1. CLAS12 Torus Magnetic Field Mapping Torus Magnetic Field - 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 Jan. 7, 2013CLAS12 Torus B-field Distortions1 / 7 Credit for this idea goes to Bernhard Mecking

2. Where are the Strictest Tracking Requirements? Jan. 7, 2013CLAS12 Torus B-field Distortions2 / 7 Here I show the trajectory* of an electron (p = 10.1 GeV/c,  = 7 o ) - elastic scattering from 11 GeV beam (highest momentum particle at 7 o ) -fractional dp/p most important at high momentum -radius ~ 42 cm * Note: plot courtesy of ced (Dave Heddle)

3. CLAS12 Tracking Resolution Simulation & Reconstruction10/30/2008 S.Procureur Momentum Resolution: Ideal B-Field DC resolution ~ 300  m Ideal DC alignment Best at small theta highest B-field dp/p Goal: ~ 0.3%

4. Torus Coil Dimensional Tolerances Aug. 12, 2014CLAS12 Torus B-field Distortions4 Most critical point for tracking Radius ~ 40 cm Most critical dimension Large radius of inner coil Stack height less critical if aligned at outer surface

5. Magnetic Field Change in Mid-plane Jan. 7, 2013CLAS12 Torus B-field Distortions5 / 7 % 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

6. Effect of Change in Coil Stack Height of 1 mm Aug. 12, 2014CLAS12 Torus B-field Distortions6 ~ 18 Gauss change in field; ~ 0.1 %

7. Effect of Coil Motion in Bore Jan. 7, 2013CLAS12 Torus B-field Distortions7

8. 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 Jan. 7, 2013CLAS12 Torus B-field Distortions8 / 7

9. 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, 2013CLAS12 Torus B-field Distortions9 / 7

10. Calculations using “RADIA” Jan. 7, 2013CLAS12 Torus B-field Distortions10 / 7 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)

11. Goal: ≤ 0.2% dB/B at R = 40 cm Aug. 12, 2014CLAS12 Torus B-field Distortions11 Coil Inner Radial position tolerance ~ 0.5 mm

12. Ideal Torus Magnetic Field Jan. 7, 2013CLAS12 Torus B-field Distortions12 / 7 This field map was calculated with nominal geometry. beam direction

13. Distortion Calculation using RADIA Jan. 7, 2013CLAS12 Torus B-field Distortions13 / 7 Coil moved radially by 2 mm Field changes by ~ 1% at 42 cm (need a better graph) - agrees roughly with my calculation

14. Magnetic Field Change in Bore Jan. 7, 2013CLAS12 Torus B-field Distortions14 / 7 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