Features of a new PDI branch observed in TST-2 during high harmonic fast wave injection

TST-2 is a spherical tokamak (ST) device, and High Harmonic Fast Wave (HHFW) injection experiments were performed to heat electrons in the spherical tokamak. In TST-2, electron heating was observed in some discharges, but when Parametric Decay Instability (PDI) occurred the injected wave energy is transferred to two daughter modes. PDI during HHFW injection in the NSTX ST device creates modes, which are presumably an Ion Bernstein wave (IBW) and a quasi mode at ion cyclotron frequency (ICQM). A similar decay was observed for low toroidal magnetic field discharges in TST-2. For higher toroidal magnetic field discharges, we found another type of decay, in which another peak in the frequency domain was detected between a pump wave (i.e., HHFW) frequency and the IBW frequency. Experiments are performed to identify this new peak. RF pickup probes are used to measure the magnetic field oscillations outside the closed magnetic surface region. Major results are as follows. The frequency of the decay waves coupled with ICQM increases with the toroidal magnetic field as expected. When Bt varies from 0.13 to 0.18 T, the frequency difference of the decay wave and the pump wave increases from 1.5 to 1.9 MHz in proportion to Bt- The new unidentified wave shows a similar dependence, and its frequency difference varies from 0.54 to 0.70 MHz in proportion to Bt- The width of this new peak is broader than that of IBWs peak. These results suggest that the new peak corresponds to the decay wave coupled with Alfven mode, because the fact that the frequency of Alfven mode is in proportional to the magnetic field, is consistent with the experimental results.