1. Why do we need to know the fields / map the magnets?
Spectrometers and FCs and CCs have different purposes:
FCs and CCs form the cooling channel
Should have been built well enough to work
Job of SS’s is to demonstrate that the cooling channel works
Field must be well known for tracker reconstruction
Relative fields (or integrals) SS1 / SS2 must be known to better than a few parts in 10,000
There is some coupling between SSs and coils in cooling chan.
Physically – fields overlap
Logically – need to time muons to first RF cavity
 Track muons from TOFs + Tracker  first cavity
Largest uncertainty in any prediction of cooling i.e. theory, is undoubtedly G4 implementation of multiple scattering
12 Apr 2012

2. Many configurations of optics x momenta
MICE VI has
18 coils
Many configurations of optics x momenta
ONLY central coil (+ maybe, End Coils) of SS will remain constant ~ 4T
Currents in other coils will change
Coils talk to each other over a few metres
e.g. ‘Uniform field’ region of SS sees field due to CC at ~ 0.5 percent
(see next page)
There is a lot of iron in the MICE hall
– but won’t make itself felt until CCs are installed (my belief).
 Cannot possibly map every configuration and use measured maps
12 Apr 2012

3. Field at centre of tracker
Effect of adjacent coils on field in Tracker Region of Spectrometer Solenoids
Configuration
Field at centre of tracker
region (z=5.2m) (Tesla)
Relative
difference
at centre
Field at end of tracker
region (z=5.7m)
Uniformity =
(End / Centre – 1)
Baseline flip mode; all coils powered
3.991
4.007
+0.004
M1 only powered off
3.967
– 6.0 x 10-3
3.999
+0.008
M2 only powered off
3.978
– 3.3 x 10-3
4.001
+0.006
Nearest Focus Coils off
3.987
– 1.0 x 10-3
4.005
+0.005
Nearest Coupling coil off
4.009
+ 4.5 x 10-3
3.994
–0.004
12 Apr 2012

4. What is to be done? Sufficient measurements to:
Determine (check) the geometry of each coil alone
A few parameters:
– thickness, length, inner & outer radii and magnetic axis
– all wrt external fiducial marks
Check linearity
Measure at a few different excitations
Check superposition of fields due coils in same module
Measure with all coils in module energised at some
nominal currents
For reconstruction & simulation use fields calculated from dimensions of coils
It’s a moot point as to whether simulations should use real or ideal fields – in principle coils should give fields which are close enough to ideal, save for effect of iron in the hall...
12 Apr 2012

5. What’s needed? Shall need:
Some code to calibrate / analyse output of CERN mapper
Not sure exactly what we get
Some means of calculating fields to sufficient precision
In 2D = (z,r)
In 3D = (x,y,z) – perhaps
Off-axis fields due to circular coils must be calculated numerically – elliptic integrals or numerical integration
Common sense
12 Apr 2012

6. Survey / calibrate coils Qualify Field qualityx
Coil
Cryostat
Fiducial marks
Mapping
grid
MAP / MEASURE
Survey wrt
fiducials
Installation /
Alignment
Survey / calibrate coils
Qualify
Field quality
FIELD CALCS
Reduce /
Parameterise … ON
Good
Enough?
YES
Simple
Model
Map for
Software
G4MICE
MAP / MEASURE FIELD IN HALL
VERIFY OPERA CALCULATIONS
LONG-RANGE FIELD ERRORS
12 Apr 2012

7. 12 Apr 2012

8. Transverse fields linear in distance from axis
and proportional to field gradient
12 Apr 2012

9. 12 Apr 2012

10. 12 Apr 2012

11. 12 Apr 2012