Exploring core-to-edge transport in Aditya tokamak by oscillations observed in the edge radiation

The understanding of transport in a Tokamak plasma is an important issue. Various mechanisms have been reported in the literature to link core phenomenon to edge phenomenon. Sawtooth and Mirnov oscillations caused by MHD instabilities are generally observed in Tokamak discharges. Observation of these effects in the visible radiation from outer edge may offer a possible means to understand transport. Oscillations in the visible radiation from outer region of the plasma have been observed during recent Aditya discharges. Percentage modulation of these oscillations vary with the Lines of Sight (LOS) of the chords and surfaces on which they terminate. This has been found in both the low frequency (about 1 kHz) oscillations that seem to correlate with sawtoothing in SXR signals and the higher frequency (about 10 kHz) oscillations that correlate well with Mirnov signals indicative of m/n 2/1 mode rotation. This suggests that the extent to which the MHD instabilities in the central region of the plasma column are reflected in the edge radiation depends on the interaction of the plasma with the surface at the extremity of the LOS. The release of particle/energy accompanying the MHD instabilities leads to a large influx of particles from such surfaces. Cross-bi-spectral analysis suggests that a mode (having frequency of about 20 kHz) is also generated due to the interaction of m/n = 1/1 (about 10 kHz, seen in SXR) and m/n = 2/1 (about 10 kHz, seen in Mirnov, Visible - Microwave Interferometer signals). By possible selection rules, this mode seems to be a m/n = 3/2 mode. This mode is seen in Mirnov, Visible - Interferometer signals. Behaviour of these oscillations on various LOS and their relation to SXR and Mirnov signals can lead to an understanding of the transport phenomenon. These observations and our interpretations will be presented. (authors)