The Hall D Photon Beam

The Hall D Photon Beam

Richard Jones, University of Connecticut

Hall D Photon Beamline-Tagger Review Jan. 23-25, 2005, Newport News

presented by

GlueX Tagged Beam Working GroupUniversity of Glasgow

University of ConnecticutCatholic University of America

2Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006

Presentation Overview

Photon beam properties Competing factors and optimization Electron beam requirements Beam monitoring and instrumentation Diamond crystal requirements


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


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::


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


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.


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.


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


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


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


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…


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)


Optimization: varying the collimator diameter

line

ar

po

lariz

atio

n

effects of collimation on polarization spectrum

collimator distance = 80 m

5

effects of collimation on figure of merit:figure of merit:

rate (8-9 GeV) * p2 @ fixed hadronic rate


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


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


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.


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


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 1 nA 1 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

total rate @ 1nA (Emin = 1 MeV) = 2 MHz


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


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:


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.


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.1010-5-5

~-4

log

Inte

nsity


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


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


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


Active Collimator Simulation

12 cm 5 cm

beam


12 cmx (mm)

y (m

m)

current asymmetry vs. beam offset

20%

40%

60%

Active Collimator Simulation


Detector response from simulation

inner ring ofpin-cushion plates

outer ring ofpin-cushion plates

beam centered at 0,0

10-4 radiatorIe = 1A


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


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.


Other Photon Beam Instrumentation

visual photon beam monitors total absorption counter safety systems


V. Diamond crystal requirements

orientation requirements limitations from mosaic spread radiation damage assessment


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


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)


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 accomodates 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


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 CEBAF @ 12GeV.

The design parameters have been carefully optimized.

The design includes sufficient beam line instrumentation to insure stable operation.


Diamond crystal: goniometer mount

temperature profile of crystalat full operating intensity

oC

The Hall D Photon Beam

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