In comments, chrismb suggested I fire the coils without the electron beam.
Great idea! This shows the effects of the coils alone… a control.
Results (same air pressure as before):
It seems the coils contribute a small positive voltage spike.
I did 3 test fires, all with similar results.
Prometheus Fusion Perfection 
Mark, nice! So, to my mind you’ve shown you are doing ‘something’ to the electrons you’re injecting, which is consistent with charge confinement.
In regards my point about the way you are using your probe, I would hazard a guess you are picking up a ‘mixed bag’ of charges by just probing with an inactive probe. That being said, there is a tantalising possibility that your wavy-pulse is an indication of ions being pulled into the space-charge formed by electrons, after all you are operating at quite a high pressure so there must be plenty of ions around (whcih you can see undergoing recombination). I don’t think this is a conclusion you can reach yet, but I’ll keep an open mind as I don’t think it is implausible.
You might also be measuring the limits of your measurement gear, in terms of impedance of the connection to the probe. Keep the lead very short, and preferably suitable coax. Also, there will almost certainly be impedance effects to your coils creating a field that does not ramp smoothly to a peak, as a function of the coils’ impedance. For this latter case perhaps you might actually find it easier to *extend* your lines to the coils, or insert some inductance into it, so as to look for a delta – just a thought, don’t hold me on that one. The other possibility is that you are initiating some form of multipacting of the electrons. I don’t have a thought on how you might elminiate that as a possibility, other than to press ahead and demonstrate that there is some ion flow going on.
So I would tend to suggest two futher approaches to generate a bit more info; first, use the probe in the more conventional Langmuir probe fashion, as mentioned before, and second try running the same thing with the same settings on the electron gun but at different chamber pressures. If you can show the same pulse but with less ‘wiggle’ in it for lower chamber pressure, then to me that’d seem consistent with showing there is some consequent ion confinement going on.
The usual caveat; these are just my own thoughts and nothing more, so make up your own mind if they are worth anything! You are in uncharted territory! Keep at it, fella!