Observation of energetic-particle-induced GAM and nonlinear interactions between EGAM, BAEs and tearing modes on the HL-2A tokamak

Abstract

In our previous letter, the geodesic acoustic mode (GAM) induced by energetic particles (EGAMs) was reported in low density ohmic plasma on HL-2A (Chen et al 2013 Phys. Lett. A 377 387). We extend the experimental results of the EGAM mode in this paper. During strong tearing modes (TMs), the beta-induced Alfvén eigenmodes (BAEs) and EGAM-induced density fluctuations are firstly measured by microwave Doppler reflectometers with different work frequencies. As predicted by theory, the measurements of magnetic probes and Doppler reflectometers suggest the EGAM magnetic oscillations have poloidal/toroidal mode numbers of m/n = 2/0, and are localized in the core with a broad radial structure. The mode frequency is less than that of the conventional GAM (i.e. fEGAM/fGAM < 1), and is constant in the radial direction. Our experimental results suggest that a density limit exists for the excitation of the EGAM in the ohmic plasma, and the density limit is improved with electron cyclotron resonance heating + neutral beam injection heating on HL-2A. The auto and cross squared bicoherences of magnetic and density fluctuations indicate that intense nonlinear interactions exist among EGAM, BAEs and strong TMs. These new observations will help us to understand the underlying physics mechanism for the excitation of fluctuations in the sub-Alfvén frequency range.

Nonlinear interaction between energetic particles and turbulence in gyro-kinetic simulations and impact on turbulence properties

“…The structure of EGAM has been little explored in the previous litterature. EGAM is believed to have a global structure [3] and has been found in simulations with imposed background distribution function to exhibit a structure constrained by the Dirichlet boundary conditions [6], with a frequency that is constant in the whole radial domain, as in experiments [9,10,26]. In addition,…”

Linear radial structure of reactive energetic geodesic acoustic modes

“…The energetic-particle-induced geodesic acoustic modes (EGAMs) are n = 0 coherent fluctuations in toroidal magnetic confined plasmas. They were first observed in DIII-D experiments [1] and later in other machines [2,3]. Their frequencies appear at half of the conventional geodesic acoustic mode (GAM) [4] frequency of the thermal plasma and the beam transit frequency.…”

Long-Distance Correlations of Geodesic Acoustic Modes in T-10

“…На большинстве установок ГАМ исследуются с помощью корреляционной рефлектометрии (KP), зондов Ленгмюра и доплеровского рассеяния, но эти методы позволяют получать информацию только из периферийной плазмы [13][14][15][16]. Самый прямой и надёжный метод исследования ГАМ в горячей зоне плазмы -это зондирование пучком тяжёлых ионов (ЗПТИ, или HIBP в английской транскрипции), который много лет используется на токамаке T-10 для измерения профилей как средних значений потенциала, так и его колебаний [17].…”