@Tom Paulson Your FFT graph of a simulated SCR spectrum is nice to look at, but rather deceptive. The log scale obscures the reality that the harmonic at 1 MHz is ~110 dB (3 X 10^-6) below the fundamental. I'm not saying that a chopped waveform doesn't have higher harmonics. However, only those below around 100 kHz have significant ) amplitude.
In addition, the self-inductance of a typical heater coil acts as a low-pass filter. I measured a corner frequency of 50 kHz for my Glowstick coils. Higher-frequency stimulus thus requires a different mechanism, perhaps rods passing through the reactor end seals. This feature is part of the GS6 design now under development.