Reply To: Focus Fusion Rail Gun

[asymmetric_implosion]

Lerner wrote: Right–we have a Rogowksi coil built into the machine to monitor the current and it does indeed show the current oscillating at the same frequency as the voltage. Although all the grounds are connected, the Rogowski coil in grounded to the ‘scope while the high voltage probe is grounded to the support structure. The ratio of current to voltage oscillations and the frequency of the oscillations combine to give us evidence that the current is moving back and forth between the top and bottom transmission plates–they have the right inductance and capacitance for this. The energy is mostly radiated out the edges of the plates, creating a big RF noise source which we spent a long time shielding against the first year of FF-1’s existence.

OK, all grounds are connected. This by definition is a ground loop. Ground loops generate noise on signals by floating the grounds at different parts of the circuit leading to noise on the ground side rather than the power side. I don’t doubt you see oscillation in the current at some level but you also have to contend with ground loops.

IF you are seeing the current oscillating at a higher frequency than the initial frequency of the system (contributions from L, C and R for short circuit) then you have to have a transmission line like system. Typically, a ~1 us pulse system does not behave like a transmission line. You usually have to be much faster. One might argue that the fast behavior of pinch can do it but you see it on the trigger as well. The trigger should be reasonably clean as there is negligible current in the system. If you see oscillation at breakdown, it tends to be high voltage noise in my experience. It could be something else but I suspect if you can make the noise go away with a better selection of the ground point of the HV probe. It is a simple experiment that could tell the difference between a paper and a problem.