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Why do we need to know the fields / map the magnets?

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

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Why do we need to know the fields / map the magnets?

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  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

  2. 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

  3. Effect of adjacent coils on field in Tracker Region of Spectrometer Solenoids

  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...

  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

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

  7. Transverse fields linear in distance from axis and proportional to field gradient

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