RF arc detection using harmonic signals

A new technique for arc detection in high power RF systems is being tested on the TFTR and DIII-D ICRF systems. Because the arc is nonlinear, it generates harmonics of the operating frequency as well as broadband low frequency noise. These bands of the spectrum can be detected from a coupled signal which passes through the appropriate frequency filter. In an RF system, when the power is split several times, such as the DIII-D system, a number of monitored locations may be necessary to ensure detection of the arc and isolation of its location. On DIII-D, both the low frequency noise and the second harmonic of the operating frequency have been monitored. A simple system measuring the presence of the second harmonic frequency has been demonstrated to be very reliable on TFTR.