tune, operators
   can find the collimator.
2. Once it is approximately centered ( ?5 mm ) an
   active collimator must provide feedback.

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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
1
2
3
4
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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

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Overview of Photon Beam Stabilization

• Monitor alignment of both beams
• BPMs 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
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Active Collimator Design

• Tungsten pin-cushion detector
• used on SLAC coherent bremsstrahlung beam line
  since 1970s
• 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

primary collimator (tungsten)
active device
incident photon beam
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Active Collimator Simulation
beam
12 cm
5 cm
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Active Collimator Simulation
current asymmetry vs. beam offset
y (mm)
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40
60
x (mm)
12 cm
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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
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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
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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
  leftright timing
• coincidences with the tagger provide a continuous
  monitor of the post-collimator photon beam
  spectrum.

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Other Photon Beam Instrumentation

• visual photon beam monitors
• total absorption counter
• safety systems

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V. Diamond crystal requirements

• orientation requirements
• limitations from mosaic spread
• radiation damage assessment

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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
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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
• RD is ongoing towards reliable operation of one
  20mm crystal (Hall B)

natural fwhm
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Diamond Crystal Lifetime

• conservative estimate (SLAC) for useful lifetime
  (before significant degradation)
• during initial running at 107 g/s this gives 600
  hrs of running before a spot move
• a good crystal accommodates 5 spot moves
• RD is planned that will improve the precision of
  this estimate.

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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.

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