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# 27th IAEA Fusion Energy Conference - IAEA CN-258

22-27 October 2018
Mahatma Mandir Conference Centre
Asia/Kolkata timezone
CONFERENCE MATERIAL NOW AVAILABLE!

## Effect of externally applied radial electric field (biased-electrode) on Geodesic Acoustic Modes in SINP tokamak

24 Oct 2018, 14:00
4h 45m
Mahatma Mandir Conference Centre

Poster

### Speaker

Mr Lavkesh Lachhvani (Institute for Plasma Research)

### Description

Geodesic Acoustic modes (GAMs), believed to play an important role in L to H transition in tokamaks, are the high frequency branch of the zonal flows and are characterized by toroidally and poloidally symmetric in potential $\phi$ (n=0, m=0) and toroidally symmetric but poloidally asymmetric in density perturbation (n =0, m=1). The coherent modes in the spectral analysis of ﬂoating potential ﬂuctuations measured in the edge plasma region of Saha Institute for Nuclear Physics tokamak (SINP-tokamak) using Langmuir probes are recently identified as geodesic acoustic modes (GAMs) having different characteristics over a wide range of qedge. The mode is radially localised in the edge plasma and have finite radial propagation. These coherent modes are simultaneously observed in density and radial electric ﬁeld ﬂuctuation spectra as well. The observed mode conclusively exhibits all the characteristics of the continuum GAM in the discharges having qedge values from 3.0 to 6.0 in normal tokamak regime. In this range of qedge, the poloidal and toroidal components of the wave-vector clearly show the $n \sim 0$, $m \sim 0$ structure of the mode and the frequency of the mode, and its variation with qedge matches quite well with that predicted by theory. In the intermediate range of qedge = 2.5 - 3, the mode exhibits the eigenmode GAM like characteristics as the frequency of modes does not depend on the local plasma parameters; however, the structures remained of $n \sim 0$, $m\sim 0$ type. Decreasing qedge below 2.5, the mode characteristics change signiﬁcantly with the poloidal wave number becoming ﬁnite. Further, these modes are observed to be affected by the externally applied radial electric fields. The radial electric fields are induced by inserting a biased electrode inside the last closed flux surface of SINP-tokomak plasma. Interestingly, it is observed that the radial electric field affect the frequency and amplitude of GAM modes. Frequency range of the typical Eigen-mode GAM widens, owing to increase in temperature of the plasma due to improved confinement. Amplitude of the mode is observed to increase with bias potential.
Country or International Organization India EX/P4-11

### Primary author

Mr Lavkesh Lachhvani (Institute for Plasma Research)

### Co-authors

Dr Joydeep Ghosh (Institute for Plasma Research) Dr Nikhil Chakrabarty (Saha Institute for Nuclear Physics) Dr Prabal Chattopadhyay (Institute for Plasma Research) Dr Rabindranath Pal (Saha Institute for Nuclear Physics) Mr Tanmay Macwan (Institute for Plasma Research)