Observation of low and high frequency spectra after turn-off of the turbulent heating pulse in TORTUR tokamak plasma

Turbulent heating is an attractive method for additional heating of tokamak plasma because of its high efficiency. It has been known, however, that there are various microinstabilities in tokamak plasma, resulting in the abnormal transport of heat or particles aross the confinement magnetic field. In the present study, a 4 mm microwave scattering observation is performed to determine the density fluctuation in TORTUR tokamak plasma. In particular, investigation is focused on the effects of turbulent heating pulses of 20 ∼ 50 kA on microinstabilities. Low-frequency spectra (up to several hundreds of kHz) and high-frequency spectra (10 ∼ 50 MHz) are observed by means of the microwave scattering method. Measurements are made to determine the relations of frequencies with the power spectrum and growth time of low-frequency oscillation. For high-frequency oscillation, the time evolution of the ratio of electron temperature to ion temperature is examined. It is shown that the low-frequency spectra observed come from drift waves having frequencies which satisfy a certain relation in the neighborhood of the electron diamagnetic drift frequency. The growth rate of the low-frequency oscillation is consistent with the theory for trapping modes in the banana region. The electron temperature reaches a maximum after the decay of the instability. In addition, investigation is made of parameters related to the high-frequency spectra observed, indicating that the ion cyclotron instability is responsible for that excited. (Nogami, K.)