Electron Beam Simulation for GlueX

Richard Jones, University of Connecticut

GlueX collaboration meeting April 27-29, 2006

presented by

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Photon Tagger Simulation

estimate background rates in focal plane countersestimate background rates in focal plane counters evaluate options for shieldingevaluate options for shielding

1. detailed field map of the tagger

2. design for the vacuum box downstream exit region

3. design for the electron beam dump

Elements needed for tagger simulation (in addition to building dim.)

3

Simulation Overview

gxtwistgxtwist hdgeanthdgeant

electron beam + tagger photon beam + detector

tagger area “HILL”hall D “HALL”

“CAVE”

e- beam dump

in use since 7/2001new since 1/2006new since 1/2006

separateexecutables

commondesign

and lotsof shared

code

currently running with Geant3, Geant4 prototype under development

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Photon Tagger Simulation

original concept, shown at the Tagger Review in January, 2006

goniometer

quadrupolemagnet

taggerdipoles vacuum

chamber to electronbeam dump

Hall D

Tagger Building

fixed arrayhodoscope

microscope

photon beam line

detailed magnetic field map (from TOSCA)350 x 30 x 1600 = 17 M points areas of interest

5

Some Simulated Events

concrete wall

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New Concept (ca. 2 / 2006)

extend the length of the entire hall bring the electron dump in closer to the tagger building connect the electron beam dump to the tagger area by a corridor

7

New Concept (ca. 2 / 2006)

extend the length of the entire hallextend the length of the entire hall bring the electron dump in closer to the tagger building connect the electron beam dump to the tagger area by a corridor

8

New Concept (ca. 2 / 2006)

extend the length of the entire hall bring the electron dump in closer to the tagger buildingbring the electron dump in closer to the tagger building connect the electron beam dump to the tagger area by a corridor

9

New Concept (ca. 2 / 2006)

extend the length of the entire hall bring the electron dump in closer to the tagger building connect the electron beam dump to the tagger area by a corridor

10

New Concept (ca. 2 / 2006)

Is the electron dump a significant source of background in the tagger?Is the electron dump a significant source of background in the tagger? What is the minimum distance from the tagger to the dump?What is the minimum distance from the tagger to the dump? What is the optimum beam pipe and shielding configuration?What is the optimum beam pipe and shielding configuration?

Questions raised by the new design:

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Tagger + beam dump simulations

the problem of statistics rate of electrons in beam (3 A): 2 x 1013 /s rate of electrons at focal plane (3 GeV e-): 2 x 108 /s rate of FP backgrounds we care about: 2 x 106 /s 101088 simulated dump events simulated dump events are needed

to measure bg at the level of 10% error 10% error

required CPU time 12 GeV electron shower simulation ~0.4s0.4s (AMD 2800+) 4 x 104 x 1077 s s (1.2 GFlops-years)

use cascading to enhance statistics

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Monte Carlo cascading scheme

Any particles passing through these virtual surfaces in the indicated direction are intensified, ie. replicated on the secondaries stack N times.

Once a particle has cascaded, it is marked so that it (or its descendants) cannot cascade from that same surface again.

Two cascade surfaces of x1000 each: 101088 MC events MC events ~~ 10 101414 e e--

x1000

x1000

III

III

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Plan of beam dump simulations

1. track 108 12 GeV electrons from radiator (no cascades).

2. same as above, with x1000 at region II → I boundary.

3. same as above, with x1000 also at region III → II boundary.

sample 1: inclusive spectrum (signal + bg)sample 1: inclusive spectrum (signal + bg)sample 2: sample 1 + (sources in region II) x 1000sample 2: sample 1 + (sources in region II) x 1000sample 3: sample 2 + (sources in region III) x 1000,000sample 3: sample 2 + (sources in region III) x 1000,000

Simulations are still in progress…

but preliminary results will be shown based on 1/5 statistics obtained so far.

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Results of beam dump simulations

The first 105 simulated events looked a lot like this:

labyrinth is doing a good job

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Results of beam dump simulations

1.1. focal plane microscopefocal plane microscope• fiber length 2 cm• fiber axis aligned with

electron trajectories• mip peak at 4 MeV4 MeV

2.2. bg is visible but smallbg is visible but small• only hits with energy

deposition are recorded• gammas suppressed

3.3. nothing seen from dumpnothing seen from dump• evidence is time of flight

(not shown)

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Results of beam dump simulations

1.1. fixed focal plane arrayfixed focal plane array• counter thickness 5 mm• counters aligned with

focal plane (not ideal)• mip peak at 6 MeV 6 MeV for

8GeV electrons, less for lower energies.

2.2. bg is visible, largerbg is visible, larger• only hits with energy

deposition are recorded• gammas must convert

3.3. nothing seen from dumpnothing seen from dump

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Results for cascaded simulations

1.1. focal plane microscopefocal plane microscope• direct electrons are still

the primary source of hits over 2 MeV.

2.2. bg is visible from region IIbg is visible from region II• rate ~ 0.5% of region I• comes from gammas

3.3. no excess visible from no excess visible from dump (100% stat. error)dump (100% stat. error)

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Results for cascaded simulations

1.1. fixed focal plane arrayfixed focal plane array• direct electrons remain

the dominant source of hits over 1 MeV.

2.2. bg is visible from region IIbg is visible from region II• rate ~ 0.5% of region I• comes from gammas

3.3. small excess seen from small excess seen from dump (at dE < 1 MeV)dump (at dE < 1 MeV)

• a fewa few 1010-4-4 of rate coming from sources in region II

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Summary

Background rates in the Hall D tagger focal plane counters have been estimated using a detailed physics simulation, distinguishing three categories of sources:

region I – tagging spectrometer, vacuum box and exit flange

region II – electron exit beam pipe and labyrinth penetrations

region III – the electron beam dump

None of the backgrounds reach levels that threaten the performance of the tagging focal plane counters.

The present dump labyrinth design from the Radcon group leads to rates in the focal plane counters of that are a factor 10a factor 10-4-4 smaller smaller than backgrounds coming from sources in regions I and II.

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Photon Tagger Resolution

Resolution in focal plane microscope in energy and emission angle

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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

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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.

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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 maximizing 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

The reviewers challenged the validity of our requirement on dE/E of 0.1% based on a missing-mass plot we showed for this reaction.

My viewMy view: this is a soft requirement (0.2% is also OK) but we get it essentially for free based on other requirementsother requirements.

post-review note

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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

Stability of 100pA beam was highlighted as a potential issue during the review process. My viewMy view: this is only needed for doing checks. Loss of beam orbit locks is an acceptable part of running at <1nA. post-review note

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Beam emittance: key to effective 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…

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d > 70 m

Optimization: radiator – collimator distanceWith decreased collimator angle: polarization grows tagging efficiencytagging efficiency drops off

< 20 r

0 < 1/3 c

c/d = 1/2 (m/E)

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Effects of emittance on photon beam quality

plot from tagger review(constant beam current) same plot, matched hadronic rate

post-review note

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new slide, added for this presentation, follows from data shown in previous slide –rtj-

Effects of emittance on photon beam quality

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

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

Figure of merit expressesthe degradation in beam quality in terms of

the comparative loss of the comparative loss of effective beam time for a effective beam time for a run of fixed duration,run of fixed duration,

or equivalent sacrifice in ultimate precision if experimental errors are systematic in nature.

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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.

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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

origin of this number: What size of beam

halo would be small enough to be safe,

without needing a detailed model?post-review note

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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

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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

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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

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High-level Parameters

Beam energy 12 GeV

Beam power 1 MW

Beam current (Hall D) 5 µA

Emittance @ 12 GeV 10 nm-rad

Energy spread @ 12 GeV 0.02%

Simultaneous beam delivery Up to 3 halls

This slide was added in proof, to counter the suggestion that GlueX planners engaged in hopeful speculation in specifying their electron beam performance requirements.

copy of slide 2, presentation to Lehmann Review 7/05, L. Harwood