Electron temperature measurements during electron cyclotron heating on PDX using a ten-channel grating polychromator

First-harmonic electron cyclotron heating (ECH) experiments (Psub(RF)approx. = 80 kW, f approx. = 60 GHz) have been conducted on the Princeton Divertor Experiment (PDX) (R=137 cm, a approx. = 40 cm). Off-axis (approx. 4 cm) location of the resonance zone resulted in heating without sawtooth or m=1 activity. However, heating with the resonance zone at the plasma centre caused very-large-amplitude sawteeth accompanied by strong m=1 activity: ΔT/Tsub(MAX) approx. = 0.41, sawtooth period approx. 4 ms, m=1 period approx. = 90 μs(11 kHz). This is the first time such intense MHD activity driven by low power (Psub(RF)/Psub(OH) <or approx. 0.35) ECH has been observed. (For both cases, there was no sawtooth activity in the Ohmic phase of the discharge before ECH.) At very low densities, there is a clear indication that a superthermal electron population is created during ECH. The electron temperature evolution was monitored using a grating polychromator which measured second-harmonic electron cyclotron emission from the low field side of the tokamak. Interference from the high-power heating pulse on the broadband detectors in the grating instrument was eliminated by using a waveguide filter in the transmisson line which brought the emission signal to the grating instrument. (author)