1. UpstreamDownstream S PECIFYING A MAGNET Key: A coil Position of upstream edge Inner radius Current in coil Conductor This is a generic magnet, it’s not meant to look like the SS or AFC. We can model any magnet we want!

2. F ITTING A FIELD MAP Mixing/Scaling fit procedure: 1.Take data in cylindrical co-ordinates a)In this case using a pre- calculated map based on SS. 2.Make two detailed field maps with parameters that “bracket” our best guess at the real paramters 3.Minimise for “best fit” parameters: a)Mixing of the detailed field maps b)Length scale of the detailed field maps c)Field scale of the detailed field maps Two methods: Mixing/scaling fit Full 20+ parameter coil fit – takes FOREVER for SS

3. F ITTING A FIELD MAP Mixing/Scaling fit procedure: 1.Take data in cylindrical co- ordinates a)In this case using a pre- calculated map based on SS. Again, a generic magnet Spectrometer Solenoid parameters

4. F ITTING A FIELD MAP Mixing/Scaling fit procedure: 2.Make two detailed field maps with parameters that “bracket” our best guess at the real parameters Original magnet “Long, thin” bracketing magnet “Short, fat” bracketing magnet

5. F ITTING A FIELD MAP Mixing/Scaling fit procedure: 2.Make two detailed field maps with parameters that “bracket” our best guess at the real parameters

6. F ITTING A FIELD MAP Mixing/Scaling fit procedure: 3.Minimise for “best fit” parameters: a)Mixing of the detailed field maps b)Length scale of the detailed field maps c)Field scale of the detailed field maps

7. A SIDE : I NTERPOLATING FIELDS How well does this work? (weakest link) Lesson: Make sure the bracketing fields are calculated on a finer grid than the data! 1e-15

8. F ITTING A FIELD MAP Few grid points Many grid points

9. F ITTING A FIELD MAP zoomed y-scale

10. F ITTING A FIELD MAP zoomed y-scale

11. I MPROVEMENTS TO COME : Need to account for longitudinal offset + two rotations This is included in the 20+ parameter fit, but is far too slow for the SS map Adding Fourier-Bessel fit to account for residual field (green plots) will improve our map further. Also helps account for Virostek plate longitudinal offset + in/out of screen rotations