Sub-harmonics ? (eigenfrequencies) Operational frequency ‘D’ Pick-up frequency ‘C’ Arc/trip ‘signature’ ? Zoom #63412 D2 trip OTL reflected voltage and spectrum Trip See spectrum time evolution 

Pre-trip spectra for three overlapping ~11  s windows Note nothing unusual at low frequencies

Trip spectra for three overlapping ~11  s windows Note broad low-frequency peak appears prior to trip

Post-trip spectra for three overlapping ~11  s windows Note low-frequency peak disappears after the trip Pick-up frequencies from modules ‘A,B,C’

Typical spectrum long before the trip Two things, which I don’t understand: Low frequency modulation throughout (~35 kHz and harmonics) - if this is not a ‘phantom’, then what value do we measure as voltage amplitude (peak?) and what are the implications (e.g. power calculation etc) Sub-harmonics peaks (short-circuited line eigenfrequencies) correspond to multiples of wavelengths, but not half-wavelengths in the line

Another pulse; no trip, but ELM same sort of modulation ! cleaner spectrum

Spectrum during ELMs ELM Relatively clean spectrum during ELMs