Identification of the energetic-particle driven GAM in the LHD

Abstract

n = 0 modes with frequency chirping have been observed by a heavy ion beam probe and Mirnov coils in the large helical device plasmas, where n is the toroidal mode number. The spatial structures of the electrostatic potential fluctuation and the density fluctuation correspond to those of the geodesic acoustic mode (GAM). The modes are observed only during the tangential neutral beam injection with the energy of 175 keV. The energy spectra of fast ions measured by a neutral particle analyzer implies that the modes are excited by the fast ions through the inverse Landau damping. The absolute values and the temperature dependence of the frequency of the mode can be interpreted by the dispersion relation taking into account the measured energy spectra of the fast ions. Therefore, the observed n = 0 modes are identified as the energetic-particle driven GAM.

Role of Helium–Hydrogen ratio on energetic interchange mode behaviour and its effect on ion temperature and micro-turbulence in LHD

“…Furthermore, the mode frequency is broadly in in the range of β-induced Alfven eigenmode (BAE) and the energetic particle GAM [22], but since v A ∼ 1/A, the the frequency should change with He concentration, which is not the case. To determine what fast particles destabilise this mode, we have to consider the condition [38]:…”

Onset condition of the subcritical geodesic acoustic mode instability in the presence of energetic-particle-driven geodesic acoustic mode

“…The experimental observations, which form the basis of the model, are described in [7,11]. The LHD is a helical system (major ( ) and averaged minor radii of plasma are 3.75 m and about 0.6 m, respectively).…”

“…The frequency was compared with the theory with plausible distribution function of fast ions. With these examinations, the excitation of EGAM was identified [11]. When the mode is excited, the chirping of frequency to higher frequency is often observed.…”

“…(7), and is substituted into Eq. (8) to give (9) where is defined as (10) By imposing the time dependence (11) in Eq. (9), one has the eigenvalue equation (12) The eigenvalue is obtained as (13) Equation (13) gives the condition for the onset of parametric instability as The mother mode has a threshold, above which the parametric instability is induced.…”

“…An energetic particle driven GAM (EGAM) [8][9][10] is observed in the NBI heated plasma in LHD [11]. The EGAM is associated with a frequency chirping, which is caused by the nonlinear instability mechanism associated with deformation of velocity distribution function [12].…”