Spectrometer & Collimation

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January 14, 2010
Spectrometer and Collimation
Spectrometer & Collimation
Kent Paschke
University of Virginia
January 14, 2010
Director’s Review, Jefferson Laboratory
Measurement of Lepton-Lepton Electroweak Reaction
MOLLER
January 14, 2010
Spectrometer and Collimation
Outline

Spectrometer Design Considera8ons

Proposed Toroidal Spectrometer Design

Asymmetric Toroid Concept

Simula8on Results

Collima8on and photon flux 

Spectrometer Beamline / Vacuum Vessel

Plan for Spectrometer Engineering
E
!
=
E
beam
2
(1 +cos
!
)
January 14, 2010
Spectrometer and Collimation
Spectrometer for MOLLER
Scattered Electron Energy (GeV
)
1 2
3 4
5 6
7 8
9 1
0 1
1
Lab Scattering Angle (mrad)
0
5
10
15
20
25
30
Backward
Forward
!
e
!
e
!
e
!
e
!
identical particles!
Asymmetry (ppb)
Center of Mass Angle
Highest figure of merit at
θ
CM
= 90
o
A
PV
dσ/dΩ
Choose the CM backangle scatters
CM angles 90
o
-120
o
11 GeV in: 2.75 to 5.5 GeV out
Lab angles ~9
mrad
- 17
mrad

separate ee from ep and neutrals

maximize acceptance

cooling for heat load
Spectrometer magnets
Concrete shielding
target
Detector
cart
E158
used dipole chicane,
4 quadrupole spectrometer
January 14, 2010
Spectrometer and Collimation
E158

Primary Collimators
August 14, 2008
Spectrometer Options
11
Acceptance Collimator
Higher

moments
Alignment
Synchrotron

spoke

August 14, 2008
Spectrometer Options
12
August 14, 2008
Spectrometer Options
Two-Bounce System
13
tar
get
3D1
3D2
3D3
3DC2C
3DC3
inner Motts (t
o lumi)
D
E
T
E
C
T
O
R
3QC1B
c
entimet
ers
3Q1
3Q2
3Q3
3Q4
3SC1
phot
on r
a
y
s
1
2
3
4
5
6
E158
January 14, 2010
Spectrometer and Collimation
Quadrupole Concept for MOLLER
The same magnets could be used again!
Many challenges, but there is one fundamental characteristic which
cannot be improved in this concept: <80% azimuthal acceptance
Target
August 14, 2008
Spectrometer Options
11 GeV Solution
6
L. Mercado
0
5 m
10 m
25 m
15 m
20 m
20 cm
10 cm
30 cm
ee (
X
&
Y
)
ep (
X
&
Y
)
Detectors
January 14, 2010
Spectrometer and Collimation
Toroidal Concept for MOLLER
COM Scattering Angle (degrees)
0 2
0 4
0 6
0 8
0
100
120
140
160
180
Scattered Electron Energy (GeV
)
0
2
4
6
8
10
Forward
Backward
Scattered Electron Energy (GeV
)
1 2
3 4
5 6
7 8
9 1
0 1
1
Lab Scattering Angle (mrad)
0
5
10
15
20
25
30
Backward
Forward
!
e
!
e
!
e
!
e
!
highly boosted
laboratory frame
identical particles!
... are collected as 
θ
CM
=[60,90] over here!
Unique concept opens
space for coils but makes
for a challenging design
All of those rays of 
θ
CM
=[90,120] that 
you don’t get here...
CM angles 60
o
-120
o
11 GeV in: 2.75 to 8.25 GeV out
Lab angles ~5
mrad
- 17
mrad
Target length 1.5 meters
Asymmetry (ppb)
Center of Mass Angle
Highest figure of merit at
θ
CM
= 90
o
A
PV
dσ/dΩ
January 14, 2010
Spectrometer and Collimation
Toroid configurations
One magnet doesn’t do it: multiple magnets can

ep / ee separation

radial focus size

~40m to fit in Hall A

beam line aperture

photon/neutral backgrounds
Bend hard track out, bend
soft tracks out and then
back
Bend hard track out hard to
meet soft tracks
- superconducting magnet
Bend hard track out softly
ep
ep
ep
ee
ee
ee
Must control integral Bdl vs. track
angle using magnet geometry
radial “fringe” fields are crucial!
I
I
January 14, 2010
Spectrometer and Collimation
Radial and Azimuthal Field
Radial field component
vs. radius
off-center of open sector
repels electrons
from coil
attracts electrons
toward coils
Effectively focuses or
defocusses azimuthally!
- radial and azimuthal focus can be
degraded by this effect
- in extreme cases, the edge of “open”
sector can be bent into magnet coil!
Inner and outer edges of
toroids have significant
radial field components
“focussing”
“defocussing”
January 14, 2010
Spectrometer and Collimation
Calculating the Field
Design prototypes were calculated using home-built code

Field was calculated using Biot-Savart law for thin current segments

Finite conductor cross-section was represented by distribution of many
thin conductors

Coil geometries were constructed from these straight line segments

Simplified descriptions of coil geometry led to some shape distortions
(no curves and non-uniform conductor cross-section)
Field calculations
Particle tracking

selected optics rays were used, spanning center-of-mass scattering
angles and vertex positions in the target for Moller and Mott scattering

rays were propagated using 4th order Runge-Kutta
Calculations were repeated with doubled tracking step size, and doubled
conductor division size, to check for errors due to finite step approximation
calculations. Granularity was sufficient to keep track deviations small (<1cm)
in process of being reproduced/
superceded by TOSCA
now superceded by GEANT4
simulation using field map
January 14, 2010
Spectrometer and Collimation
“Nested” Toroidal coils
4 separate current returns so downstream end
of magnet is much stronger than upstream end
Tracks here are
defocused in phi by
radial field
Tracks here receive
extra B.dl from long
path length
29160
A/coil
16860
A/coil
10600
A/coil
7750
A/coil
204,120 A total
4.3 cm
10 m
11 m
12 m
13 m
14 m
14.5m
16.5m
38 cm
height
100% azimuth “line current” near beamline
14-17 degree azimuthal fill for coil limbs
System needs a first toroidal magnet:

pushes highest angle tracks above
high field region in 2nd magnet


focuses
other tracks in azimuth
Magnet geometry used to control integral
Bdl without excessive defocusing
Very modest power consumption, ~600 kW
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January 14, 2010
Spectrometer and Collimation
Spectrometer Ray Traces
Radial focus ~ 1 m, @ z ~ 28 m
note: aspect ratio is 1:4
long and skinny
First toroidal magnet:

pushes highest angle tracks above high field
region in 2nd magnet


focuses
other tracks in azimuth
Defocusing results in
population of full azimuth
lab angle (radians)
January 14, 2010
Spectrometer and Collimation
Toroid design concept
meters
ee
ep
meters

 ep

 ee, 60
o
‐75
o

 ee, 75
o
‐105
o

 ee, 105
o
‐120
o
r (m)
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3
rate (GHz)
0
2
4
6
8
10
ee
ep
r (m)
“blocked” segment
half of
“open”
segment
neighboring
“open”
segment
meters
meters
(millirad)
la
b
!
0 5
10
15
20
25
rate (GHz)
0
1
2
3
4
5
laboratory scattering angle
January 14, 2010
Spectrometer and Collimation
Azimuthal Variation
wrap
!
!
20
!
10
0 1
0 2
0
rate (GHz)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
3 different phi distributions
one-seventh of the azimuth
open sector
behind
primary
collimator
behind
primary
collimator
r (m)
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
rate (GHz)
0
2
4
6
8
10
elastic e-p
e-e
January 14, 2010
Spectrometer and Collimation
Transverse Asymmetry
Interference between one- and
two-photon exchange
electron beam polarized
transverse to beam direction
Theory References:
1. A. O. Barut and C. Fronsdal, (1960)
2. L. L. DeRaad, Jr. and Y. J. Ng (1975)
3. Lance Dixon and Marc Schreiber:hep/ph-0402221
Measured at E158
For identical particles: magnitude
of asymmetry must be odd around
90 degrees in the center of mass
Potential systematic error in A
PV
.
Suppressed by
- small transverse polarization
- azimuthal acceptance symmetry
- acceptance symmetry in c.m.s.
polar angle
January 14, 2010
Spectrometer and Collimation
Transverse Polarization
detector number
0 5 10 15 20 25
(ppm)
T
A

10

5
0
5
10
Average transverse asymmetry
Average transverse asymmetry
Initial beam setup ~ 1-2 degree
unique “signature” of transverse beam polarization
Over full run: feedback will hold transverse polarization to be
small (<<1 degree), azimuthal cancellation suppresses effect
wrap
!
!
20
!
10
0 1
0 2
0
rate (GHz)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
(millirad)
la
b
!
0 5
10
15
20
25
rate (GHz)
0
1
2
3
4
5
laboratory scattering angle
expected grand average
for the simulated
experimental acceptance
50 ppb error on A
T
*P
b
in 4 hours: 1 degree precision
January 14, 2010
Spectrometer and Collimation
inelastic background
Inelastic ep Z-couplings
poorly known at these
kinematics
A
PV
as function of detector
r & φ helps bound inelastic
asymmetry contributions
“red”
“green”
“blue”
Expect: 4% correction, 0.4% error
wrap
!
!
20
!
10
0 1
0 2
0
rate (GHz)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
ee elastic
ep elastic
ep inelastic
Azimuthal segmentation gives
independent measurements
around the ee elastic peak with
varying inelastic fraction
center
open
edge
center
closed
P. Souder
January 14, 2010
Spectrometer and Collimation
Collimation
detector
target
GEANT4 studies of
backgrounds and scatters
Goals:

power dissipation

minimize one-bounce edges

beamline design
Tungsten collimation near beamline
Taper
2.3
3.4
2.397
3.477
January 14, 2010
Spectrometer and Collimation
Beamline Collimation
target
Toroid 1
Toroid 2
Collimator 5
(defines lower angle
of acceptance)
Collimator 4
Collimator 2
Energy Deposited per collimator
Collimator
e-
e+
photon
2
1350 W
202 W
126 W
4
945 W
172 W
74 W
~150 kW photons from 18% R.L. target
Taper
2.3
3.4
2.397
3.477
January 14, 2010
Spectrometer and Collimation
Beamline Collimation
target
Toroid 1
Toroid 2
Collimator 5
(defines lower angle
of acceptance)
Collimator 4
Collimator 2
Energy Deposited per collimator
Collimator
e-
e+
photon
2
1350 W
202 W
126 W
4
945 W
172 W
74 W
~150 kW photons from 18% R.L. target
January 14, 2010
Spectrometer and Collimation
Photon background
Preliminary design:
~1.3% total
energy at detector from soft photons

Magnet material not yet included (will
provide more shielding)

Additional collimation not yet explored

In this first attempt, ~12 low energy photons
(~6 MeV average) per signal electron.

Further optimization underway.
Preliminary design
January 14, 2010
Spectrometer and Collimation
TOSCA
January 14, 2010
Spectrometer and Collimation
TOSCA Model of First Toroid
Verified design field and current density (<1100 A/cm
2
).
Started thinking in more detail about conductor cross-section, winding, cooling, etc.
January 14, 2010
Spectrometer and Collimation
Field magnitude at
midplane, first toroid
January 14, 2010
Spectrometer and Collimation
First attempt:
We know how to use
TOSCA, but in the first
attempt the inner limb of
the toroid didn’t cover the
specified radial range...
correcting the geometry
reproduced the design field
Radial field
component
Total field magnitude vs. radius
January 14, 2010
Spectrometer and Collimation
Beamline
Goal: no vacuum windows or flanges
for accepted events until detector
January 14, 2010
Spectrometer and Collimation
Outlook

Launch magnet advisory group:

George Clarke (TRIUMF), Ernie Ihloff & Jim Kelsey 
(MIT‐Bates), Dieter Walz (SLAC), Paul Brindza 
(JLab)

Plans 

Model realis8c hybrid toroid in TOSCA with 
expert input

Begin detailed evalua8on the mechanical 
challenges of this magnet
beam
Second toroid is a challenge
DOE supporting UMass postdoc Juliette Mammei
primarily on this topic, starting Jan ’10
Spectrometer op8cs / collima8on / beamline 
must be approached coherently

work with engineers to op8mize op8cs, collima8on and 
magnet engineering
Robust design concept fulfills experimental requirements
No detailed engineering has yet been done