Coherent to turbulence transition, enhanced flow and confinement in a simple toroidal plasma

Abstract

In a simple toroidal plasma, self consistent poloidal flow generation at different toroidal fields is observed. On changing the magnitude of toroidal field, transition occurs in the nature of fluctuation from highly coherent modes at low magnetic field to a turbulent broadband spectrum at high field accompanied by significant improvement in the confinement. Interestingly, though the coherence is reduced with the increasing field, the fluctuation driven poloidal flux remains comparable. The observed increase in the densities on high field side with the increase in magnetic field is simultaneously accompanied by the enhanced net poloidal flow. The present work demonstrates that in a toroidal compressible plasma, an intimate relationship exists between the fluctuations, self consistently generated flows and enhanced confinement. V C 2012 American Institute of Physics.

Role of ion mass in the generation of fluctuations and poloidal flows in a simple toroidal plasma

“…In the case of argon, with the average poloidal wave number k h $ 16 m À1 for mode number m ¼ 1, the fundamental frequencies are found to be dominated by the plasma rotation frequency at all magnetic field values. 24 In the present case with various ion masses, considering 3 kHz as the fundamental frequency for argon, the fundamental frequency scale with the net poloidal flow velocity. The dominant frequency for argon is, however, comparable to the ion cyclotron frequency at the limiter edge on LFS (f ci $ 7 kHz).…”

“…Measurement of the parallel wave number ðk k Þ can clearly distinguish between the drift and flute kind of modes; however, the experimental determination of ðk k Þ is a difficult task. 24,38 At present, vertical arrays of probes which can be precisely aligned on a single magnetic field to determine k k , sitting in different poloidal planes are not available in our experimental apparatus. The possible instabilities are, therefore, identified using the estimated cross-phase ðh n/ Þ, coherence ðc n/ Þ of density and potential fluctuations.…”

“…On the other hand, in a simple toroidal device, the existence of large fluctuations and the consequent poloidal flows have been found to lead to improved densities on high field side. 23,24 A complete understanding of the confinement can be obtained by addressing the two competing effects: (1) poloidal flow due toẼ r  B leading to a good "effective rotational transform" and (2) the radial flow due toẼ h  B which tries to degrade the confinement. However, here we are addressing only the first part of the problem; that is, experimental determination ofẼ r  B flow only.…”

“…37 The analysis method used for the estimation of auto-power and cross-power spectra is described in Ref. 24.…”

“…A transition in the nature of the fluctuations with toroidal magnetic field strength, an enhanced poloidal flow and improved confinement, has also been reported recently. 24 At a low magnetic field, flute-like coherent fluctuations, due to Rayleigh-Taylor instability are observed on low field side (LFS). At higher magnetic fields, the fluctuations are non-flute like on LFS.…”