P-18 · Study of nonlinear phenomena in a tokamak plasma using a novel Hilbert Transform technique
Raju, Daneil1)
1) Institute for Plasma Research, Bhat, Near Indira Bridge, Gandhinagar- 382 428, INDIA
Abstract
Tokamak plasma is rich in non-linearities of various kinds. The interacting low frequency long
wavelength coherent modes are dominant in the core and the confinement regions whereas
modes in a broad range of frequencies and wavelengths typically characterize the edge plasma.
These interactions have been studied conventionally using a varieties of techniques including
Fourier and wavelet transforms.
Recently a new technique as empirical mode decomposition (EMD) has been introduced which
allows extraction of a finite number of intrinsic modes from the data. The Hilbert transform of
such modes help to determine instantaneous frequencies and sharp changes in the instantaneous
frequencies are identified as a signature of nonlinear phenomena in the data. This method is
suitable for studying non-linearity present in the transient events.
The plasma transients during start-up and current termination phases in ADITYA tokamak have
been studied using this technique. The analysis of signals from an array of Mirnov coils shows
that nonlinear interaction among low frequency long wavelength modes plays an important role
in current penetration during the start-up phase. On the other hand, interaction among low m
modes lead to disruption during current termination phase.
Langmuir probe data from the turbulent edge plasma have also been analyzed using this
technique. The data show signatures of intermittency in the form of sporadic bursts of mode
energy. The Hilbert spectrum also allows evaluation of the degree of non-stationarity. It is
observed that only high frequency signals (exceeding 20 kHz) are non-stationary.
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Abstract (pdf)