1. CLAS12 Torus Magnetic Field Mapping
- qualitative look at the field distribution
How well do we need to know the B-field?
- momentum resolution with ideal torus
What are the effects of construction inaccuracies?
- misplacement of coils  distortion of Bfield
How can we measure the distortions?
- measure distortion field
know coil position
calculate true field
I am going to outline a strategy for dealing with non-ideal torus geometry.
The non-ideal geometry results in a non-ideal magnetic field which in turn results in shifts in track parameters; particularly the momentum. This is easy to see: if you reconstruct a track to obtain its emission angles (phi and theta) and its curvature and you multiply the inverse curvature (“stiffness”) by the integral B-dl, you obtain the momentum. If you make a 1% mistake on the integral B-dl you have a 1% error
Credit for this idea goes to Bernhard Mecking
Jan. 7, 2013
CLAS12 Torus B-field Distortions

2. Where are the Strictest Tracking Requirements?
Here I show the trajectory* of an electron (p = 10.1 GeV/c, q = 7o)
- elastic scattering from 11 GeV beam (highest momentum particle at 7o )
fractional dp/p most important at high momentum
radius ~ 42 cm
* Note: plot courtesy of ced
(Dave Heddle)
I am going to outline a strategy for dealing with non-ideal torus geometry.
The non-ideal geometry results in a non-ideal magnetic field which in turn results in shifts in track parameters; particularly the momentum. This is easy to see: if you reconstruct a track to obtain its emission angles (phi and theta) and its curvature and you multiply the inverse curvature (“stiffness”) by the integral B-dl, you obtain the momentum. If you make a 1% mistake on the integral B-dl you have a 1% error
Jan. 7, 2013
CLAS12 Torus B-field Distortions

3. CLAS12 Tracking Resolution
Momentum Resolution:
Ideal B-Field
DC resolution ~ 300 mm
Ideal DC alignment
Best at small theta
highest B-field
dp/p Goal: ~ 0.3%
Simulation & Reconstruction 10/30/ S.Procureur

4. Torus Coil Dimensional Tolerances
Most critical point for tracking
Radius ~ 50 cm
Most critical dimension
Large radius of inner coil
Aug. 12, 2014
CLAS12 Torus B-field Distortions

5. Torus Mapping Location
Magnetic mapping Points
Radius = 50 cm, 100 cm
Angle = -15, 0, 15o
measure B-field to 0.1%
need to measure radius to 0.1%
0.5 mm at 50 cm
Aug. 12, 2014
CLAS12 Torus B-field Distortions

6. Torus Magnetic Mapping vs. Z
Measurement Tubes
non-magnetic metal
~ 1” diameter
Rotating Coil
~ 2.5 X 2.5 cm
100 turns, 10 Hz  1V
measure ~ every 5cm in Z
Digital Voltage Integrator
Jan. 7, 2013
CLAS12 Torus B-field Distortions

7. Torus Mapping: Next Steps
need very good survey and good mapping
accelerator group is currently developing a mapping device similar to what we need
set up meeting with
Ken Bagget, Joe Meyers (accelerator mag-mapping)
Chris Curtis (head of metrology)
Hall B representation
Jan. 7, 2013
CLAS12 Torus B-field Distortions

8. Magnetic Field Change in Mid-plane
% Change in B-field:
plotted vs. radius
coil stack too large by 2mm
coil moved down, centered, up
2 mm increase in stack size ok if coil is moved inward to compensate
large radius surface of inner coil is most critical dimension
survey of total stack height important
adjust (shim) to compensate
I am going to outline a strategy for dealing with non-ideal torus geometry.
The non-ideal geometry results in a non-ideal magnetic field which in turn results in shifts in track parameters; particularly the momentum. This is easy to see: if you reconstruct a track to obtain its emission angles (phi and theta) and its curvature and you multiply the inverse curvature (“stiffness”) by the integral B-dl, you obtain the momentum. If you make a 1% mistake on the integral B-dl you have a 1% error
Jan. 7, 2013
CLAS12 Torus B-field Distortions

9. Effect of Change in Coil Stack Height of 1 mm
~ 18 Gauss change
in field; ~ 0.1 %
Aug. 12, 2014
CLAS12 Torus B-field Distortions

10. Effect of Coil Motion in Bore
Jan. 7, 2013
CLAS12 Torus B-field Distortions

11. CLAS12 Torus B-field Distortions
Recommendations
Torus Construction
- concentrate on critical dimensions
radial placement: < 1 mm
out-of-plane: < 1cm (preliminary)
Plan a Magnetic Mapping Strategy
- measure distortion field in bore for radial displacement
- sample points in fiducial area?
 measure coil displacements  calculate true field
I am going to outline a strategy for dealing with non-ideal torus geometry.
The non-ideal geometry results in a non-ideal magnetic field which in turn results in shifts in track parameters; particularly the momentum. This is easy to see: if you reconstruct a track to obtain its emission angles (phi and theta) and its curvature and you multiply the inverse curvature (“stiffness”) by the integral B-dl, you obtain the momentum. If you make a 1% mistake on the integral B-dl you have a 1% error
Jan. 7, 2013
CLAS12 Torus B-field Distortions

12. CLAS12 Torus Magnetic Field Mapping Mac Mestayer
I am going to outline a strategy for dealing with non-ideal torus geometry.
The non-ideal geometry results in a non-ideal magnetic field which causes shifts in track parameters; particularly the momentum.
This is easy to see: if you reconstruct a track to obtain its emission angles (phi and theta) and its curvature, and you multiply the inverse curvature (“stiffness”) by the integral B-dl, you obtain the momentum. If you make a 1% mistake on the integral B-dl you have a 1% error on the calculated momentum.
Jan. 7, 2013
CLAS12 Torus B-field Distortions

13. Calculations using “RADIA”
This field map was calculated by Burin Asavapibhop using the program “RADIA” * and a description of the coils as four straight lines and four circular arcs
*(supplied by Lionel Quettier)
Preliminary (received Jan.2)
I am going to outline a strategy for dealing with non-ideal torus geometry.
The non-ideal geometry results in a non-ideal magnetic field which in turn results in shifts in track parameters; particularly the momentum. This is easy to see: if you reconstruct a track to obtain its emission angles (phi and theta) and its curvature and you multiply the inverse curvature (“stiffness”) by the integral B-dl, you obtain the momentum. If you make a 1% mistake on the integral B-dl you have a 1% error
Jan. 7, 2013
CLAS12 Torus B-field Distortions

14. CLAS12 Torus B-field Distortions
Goal: ≤ 0.2% dB/B at R = 40 cm
Coil Inner Radial
position tolerance
~ 0.5 mm
Aug. 12, 2014
CLAS12 Torus B-field Distortions

15. Ideal Torus Magnetic Field
This field map was calculated with nominal geometry.
beam direction
I am going to outline a strategy for dealing with non-ideal torus geometry.
The non-ideal geometry results in a non-ideal magnetic field which in turn results in shifts in track parameters; particularly the momentum. This is easy to see: if you reconstruct a track to obtain its emission angles (phi and theta) and its curvature and you multiply the inverse curvature (“stiffness”) by the integral B-dl, you obtain the momentum. If you make a 1% mistake on the integral B-dl you have a 1% error
Jan. 7, 2013
CLAS12 Torus B-field Distortions

16. Distortion Calculation using RADIA
Coil moved radially by 2 mm
Field changes by ~ 1% at 42 cm (need a better graph)
- agrees roughly with my calculation
I am going to outline a strategy for dealing with non-ideal torus geometry.
The non-ideal geometry results in a non-ideal magnetic field which in turn results in shifts in track parameters; particularly the momentum. This is easy to see: if you reconstruct a track to obtain its emission angles (phi and theta) and its curvature and you multiply the inverse curvature (“stiffness”) by the integral B-dl, you obtain the momentum. If you make a 1% mistake on the integral B-dl you have a 1% error
Jan. 7, 2013
CLAS12 Torus B-field Distortions

17. Magnetic Field Change in Bore
Radial component of the B-field measured at radii = 4, 2, 0 cm
Black: ideal (no offset)
Light: one coil offset 2mm (rad)
40 Gauss dipole field on axis
azimuth determines which coil
I am going to outline a strategy for dealing with non-ideal torus geometry.
The non-ideal geometry results in a non-ideal magnetic field which in turn results in shifts in track parameters; particularly the momentum. This is easy to see: if you reconstruct a track to obtain its emission angles (phi and theta) and its curvature and you multiply the inverse curvature (“stiffness”) by the integral B-dl, you obtain the momentum. If you make a 1% mistake on the integral B-dl you have a 1% error
Jan. 7, 2013
CLAS12 Torus B-field Distortions