Important issues related to the temperature gradient driven instabilities are investigated in the Columbia Linear Machine (CLM). The main purpose of this research is to produce, definitively identify and elucidate the basic physics of these instabilities.
The first part of the thesis is about the study of Zonal flows (ZF) associated with the ion temperature gradient (ITG) modes. The difficult problem of the ZF detection has been solved via a novel diagnostic using the paradigm of frequency modulation (FM) in radio transmission. Through the discrete short time Fourier transform (STFT) analysis, the most important ZF characteristics such as low frequency (∼ 2kHz), poloidal symmetry ( m = 0), toroidal symmetry (k‖ = 0) and radial structure (kr ≠ 0) have been identified directly in the experiment. Furthermore, the ZF saturation physics has been investigated through unique feedback control diagnostics. Finally, the experimental results are compared with various existing theoretical models.
The second part of the thesis is about the research on the electron temperature gradient (ETG) mode. ETG modes, which are believed to be one of the strongest candidates for the anomalous electron energy transport in plasmas, is difficult to detect in experiments because of its high frequency (∼ MHz ) and short wave length (k⊥ρ e ≈ 1). Using a DC bias heating scheme of the core plasma, we are able to produce a sufficiently strong electron temperature gradient for exciting ETG modes in the CLM. A high frequency mode at ∼ 2MHz, with azimuthal wave number m ∼ 14–16 and parallel wave number k‖ ≈ 0.01cm-1, has been observed. This frequency is consistent with the result of a kinetic dispersion relation of slab ETG modes with an appropriate [special characters omitted] Doppler shift. The scaling of its fluctuation level with the temperature gradient scale length and the radial structure are found to be roughly consistent with theoretical expectations. With all the parametric signatures described above, ETG modes have been produced and definitively identified for the first time in a basic physics experiment.
|Adviser||Amiya K. Sen|
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