Parametric decay instability (PDI) is often observed in the TST-2 spherical tokamak during high harmonic fast wave heating by rf pickup probes. The frequency spectrum exhibits lower and upper sideband peaks in addition to the pump wave at f0 =21 MHz. Two types of PDI are observed. One is the well-known decay into the ion-cyclotron quasimode (n fci) and the ion Bernstein wave (f0 -n fci). The other is a newly found decay with the sideband frequency between f0 and f0 - fci. The frequency difference between this sideband and the pump increases in proportion to Bt. Moreover, high-speed visible light measuring systems with photomultiplier tubes or hybrid photodetectors viewing the plasma core detected oscillation of light emission at around f0.
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