Simulation study of high-frequency energetic particle driven geodesic acoustic mode

Abstract

High-frequency energetic particle driven geodesic acoustic modes (EGAM) observed in the large helical device plasmas are investigated using a hybrid simulation code for energetic particles and magnetohydrodynamics (MHD). Energetic particle inertia is incorporated in the MHD momentum equation for the simulation where the beam ion density is comparable to the bulk plasma density. Bump-on-tail type beam ion velocity distribution created by slowing down and charge exchange is considered. It is demonstrated that EGAMs have frequencies higher than the geodesic acoustic modes and the dependence on bulk plasma temperature is weak if (1) energetic particle density is comparable to the bulk plasma density and (2) charge exchange time (τcx) is sufficiently shorter than the slowing down time (τs) to create a bump-on-tail type distribution. The frequency of high-frequency EGAM rises as the energetic particle pressure increases under the condition of high energetic particle pressure. The frequency also increases as the energetic particle pitch angle distribution shifts to higher transit frequency. It is found that there are two kinds of particles resonant with EGAM: (1) trapped particles and (2) passing particles with transit frequency close to the mode frequency. The EGAMs investigated in this work are destabilized primarily by the passing particles whose transit frequencies are close to the EGAM frequency.

The systematic investigation of energetic-particle-driven geodesic acoustic mode channeling using MEGA code

“…10 of Ref. [39], but in the present work, the mode frequency changes only approximately 10% by changing Λ peak , while in Fig. 10 of Ref.…”

“…10 of Ref. [39], the frequency changes approximately 40%. In the present work, the simulated EGAM is the low-frequency branch, while the mode in Ref.…”

“…In the present work, the simulated EGAM is the low-frequency branch, while the mode in Ref. [39] is the highfrequency branch. The high-frequency branch is a kind of beam branch [16], it is weakly coupled to the GAM continuum and more easily to be affected by energetic particles.…”

Observation of subcritical geodesic acoustic mode excitation in the large helical device

“…The GAM frequency [34] calculated with the parameters of the bulk plasma, when assumed to be pure hydrogen plasma, is ± 56 10 kHz. Although the frequency of 41.5 kHz is smaller than the calculated GAM frequency, the difference can be explained by the effect of energetic particles [15,33,35] and of impurity ions. The abruptly excited mode at 41.5 kHz has been identified as a GAM because the frequency and the spatial structures of the electric potential fluctuation and the density fluctuation associated with the mode agree with those of the GAM, as shown in [14].…”

Simulation of energetic particle driven geodesic acoustic modes and the energy channeling in the Large Helical Device plasmas

“…In LHD, the EGAM frequency should be similar to that in tokamaks [3,4]. The energy of neutral beam injection (NBI) is high, thus it is possible to excite a high frequency branch of EGAM under the condition of bumpon-tail distribution [36,37], while the high frequency branch has not been observed in tokamaks. The following six parameters for the EGAM simulation are based on an LHD experiment [42]: 1) The plasma major radius R 0 = 3.75 m. 2) The magnetic field strength on the magnetic axis B 0 = 1.5 T. 3) The electron density in plasma center n e = 0.072 × 10 19 m −3 , and density profile is the same as experiment.…”