Add Regression Test for Low Energy Tracking Through Buncher Cavity

[electronsandstuff]

This PR adds an accompanying regression test and minimum working example for #1762.

[DavidSagan]

@electronsandstuff Looks like there are conflicts there that need to be resolved. Note: I did just add a test for this on my side.

[electronsandstuff]

OK, resolved if you want to add this one too.

[DavidSagan]

@electronsandstuff Looks like now there are failing tests...

[electronsandstuff]

I am checking on my end and there still seems to be an issue in the cavity. That's why the new unit test is failing. Here is Bmad 20251016.0 on the test lattice.

Here is Bmad 20250915.0

I think something is still going on with the energy calculations, but I also don't have permissions to re-open the issue.

[DavidSagan]

I assume by "issue" you mean #1762. See my comment at the end of this issue. The fact that there is a difference now as compared to 20250915.0 is to be expected.

[electronsandstuff]

Sorry if I am misunderstanding something, but I am still confused.

I know that E_tot is the reference energy, but since the lattice doesn't have any pz orbit in either Bmad version, it is also the energy of the bunch.

In the older Bmad version, the code says that the bunch gains 100keV through the cavity. In the latest Bmad, the bunch's energy doesn't change (no change in E_tot or orbit pz). Outside of convergence issues, I would expect there to be a consistent value for the beam's energy after the cavity. Maybe I am calculating it incorrectly or missing something?

[DavidSagan]

Yes there has been a change and the results are not consistent. What the code did in the past was not what a User would expect so this has changed.

[DavidSagan]

If you want we can zoom and I can explain in detail.

[electronsandstuff]

OK, thanks. I will reach out to you over slack. In the meantime it sounds like this test isn't correct and I can close out the PR.