Tagger Magnetic Field Mapping

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Detailed Proposal

Tagger mapping boxes-8-8-2013.png

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

  1. 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.
    Tagger electron orbits map.png
  2. 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
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 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:

  1. 6 kG
  2. 9 kG
  3. 12 kG
  4. 14 kG
  5. 15 kG
  6. 16 kG
  7. 17 kG