Effect of ECH/ECCD on Sawtooth Oscillations in NB-heated Plasmas in JT-60U

Sawtooth period and amplitude during electron cyclotron (EC) wave injection to a neutral beam heated plasma are investigated. Significant decrease in sawtooth period is observed for EC injection inside the inversion radius in co-direction: the normalized sawtooth period 251/7"(0)l'5 reaches 1/10 of that in an ohmic heating phase, which shows a strong destabilization effect (here, f51 and 7"(0) are sawtooth period and the central electron temperature, respectively). While time scale for the change in the sawtooth amplitude is comparable to that in the inversion radius, time scale for the change in the sawtooth period is much shorter than that in the inversion radius. This suggests that the change in the sawtooth amplitude is related to the current profile, and that the strong destabilization effect of the sawtooth period occurs independently of the change in the current profile. Keywords: sawtooth oscillation, magnetohydrodynamic instability, electron cyclotron heating, electron cyclotron current drive, neutral beam, JT-60U, tokamak 1