Charge Deposition: Fix Sign

[ax3l]

This fixes the charge deposition. Electrons should deposit negative charge :-)

[cemitch99]

Great!

[RemiLehe]

@ax3l It seems that you will need to modify the test to replace:

assert math.isclose(beam_charge, 1.0e-9)

with

assert math.isclose(beam_charge, -1.0e-9)

[ax3l]

correct, wanted to quickly see if this fails now as intended.

It does, but I also need to increase the tolerance a little now: beam_charge=-1.0000000087381137e-09

[cemitch99]

@ax3l It seems that you will need to modify the test to replace:

assert math.isclose(beam_charge, 1.0e-9)

with

assert math.isclose(beam_charge, -1.0e-9)

This is a good point. In Impact-Z, the sign problem is avoided by treating all charges as positive. (So electrons are treated as positrons.) This makes life a bit easier, and it does not affect tracking in the external fields because the field strength is given in a normalized form (scaled by signed magnetic rigidity p/q). Either option is fine, as long as the sign of the pushed single-particle charge is the same as the sign of the deposited charge of the bunch.

[ax3l]

I like that we have both, would allow us to do some (short beamline) tests with partly neutral beams as come out from some plasma sources.