the gluex photon beam richard jones, university of connecticut gluex photon beamline-tagger...

The GlueX Photon Beam

Richard Jones, University of Connecticut

GlueX 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, GlueX 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 separationforward mX production up to 2.8GeV/c2.8GeV/c22

adequate for resolving resonances with opposite dominant exchangesopposite dominant exchanges

provides sufficent statistics for PWA PWA

on major channels with ~1yr running

matches resolution of the GlueX

spectrometer in all-charged modesall-charged modes


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?).

With the closure of SLAC, Jefferson Lab is the unique place where high-energy polarized photons can be found.

The continuous beams from CEBAF are ideal for an experiment seeking to detect high-multiplicity 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

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

Circular polarization is potentially useful as well, but not a requirementnot a requirement to achieve the physics goals of GlueX.


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 GeV8 GeV, 10 GeV10 GeV would be better for several reasons,

for a fixed endpoint of 12 GeV, the coherent gain factor is a steep function of peak energysteep function of peak energy.

CB polarization is a key factor in the choice of a peak energy of 9 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.


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…


Optimization: radiator – collimator distance

< 20 r0 < 1/3 c

d > 70 m

With increased collimator distance: polarization grows low-energy backgrounds shrink tagging efficiencytagging efficiency drops off


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: < 0.01 % r.m.s.0.01 % r.m.s. compares favorably with best estimate: 0.06 %0.06 %

absolute energy scale determination to 0.1% via known meson masses in the GlueX detector: eg. (1020)

p + + - n

1. tied to the energy resolution requirement for the tagger

2. derived from optimizing the missing-mass resolution in the channels with a missing final-state particle.

Typical channel analyzed usingTypical channel analyzed usingthe missing mass technique:the missing mass technique:


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 100 pA 100 pA is required to permit accurate measurement of the tagging efficiency using a in-beam total absorption countertotal absorption counter during special low-current runs.

similar to low-current operation in Hall B

Range of Required Beam Currents


Electron Beam Emittance

requirement : << 1010-8-8 m m••rr all expressions 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 ~100m upstream~100m 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:

r /

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


Photon Beam Position Controls

conventional BPM’s provide coarse centering

position resolution 100100m r.m.s.m r.m.s. a pair separated by 2m : 4mm r.m.s. at the collimator4mm 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 30mm30mm from beam axis maximum sensitivity of 200200m r.m.s.m r.m.s. within 2mm


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


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 centroil 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

how much to say here? should probably be brief


Diamond Crystal Properties

limits on thickness from multiple-scattering rocking curve from X-ray scattering

natural fwhm


Diamond crystal: goniometer mount

temperature profile of crystalat full operating intensity

oC


Summary

review highlights

the gluex photon beam richard jones, university of connecticut gluex photon beamline-tagger...

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