The Hall D Photon Beam
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Transcript of The Hall D Photon Beam
The Hall D Photon Beam
Richard Jones, University of Connecticut
Hall D Tagger-Photon Beamline Review Jan. 23-24, 2005, Newport News
presented by
GlueX Tagged Beam Working GroupUniversity of Glasgow
University of ConnecticutCatholic University of America
2Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Presentation Overview
Photon beam properties Competing factors and optimization Electron beam requirements Beam monitoring and instrumentation Diamond crystal requirements
3Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
I. Photon Beam PropertiesDirect connections with the physics goals of the GlueX experiment:
Energy
Polarization
Intensity
Resolution
9 GeV
40 %
107 /s
10-3 EE
solenoidal spectrometer
meson/baryon resonance separationlineshape fidelity up to m mXX==2.8GeV/c2.8GeV/c22
adequate for distinguishing reactionsinvolving opposite parity exchangesopposite parity exchanges
provides sufficient statistics for PWA PWA
on key channels in initial three years
matches resolution of the GlueX
spectrometer tracking system
4Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
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 <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.
UniqueUnique::
5Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Kinematics of Coherent Bremsstrahlung
effects of collimation at 80 m distance from radiatorincoherent (black) and coherent (red) kinematics
effects of collimation: to enhance high-energy flux and increase polarization
6Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
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.
7Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Photon Beam Intensity Spectrum
4
nominaltagginginterval
Rates based on:• 12 GeV endpoint• 20m diamond crystal• 100nA electron beam
Leads to 107 /s on target
(after the collimator)
Design goal is to build an experiment with ultimate rate capability as high as 108 /s on target.
8Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
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
9Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Optimization: chosing a photon energy
A minimum useful energy for GlueX is 8 GeV;8 GeV; 9-10 GeV9-10 GeV is better for several reasons,
for a fixed endpoint of 12 GeV, the peak polarizationpeak polarization and the coherent gain factorcoherent gain factor are both steep functions of peak energysteep 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
10Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Optimization: choice of diamond thickness
Design calls for a diamond thickness of 2020mm which is approximately 1010-4-4 rad.len rad.len.
Requires thinningthinning: special fabrication steps and $$.
Impact from multiple-scattering is significant.
Loss of rate is recovered by increasing beam current,
up to a point…up to a point…
The choice of 20m is a trade-off between MS and radiation damage.
-3
-4
11Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
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 virtualvirtual electron spot electron spot on the collimator face.
It must be much smaller than the real photonspot size for collimation to be effective
but
the convergence angle a must remain smallto preserve a sharp coherent peak.
Putting in the numbers…
12Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
d > 70 m
Optimization: radiator – collimator distance
With decreased collimator angle: polarization grows tagging efficiencytagging efficiency drops off
< 20 r
0 < 1/3 c
c/d = 1/2 (m/E)
13Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Optimization: varying the collimator diameter
line
ar
po
lariz
atio
n
effects of collimation on polarization spectrum
collimator distance = 80 m
effects of collimation on figure of merit:figure of merit:
rate (8-9 GeV) * p2 @ fixed hadronic rate
14Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
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)
Results: summary of photon beam properties
1. Rates reflect a beam current of 3A which corresponds to 108 /s in the coherent peak, which is the maximum currentthe maximum current foreseen to be used in Hall D. Normal GlueX running is planned to be at a factor of 10 lowera 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 almostindependent of coherent peak energy, comes mostly from incoherent part.
2,3
1
1
1
15Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
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
16Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Electron Beam Energy
effects of endpoint energy on figure of merit:
rate (8-9 GeV) * p2 @ 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.
17Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Electron Beam Energy Resolution
beam energy spread E/E requirement: 0.1 % r.m.s.0.1 % r.m.s. compares favorably with best estimate: 0.06 %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 theTypical channel where one of theparticles might escape detectionparticles might escape detection
18Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
upper bound of 3 3 A A projected for GlueX at high intensity corresponding to 108 /s on the GlueX target.
with safety factor, translates to 5 5 AA for the maximum current to be delivered to the Hall D electron beam dump
during running at a nominal rate of 107 /s : I =I = 300 nA300 nA
lower bound of 0.11 nA nA is required to permit accurate measurement of the tagging efficiency using a in-beam total absorption countertotal absorption counter during special low-current runs.
Range of Required Beam Currents
19Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Electron Beam Emittance
requirement : << 1010-8-8 m m••rr 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 Optics study: goal is achievable, but close to the limits according to 12 GeV machine models
20Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Hall D Optics Conceptual Design Study
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
Summary of key results:
21Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Must satisfy two criteria:1.1. The virtual electron spot must beThe virtual electron spot must be centered on centered on
the collimator.the collimator.
2.2. A significant fraction of the real electron A significant fraction of the real electron beam must beam must pass through the diamond crystal.pass through the diamond crystal.
criteria for “centering”: x < x < mm
controlled by steering magnets ~100 m upstream~100 m upstream
Electron Beam Position Controls
1.1. Using upstream BPM’s and a known tune, Using upstream BPM’s and a known tune, operators can “find the collimator”.operators can “find the collimator”.
2.2. Once it is approximately centered ( Once it is approximately centered ( 5 mm ) 5 mm ) an active collimator must provide feedback.an active collimator must provide feedback.
22Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Electron Beam Halo
two important consequences of beam halo:1.1. distortion of the active collimator response matrixdistortion of the active collimator response matrix
2.2. backgrounds in the tagging countersbackgrounds in the tagging counters
Beam halo model: central Gaussian power-law tails
Requirement:
Further study is underwayr /
central Gaussianpower-law tailcentral + tail
1 2 3 4 5
Integrated tail current is less than
of the total beam current.1010-6-6
~-4
log
Inte
nsity
23Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Photon Beam Position Controls
electron Beam Position Monitors provide coarse centering
position resolution 100 100 m r.m.s.m r.m.s. a pair separated by 10 m : ~~1 mm r.m.s. at the collimator mm r.m.s. at the collimator matches the collimator aperture: can find the collimator can find the collimator
primary beam collimator is instrumented
provides “active collimation” position sensitivity out to 30 mm30 mm from beam axis maximum sensitivity of 200 200 m r.m.s.m r.m.s. within 2 mm
24Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
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
1contour of electron beam at radiator
25Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
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) 33-37
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
26Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Active Collimator Simulation
12 cm 5 cm
beam
27Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
12 cmx (mm)
y (m
m)
current asymmetry vs. beam offset
20%
40%
60%
Active Collimator Simulation
28Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Detector response from simulation
inner ring ofpin-cushion plates
outer ring ofpin-cushion plates
beam centered at 0,0
10-4 radiatorIe = 1A
29Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Active Collimator Position Sensitivityusing inner ring only for fine-centering
±200 m of motionof beam centroid onphoton detector
corresponds to
±5% change in theleft/right currentbalance in the innerring
30Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
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.
31Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Other Photon Beam Instrumentation
visual photon beam monitors total absorption counter safety systems
32Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
V. Diamond crystal requirements
orientation requirements limitations from mosaic spread radiation damage assessment
33Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Diamond Orientation
orientation angle is relatively large at 9 GeV: 3 mr3 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)
alignmentzone
operatingzone
fixed hodoscope
microscope
34Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Diamond Crystal Quality
rocking curve from X-ray scattering
natural fwhm
reliable source of high-quality synthetics from industry (Univ. of Glasgow contact)(Univ. of Glasgow contact)
established procedure in place for selection and assessment using X-rays
R&D is ongoing towards reliable operation of one 20m crystal (Hall B)(Hall B)
35Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
conservative estimate (SLAC) for useful lifetime (before significant degradation):
during initial running at 107 /s this gives 600 hrs of running before a spot move
a “good” crystal accommodates 5 spot moves
R&D is planned that will improve the precision of this estimate.
Diamond Crystal Lifetime
0.25 C / mm0.25 C / mm22
36Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
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.
37Richard Jones, Hall D Tagger-Photon Beamline Review, Newport News, Jan 23-24, 2006
Diamond crystal: goniometer mount
temperature profile of crystalat full operating intensity
oC