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GlueX collaboration meeting April 27-29, 2006. Electron Beam Simulation for GlueX. presented by. Richard Jones, University of Connecticut. Photon Tagger Simulation. estimate background rates in focal plane counters evaluate options for shielding detailed field map of the tagger - PowerPoint PPT Presentation
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Electron Beam Simulation for GlueX
Richard Jones, University of Connecticut
GlueX collaboration meeting April 27-29, 2006
presented by
2
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
4
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
6
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
12
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
13
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.
14
Results of beam dump simulations
The first 105 simulated events looked a lot like this:
labyrinth is doing a good job
15
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)
16
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
17
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)
18
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
19
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.
20
Photon Tagger Resolution
Resolution in focal plane microscope in energy and emission angle
21
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
22
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.
23
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
24
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
25
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…
26
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)
27
Effects of emittance on photon beam quality
plot from tagger review(constant beam current) same plot, matched hadronic rate
post-review note
28
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.
29
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.
30
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
31
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
32
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
33
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
34
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