The Hall D Photon Beam Overview

The Hall D Photon Beam Overview

Richard Jones, University of Connecticut

Hall D Tagger and Beamline Review Nov. 19-20, 2008, Newport News

presented by

GlueX Tagged Beam Working GroupJefferson Laboratory

University of ConnecticutCatholic University of America

University of Glasgow

Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News 2

Outline

Photon beam requirements Photon beam collimation Beam rates and polarization Electron beam requirements Diamond crystal requirements Beam monitoring and instrumentation


I. Photon Beam Requirements

Direct connections with the physics goals of the GlueX experiment:

Energy

Polarization

Intensity

Resolution

8.4-9.0 GeV

40 %

107 /s

0.5% EE

solenoidal spectrometer

meson/baryon resonance separationlineshape fidelity up to m mXX= = 2.5 GeV/c2.5 GeV/c22

adequate for distinguishing reactionsinvolving opposite parity exchangesopposite parity exchanges

provides sufficient statistics for PWA PWA

on reactions down to 100nb in 5 years†

better than resolution of the GlueX

calorimeters and tracking system

†† Assumes 10Assumes 1077 events and 20% acceptance. Design goal is events and 20% acceptance. Design goal is 101088 /s/s – factor 10 higher luminosity. – factor 10 higher luminosity.


Photon Beam Requirements, continued

Tagger coverage – 3 ranges

Tagging efficiency†

Energy calibration

Polarization measurement

Tagger backgrounds

tagging within the coherent peaki. 8.3 – 9.1 GeV ii. 3.0 – 9.0 GeViii. 9.0 – 11.7 GeV

70% in coherent peak

< 60 MeV r.m.s. absolute

< 3% r.m.s. absolute

< 1% of tagging rate

crystal alignment, spectrum monitoring

endpoint tagging, spectrum monitoring

†Defined as the ratio of tagged photons on target to tagged electrons in the tagger focal plane


II. Coherent Bremsstrahlung Beam Line

Coherent bremsstrahlung beam contains both coherent and incoherent components.

Only the coherent component is polarized. Incoherent component is suppressed by narrow collimation.


Effects of Collimation

effects of collimation at 80 m distance from radiatorincoherent (black) and coherent (red) kinematics

Purpose: to enhance high-energy flux and increase polarization

diameter

bre

ms

str

ah

lun

g a

ng

le


Photon Beam Collimation Geometry

1. Determine constraints from beam emittance, radiator size, and radiator quality on collimator geometry.

2. Optimize collimation angle as a compromise between high beam polarization and high tagging efficiency.

Steps taken to fix the collimator geometry:


v


radiator

D

nominal beam axis

electronbeam dump

C

cr

: beam emittance (rms)e : electron beam divergence angleC: characteristic bremsstralung angle (1) = v e (2) r = D e (3) c = D C / 2

v << c << r C / 2

<< 3 x 10-8 m.r

e

collimator

(vertical scale is expanded ~105)



radiator collimator

D

nominal beam axis

electronbeam dump

C

cvr

(1) = v e (2) r = D e (3) c = D C / 2

Length scale for D: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 mm D = 75 m

and thus

e

(vertical scale is expanded ~105)


As collimator aperture is reduced: polarization grows tagging efficiencytagging efficiency

drops off

Photon Beam Collimation Angle

diameter

m = mass of electronE = electron beam energym/E = characteristic bremsstrahlung angle


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

Polarization and Tagging Efficiency Limits

collimator diameter

curves end where tagging efficiency < 30%


tagginginterval

Rates based on:• 12 GeV endpoint• 20 m diamond crystal• 2.2 A electron beam

Leads to 108 /s on target

(after the collimator)

Design goal is to build a photon source with 108 /s in the range 8.4 – 9.0 GeV and peak linear polarization 40%.

III. Beam Rates and Polarization


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)

Summary of Collimated Beam Properties

1. Rates reflect a beam current of 2.2 A which corresponds to 108 /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.

4. Does not include 30% improvement obtained by selecting one fiber row in the microscope.

2,3

1

1

1

4


IV. Electron Beam Requirements

beam energy and energy spread

range of deliverable beam currents

beam emittance beam position

controls upper limits on beam

halo

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 <10-5 @ r>5mm

Summary of key results:


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 with 20 micron crystal at 108 /s :

I =I = 2.2 2.2 AA

lower bound of 0.77 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.

Electron Beam Requirements: current


Electron Beam Requirements: halo

two important consequences of beam halo:1.1. impact active collimator accuracyimpact active collimator accuracy

2.2. backgrounds in the tagging countersbackgrounds in the tagging counters

Beam halo model: central Gaussian power-law tails

Requirement:

Definition: “tails” are whatever extends outsider = 5 mm from the beam axis.

Integrated tail current is less than

of the total beam current.1010-5-5

r /

central Gaussianpower-law tailcentral + tail

1 2 3 4 5

log

Inte

nsity


V. Diamond crystal requirements

orientation requirements mosaic spread requirement thickness requirements radiation damage lifetime mount and heat relief


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.

alignmentzone

operatingzone

fixed hodoscope

microscope

Diamond crystal requirements: orientation

(mr)

translation step: 200 μm horizontal25 μm target ladder (fine tuning)

rotational step: 1.5 μrad pitch and yaw3.0 μrad azimuthal rotation


Diamond crystal requirements: mosaicrms angular deviation = “mosaic spread”

mosaic of quasi-perfect domainsmosaic 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”


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

= 2 d sin()

d


rocking curve from X-ray scattering

natural width(fwhm)

Example rocking Example rocking curvecurve

Actual measurement of a high-quality synthetic diamond from industry (Element Six)(Element Six)

X-ray rocking curve measurements require a synchrotron light source

Daresbury, UK (SRS) – now phased out

Cornell, NY (CHESS) – present facility of choice

Diamond crystal requirements: mosaic

inte

ns

ity


Design calls for a diamond thickness of 20 20 mm which is approx. 1.71.7 x 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…

Choice of thickness is a trade-off between MS and radiation damage.

-3

-4

Diamond crystal requirements: thickness


conservative estimate (SLAC) for useful lifetime (before significant degradation):

conservative estimate: 3-6 crystals / year3-6 crystals / year of full-intensity running

More details provided in a later talk.More details provided in a later talk.

0.25 C / mm0.25 C / mm22

Diamond crystal requirements: lifetime


temperature profile of crystalat full intensity, radiation only

oC

Diamond crystal requirements: mounting

diamond-graphite transition sets in ~800oC

Heat dissipation specificationfor the mount is not required.

x (mm)

y (mm) translation step: 200 μm horizontal25 μm target ladder (fine tuning)

rotational step: 1.5 μrad pitch and yaw3.0 μrad azimuthal rotation


1.1. The virtual electron spot The virtual electron spot must be centered on the must be centered on the collimator.collimator.

2.2. Tolerance set by effect Tolerance set by effect of offset on collimated of offset on collimated intensity spectrumintensity spectrum

Photon beam position is controlled by steering magnets ~100 m upstream~100 m upstream

Feedback from active Feedback from active collimator to electron beam collimator to electron beam position stabilization system position stabilization system is planned.is planned.

VI. Beam Monitoring – Photon Beam Position

x < 200 x < 200 mm

Specification for the “active collimator” photon beam position monitor


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

active device

primary collimator (tungsten)

incident photon beam


12 cmx (mm)

y (m

m)

current asymmetry vs. beam offset

20%

40%

60%

Active Collimator Simulation

beam

tungsten plates

tungsten pins


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

Monte Carlo simulation


coherent bremsstrahlung beamcoherent bremsstrahlung beam end-point energy 5.05 GeVend-point energy 5.05 GeV

two opposing inner sectors instrumented in prototype

collimator was swept across the beam in steps of 0.5 mm

beam intensity ~ 1% of full intensity in Hall D.

Active Collimator Prototype Beam Tests

inner wedges, raw data

innercableouter

Beam test in Hall B during G11 run, April 2007

0Intensity in good agreement withMonte Carlo simulations.


Photon Beam Spectrum Monitoring

tagger broad-band counter array necessary for crystal alignment during setup provides a continuous monitor of beam/crystal stability

electron pair spectrometer measures post-collimated photon beam spectrum 10-3 radiator located upstream of pair spectrometer

enables continuous monitoring during normal running essential for determination of the beam polarizationdetermination of the beam polarization


Photon Beam Polarimetry Method: CBSA – Coherent Coherent

Bremsstrahlung Spectrum AnalysisBremsstrahlung Spectrum Analysis Measure both the pre-collimated and post-

collimated beam spectra. Fit primary peak region in both spectra to a

model of the source + collimation system. Model gives polarization spectrum

Comparison between CBSA polarization spectrum and

measurement with pair polarimeter at Yerevan Synchrotron (NIMA 579 (2007) p.973–978)

CBSA prediction

direct measurement

spectrum measured in pair polarimeter

data pointsmodel fit curve

5% stat.2-3% syst.


Other Photon Beam Instrumentation

visual photon beam monitors total absorption counter safety systems


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 design parameters have been carefully optimized for operation with 40% polarization at 9 GeV.

The implications of the photon source design for the 12 GeV electron beam have been worked out and shown to be compatible with the 12 GeV accelerator design.

Quality assurance procedures for selection and procurement and of thin diamond crystals have been developed that can ensure a supply of radiators with the required properties.

The design includes sufficient beam line instrumentation to insure stable operation, with polarization uncertainty < 3%.


backup slides


For a fixed electron beam energy of 12 GeV, the peak peak polarizationpolarization and the coherent coherent gain factorgain factor are both steep steep functions of peak energyfunctions 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.

Coherent Bremsstrahlung Source – Flexibility


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::


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

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.


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


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 photon beam position measurement 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


Active Collimator Simulation

12 cm 5 cm

beam


Detector response from simulation

inner ring ofpin-cushion plates

outer ring ofpin-cushion plates

beam centered at 0,0

10-4 radiatorIe = 1A

The Hall D Photon Beam Overview

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