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Published byPrimrose Flynn Modified over 9 years ago
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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
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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)
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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%
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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
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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
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Effect of Change in Coil Stack Height of 1 mm Aug. 12, 2014CLAS12 Torus B-field Distortions6 ~ 18 Gauss change in field; ~ 0.1 %
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Effect of Coil Motion in Bore Jan. 7, 2013CLAS12 Torus B-field Distortions7
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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
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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
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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)
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Goal: ≤ 0.2% dB/B at R = 40 cm Aug. 12, 2014CLAS12 Torus B-field Distortions11 Coil Inner Radial position tolerance ~ 0.5 mm
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Ideal Torus Magnetic Field Jan. 7, 2013CLAS12 Torus B-field Distortions12 / 7 This field map was calculated with nominal geometry. beam direction
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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
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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
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