@me356
What you are describing doesn't sound like RFI. It sounds more like conducted interference on the AC power line. Switching the AC power line is a very nasty source of interference if power line filters are not present. Without filters, this chopped AC interference can get into everything. That is why I have recommended use of a variac or DC power supply (which I use) to drive the heater. Since Alan's AC chopping was on the secondary side of an isolation transformer, the transformer itself filtered out a lot of that potential interference.
@axil
The RFI that Defkalion found was clearly due to their high power capacitive discharge sparking provided to their reactor. Those repetitive impulses going to the reactor created huge broadband RF noise. It is not the same case here, and I suspect what ME356 is describing is not radiated, but conducted noise. The Defkalion impulses were fast enough that the lead wires could become effective radiators for broadband emission out to 100 MHz. With ME356's AC chopping, I suspect the harmonics are less than 1 MHz and the connecting wires carrying that current are short enough compared to a wavelength that little RFI power is radiated. Since he is still seeing ubiquitous interference, I suspect conducted interference through the power lines.