fix B-field normalization in torus pgen

[Yurlungur]

…Bsq calc in tmunu and torus

PR Summary

What it says on the can. The issue is that the density is already normalized when the vector potential is computed. Dividing by an additional factor of rho_rmax shrinks A and, thanks to the -0.2 in the definition, kills A, and thus B, at the edges of the torus.

PR Checklist

• [x] Adds a test for any bugs fixed. Adds tests for new features.
• [x] Format your changes by calling scripts/bash/format.sh.
• [x] Explain what you did.

[Yurlungur]

There was also a missing factor of alpha^2 in the calculation of bsq in the normalization routine that sets beta, meaning beta was incorrectly computed.

[jdolence]

good catch. trying to understand the substantive changes here. From my read, the only thing actually changing is that we were erroneously dividing by rho_rmax, which this removes. The rest of the edits don't look like they actually change anything. Is that right?

[Yurlungur]

There was also a missing alpha^2 in the calculation of bsq used when computing beta but I think it probably didn't matter because b0 should be zero in the initial conditions.

[jdolence]

There was also a missing alpha^2 in the calculation of bsq used when computing beta but I think it probably didn't matter because b0 should be zero in the initial conditions.

I don't think the alpha^2 was missing because that wasn't actually b[0] it was W * Bdotv. Anyway, I guess you're right that it won't matter for this setup, but it would matter later for other setups (like toroidal fields).

[Yurlungur]

Oh you're right. Well in that case all I did there was clarify things a little for own sanity.

The rho_rmax was the only substantive change.