This afternoon we ran the ring heater on ETMY for ~3.5 h, taking cavity scans to attempt to quantify the change in modal spacing with heating.
There's a clear change in the behavior in the 55-60 kHz region (associated with the modulation sidebands), but getting a clear signal for a higher-order cavity mode has been difficult. I also don't understand the downward slope of the phase response curve. I'm hoping to make better measurements tomorrow with a cold cavity to get consistent, clear numbers for the cavity mode frequencies, then try this measurement again.
The adoption of Phase - Phase at 30 kHz for the y-axis is purely to put the traces closer together visually for side-by-side comparisons of the sidebands. The "hot" trace starts at 166.7417, the "cold" trace starts at -162.2308.
I compared one of this afternoon's heated cavity scans with a cold cavity scan from last week. The nominal frequency of the TEM10 mode is shown (around 66kHz) [higher order frequency spacing = 28.5kHz, FSR = 37.5kHz, sum = 66kHz). There's clearly a shift in the TEM10 mode of a few hundred Hz.
