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Status report about the e- identifier
Study of shielding against the stray magnetic field at the downstream end of the spectrometer
F. Huveneers, Gh. Grégoire, M. Cyamukungu
September 10, 2003
• The photomultipliers of the Cerenkov detector have to be shielded.
• Shielding is made of a large and thick iron slab provided with a central hole (diam. 50 cm) located around the beam axis.
• The z-position of (the upstream face of) this slab is presently located 20 cm downstream of the last correction coil of the spectrometer.
The present study estimates the residual field at the off-axis position of the photomultipliers.
Active magnetic elements
From R. Palmer,Version 5
Compute stray field in r-z space after the last correction coil with/without an iron shielding of variable thickness
r
z
Current densities
3D view of spectrometer with magnetic shielding
Correction coils
Solenoid
Soft iron shieldingMatchingFocus
Coupling
Computations
Field equations in the presence of ferromagnetic materials are solved with Poisson/Superfish
http://laacg1.lanl.gov/laacg/services/possup.html
Field lines without shielding
;CAS 0 : pas de blindage
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0
20
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-20
0
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0 20 40 60 80 100 120 140 160 180
z (cm)
r (cm)Beam axis
diam. 50 cm
Correction coil
End of solenoid
O
With a 5 cm thick shielding
;CAS 3 : blindage 3
C:\PUBLIC\FH_STAGE\LANL\ESSAIS\MICE\CAS3.AM 8-18-2003 14:21:24
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z (cm)
r (cm)
Particle flight path
5 cm thick soft iron
diam. 50 cm
20 cm
O
With 15 cm thick iron shield
;CAS 1 : blindage 1
C:\PUBLIC\FH_STAGE\LANL\ESSAIS\MICE\CAS1\CAS1.AM 9-08-2003 10:20:30
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0 20 40 60 80 100 120 140 160 180
10 cm thick soft ironz (cm)
r (cm)
Particle flight path
diam. 50 cm
20 cm
O
Field along beam axis
End of correction coil
Bz along beam axis (r = 0)
0
15000
30000
45000
-80 -60 -40 -20 0 20 40 60 80 100
z [cm]
Bz
[Gau
ss] No shield
Shield 15 cm thick
10 cm
5 cm
Field inside solenoid
Location of the e- identifier – Look at next transparency for detailed view
Bz along beam axis (r = 0)
0
2000
4000
6000
8000
35 45 55 65 75 85 95
z [cm]
Bz
[Gau
ss]
No shield Shield 15 cm thick 10 cm iron 5 cm iron
Stray field downstream along beam axis
Approximate z-position of photomultiplier axis
1 kGauss limit
Influence of the shielding
-2500
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0
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2500
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Z [cm]
Bz
(with
sh
ield
) -
B z (
with
ou
t sh
ield
) [G
au
ss]
Shield 15 cm thick
10 cm
5 cm
Shown here: modifications of Bz along beam axis (r=0)
Conclusions
A soft iron shield ~15 cm thick with a central hole of ~50 cm diameter is needed to reduce to 1 kGauss the stray field at the PM positions.
- For thinner shields, the saturation of iron is reached around the central hole.
The subsequent field modifications affect:
- the tracking of particles and the simulations both upstream and downstream
- the construction of the containment vessel for the correction coil and the solenoid.
They should be hopefully taken into account in close cooperation with the relevant MICE experts.
- The residual field is further reduced by a specific mumetal shield around the photomultipliers
- the magnetic field configuration of the solenoid
Work underway
1. Optimization of the shielding parameters (thickness, central opening, shield saturation …)
2. Generation of a C++ program to evaluate field components at any point downstream the last correction coil and the shielding
3. Implementation in Geant 4 + for downstream tracking
We are waiting for comments and suggestions from the experts …
