dipole fringe field

[nadolski]

Dear All,

Concerning the setting of the dipole fringe field, I feel a little confused with the documentation.
The doc reads that EDGE_ORDER is ignored for EDGE1_EFFECTS = 3 or 4.

This is not the case.
- it seems that that EDGEORDER need to be set to at least 2 in order to profit from the options EDGE1_EFFECTS = 3 or 4
- so EDGEORDER is not ignored. Its value should be change from the default value 1 to 2


https://ops.aps.anl.gov/manuals/elegant ... 3600010.19:

Several models are available for edge (or fringe) effects. Which is used depends on the settings of the EDGE_ORDER, EDGE1_EFFECTS, and EDGE2_EFFECTS parameters EDGE1_EFFECTS controls entrance edge effects while EDGE2_EFFECTS controls exit edge effects, as follows:

1: — Edge effects using non-symplectic method [3].
EDGE_ORDER<2 — linear edge focusing with δ-dependence to all orders. Generally not recommended if symplecticity is important, though when the edge effects are weak it appears acceptable.
EDGE_ORDER>=2 — second-order matrix edge focusing with δ-dependence to all orders. Use of this model is strongly discouraged when symplecticity matters.
2: — Edge effects using K. Hwang’s method [45]. Note that there will be a trajectory offset when using this method that is particularly evident for small bending radii, due to extension of the fringe field outside the body of the magnet. To suppress this, adjustment of the FSE parameter can be performed automatically if FSE_CORRECTION is set to a non-zero value. If FSE_CORRECTION=1, the path-length is adjusted to match the nominal length, which is not physical; this behavior can be suppressed by setting FSE_CORRECTION=2.
EDGE_ORDER<2 — include only terms linear in transverse coordinates, but δ-dependence to all orders. Recommended for applications where symplecticity matters. This method is symplectic.
EDGE_ORDER>=2 — include all terms. This settings has been observed to produce emittance damping in some cases (particularly with large emittance and small bending radii), so users are advised to be cautious. This method is non-symplectic.
3: — Edge effects using symplectic method similar to [3]. The value of EDGE_ORDER is ignored. Recommended for applications where symplecticity matters.
4: —- Edge effects using symplectic method developed by R. Lindberg based on K. Hwang’s method [45]. Note that there will be a trajectory offset when using this method that is particularly evident for small bending radii, due to extension of the fringe field outside the body of the magnet. To suppress this, adjustment of the FSE parameter can be performed automatically if FSE_CORRECTION is set to a non-zero value. If FSE_CORRECTION=1, the path-length is adjusted to match the nominal length, which is not physical; this behavior can be suppressed by setting FSE_CORRECTION=2. The EDGE_ORDER parameter is ignored.
Other: — No edge effects.

[michael_borland]

Laurent,

I agree that the manual is unclear. What it should say is that the edge order doesn't matter for tracking. For computation of chromaticities, for example, the edge order is relevant since it determines the order of the matrices that are used. I should probably just set edge_order=2 when those models are used to make things as consistent as possible.

You may want to set TRACKING_MATRIX=2 on the CSBEND elements to force elegant to use fully consistent models for tracking and lattice parameter computations. Otherwise, the latter use the old Karl Brown matrices.

--Michael

[nadolski]

Thanks