Revive Relativistic Boris Push

[JaydenR2305]

Reimplements the relativistic Boris push integrator defined in #979 using the new ParticleTracker framework

Must be merged after #2704

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[JaydenR2305]

figured it out! There was some dependence on the initial velocity. I've set the initial velocity to zero for now until we can figure out a better alternative (setting it directly to some fraction of the final drift velocity wasn't working...) This just means that we have to discard the first 20 points or so to allow the particle to reach the expected drift velocity.

edit: disregard, this is false. I was looking at plots of the relative errors

[pheuer]

figured it out! There was some dependence on the initial velocity. I've set the initial velocity to zero for now until we can figure out a better alternative (setting it directly to some fraction of the final drift velocity wasn't working...) This just means that we have to discard the first 20 points or so to allow the particle to reach the expected drift velocity.

Awesome! Kind of concerning it doesn't work with an initial velocity thought... any idea why that is? Want to make sure that's not an issue with the analytic solution or the pusher or something.

[JaydenR2305]

Awesome! Kind of concerning it doesn't work with an initial velocity thought... any idea why that is? Want to make sure that's not an issue with the analytic solution or the pusher or something.

I think the derivation makes the assumption that the particle is starting from rest (taken from https://journals.aps.org/pre/pdf/10.1103/PhysRevE.72.026603)

Though it looks like it would be as simple as picking up a phase difference (plus the amplitude difference picked up by gamma)?

[pheuer]

I think the derivation makes the assumption that the particle is starting from rest

Ah, got it, ok that's expected then! In that case I'd say this test looks good enough?

[pheuer]

Looking good! Few minor comments before we merge

[pheuer]

This looks great, thanks for polishing off this neglected old PR!!