# Tagger Magnetic Field Mapping

## Contents

### Detailed Proposal

- Updated tagger mapping requirements from Dan Sober, August 12, 2013
- Updated study of ray-tracing through the tagger field, Dan Sober, August 14, 2013
- Updated boundaries for tagger dipole field map, Dan Sober, August 15, 2013
- Displacement of rays on focal plane vs emission angle, Dan Sober, August 26, 2013
- Updated boundaries for tagger dipole field map, Dan Sober, October 7, 2013
- Plot of the estimated fringe fields, Dan Sober, October 14, 2013

For guidance in mapping, here is a plot of electron orbits for 0.3, 1, 3, 6, 9, 12 GeV in mapping coordinates. The x and y scales differ by about a factor of 10 for ease in reading. The rectangle is the pole root (base of chamfer.) For example, while we need field values extending to x = -14.5 cm to cover the full-energy orbit, we need measure only to about x = -10 cm for y > 200 cm or y < -200 cm. Unfortunately, in the hard-to-measure region behind the flange near y = -300 cm, x > 0 we can use all the points we can get.

The above documents supersede the draft proposals described below.

### Mapping requirements

- We need to map every region of the field where electrons are transported, but can omit corners where no trajectories go. I attach a crude picture of electron orbits. Maybe Sascha has a better one.
- By my previous experience ray-tracing in the Hall B tagger magnet (6 cm gap), I would propose the following.
- Uniform field region inside gap (at least 10 cm or 4" away from a field boundary:
- 1" by 1" grid

- In the vicinity of the long field boundary:
- 1" (parallel to edge) by 0.25" transverse from -10 cm inside to +15 cm outside
- 1" (parallel to edge) by 1" transverse from +15 cm to +30 cm outside

- In the vicinity of the entry face and the full-energy beam exit, cover a region at least 6" (i.e. +/- 3") wide around the trajectories:
- 1" by 1" from 30 cm to 15 cm outside,
- 1" by 0.25" transverse from 15 cm outside to 10 cm inside

- Uniform field region inside gap (at least 10 cm or 4" away from a field boundary:

- A step of 0.25" near the field boundary means that the maximum field variation per step (which occurs just inside the nominal field boundary) is about 15%. This should be acceptable. If the time required for the above is excessive, we could take 2" steps parallel to the long field edge instead of 1".

### Proposal from Tim Whitlatch - Mapper dwgs

We are currently planning 1 cm steps all along the transverse direction and 2.5 cm steps along the long edge. This is what our fixture is designed to do since we have multiple hall probes 1 cm apart. It appears we can cover all your orbits shown.

- Mapping fixture drawings

### Mapping coordinate system

The map coordinate system used by Yang for reporting TOSCA results is awkward because it is left-handed and uses a different convention for the origin in the x and z directions. For the empirical field maps, *we will not use Yang's convention*. Instead, we will use a right-handed coordinate system, defined as follows.

- z points vertically upward
- y points along the long axis of the magnet
- x points along the short axis of the magnet in the bend plane, with x increasing in the direction that incident electrons bend in the field.
- The origin (0,0,0) is the geometric center of the magnetic poles, located in the middle of the gap.

### Fields at which maps should be taken

Dan Sober proposes mapping the dipole at the following central field values:

- 6 kG
- 9 kG
- 12 kG
- 14 kG
- 15 kG
- 16 kG
- 17 kG