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

2. Outline Photon beam requirements Photon beam collimation
Beam rates and polarization
Diamond crystal requirements
Beam monitoring and instrumentation
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

3. I. Photon Beam Requirements
Direct connections with the physics goals of the GlueX experiment:
Energy
Polarization
Intensity
Resolution
solenoidal spectrometer
meson/baryon resonance separation
lineshape fidelity up to mX= 2.5 GeV/c2
GeV
40 %
adequate for distinguishing reactions
involving opposite parity exchanges
107 g/s
provides sufficient statistics for PWA on reactions down to 100nb in 5 years†
0.5% dE E
better than resolution of the GlueX calorimeters and tracking system
† Assumes 107 events and 20% acceptance. Design goal is 108 g/s – factor 10 higher luminosity.
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

4. I. Requirements, continued
Tagger coverage – 3 ranges
Tagging efficiency†
Energy calibration
Polarization measurement
Tagger backgrounds
tagging within the coherent peak
8.3 – 9.1 GeV
3.0 – 9.0 GeV
9.0 – 11.7 GeV
crystal alignment, spectrum monitoring
endpoint tagging, spectrum monitoring
70% in coherent peak
< 60 MeV r.m.s. absolute
< 3% r.m.s. absolute
< 1% of tagging rate
†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 News

5. II. Coherent Bremsstrahlung Beam Line
needs a better figure
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 News

6. Effects of Collimation
Purpose: to enhance high-energy flux and increase polarization
incoherent (black) and coherent (red) kinematics
effects of collimation at 80 m distance from radiator
diameter
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

7. Photon Beam Collimation Geometry
Steps taken to fix the collimator geometry:
Determine constraints from beam emittance, radiator size, and radiator quality on collimator geometry.
Optimize collimation angle as a compromise between high beam polarization and high tagging efficiency.
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

8. Photon Beam Collimation Geometry
(vertical scale is expanded ~105)
: beam emittance (rms)
e : electron beam divergence angle
C: characteristic bremsstralung angle D r v c
nominal beam axis
(1)  = v e C e
(2) r = D e
(3) c = D C / 2
v << c
 << r C / 2
radiator
electron
beam dump
collimator
 << 3 x 10-8 m.r
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

9. Photon Beam Collimation Geometry
(vertical scale is expanded ~105)
(1)  = v e D
(2) r = D e
(3) c = D C / 2 r v c
Length scale for D:
e convoluted with crystal mosaic spread m sets scale for smearing of coherent edge.
nominal beam axis C e
m ~ 20 µr
e = 20 µr
radiator
electron
beam dump
collimator
and thus
r = 1.5 mm
D = 75 m
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

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

11. Polarization and Tagging Efficiency Limits
effects of collimation on polarization spectrum
collimator distance = 80 m
effects of collimation on figure of merit:
rate (8-9 GeV) * p2 @ fixed hadronic rate
collimator diameter
linear polarization
curves end where tagging efficiency
e < 30%
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

12. III. Beam Rates and Polarization
Rates based on:
12 GeV endpoint
20mm diamond crystal
2.2 nA electron beam
Leads to 108 g/s on target
(after the collimator)
tagging
interval
Design goal is to build a photon source with 108 g/s in the range 8.4 – 9.0 GeV and peak linear polarization 40%.
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

13. Electron Beam Requirements
Summary of key results:
energy 12 GeV
r.m.s. energy spread < 60 MeV
transverse x emittance < 10 mm µr
transverse y emittance < 2.5 mm µr
minimum current 700 pA
maximum current 5 µA
x spot size at radiator 0.8–1.6 mm r.m.s.
y spot size at radiator 0.3–0.6 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 r>5mm
beam energy and energy spread
range of deliverable beam currents
beam emittance
beam position controls
upper limits on beam halo
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

14. Electron Beam Requirements: current
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 with 20 micron crystal at 108 g/s :
I = 2.2 A
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.
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

15. Summary of Collimated Beam Properties
peak energy GeV 9 GeV 10 GeV 11 GeV
N in peak M/s 100 M/s 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 W W W 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 4 1 1
2,3
Rates reflect a beam current of 2.2 mA which corresponds to 108 g/s in the coherent peak.
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.
4. Does not include 30% improvement obtained by selecting one fiber row in the microscope.
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

16. IV. 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 News

17. Diamond crystal requirements: 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.
(mr)
alignment
zone
operating
zone
microscope
translation step: 200 μm horizontal
25 μm target ladder (fine tuning)
rotational step: 1.5 μrad pitch and yaw
3.0 μrad azimuthal rotation
fixed hodoscope
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

18. 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 News

19. 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
l = 2 d sin(q) q q d
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

20. Diamond crystal requirements: mosaic
Example rocking curve
Actual measurement of a high-quality synthetic diamond from industry (Element Six)
Measurements performed at a synchrotron light source
Daresbury, UK (SRS) – now phased out
Cornell, NY (CHESS) – present facility of choice
rocking curve from X-ray scattering
intensity
natural fwhm
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

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

22. Diamond crystal requirements: lifetime
conservative estimate (SLAC) for useful lifetime (before significant degradation):
conservative estimate: 3-6 crystals / year of full-intensity running
More details provided in a later talk.
0.25 C / mm2
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

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

24. V. Beam Monitoring – Electron Beam Position
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 < s / 2  200 mm
controlled by steering magnets ~100 m upstream
Using upstream BPM’s and a known tune, operators can “find the collimator”.
Once it is approximately centered ( 5 mm ) an active collimator must provide feedback.
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

25. Electron Beam Halo two important consequences of beam halo:
impact active collimator accuracy
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.
central Gaussian
power-law tail
central + tail
log Intensity
Integrated tail current is less than
of the total beam current.
10-5
r / s 1 2 3 4 5
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

26. 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 photon beam position measurement
position sensitivity out to 30 mm from beam axis
maximum sensitivity of 200 mm r.m.s. within 2 mm
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

27. Active Collimator Design
Tungsten pin-cushion detector
reference: Miller and Walz, NIM 117 (1974) 33-37
measures current due to knock-ons in EM showers
performance is known
primary collimator (tungsten)
active device
incident photon beam
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

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

29. 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
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

30. 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
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

31. 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 Tagger and Beamline Review, Nov , 2008, Newport News

32. II. Coherent Bremsstrahlung Source – Flexibility
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.
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

33. 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.
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

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

35. 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 <10-4 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.
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

36. Coherent Bremsstrahlung Source Polarization
Linear polarization arises from the two-body nature of the CB kinematics
linear polarization
determined by crystal orientation
vanishes at end-point
independent of 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.
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

37. 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.
3.5 mm
1s contour of electron beam at radiator
1.1 mm
Hall D Tagger and Beamline Review, Nov , 2008, Newport News

38. Active Collimator Simulation
beam
12 cm
5 cm
Hall D Tagger and Beamline Review, Nov , 2008, Newport News