2016
DOI: 10.1103/physrevlett.116.095004
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Energetic Geodesic Acoustic Modes Associated with Two-Stream-like Instabilities in Tokamak Plasmas

Abstract: An unstable branch of the energetic geodesic acoustic mode (EGAM) is found using fluid theory with fast ions characterized by their narrow width in energy distribution and collective transit along field lines. This mode, with a frequency much lower than the thermal GAM frequency ω_{GAM}, is now confirmed as a new type of unstable EGAM: a reactive instability similar to the two-stream instability. The mode can have a very small fast ion density threshold when the fast ion transit frequency is smaller than ω_{GA… Show more

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Cited by 14 publications
(27 citation statements)
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“…The smallness of crit Y for the bump-tail distribution implies fast excitation of EGAM after NBI heating is turned on. This result is consistent with observed fast excitation in the DIII-D experiments and is similar to the previous analytic results [12,[22][23]. .…”
Section: B Dependence On Energetic Particle Pressure: Stability Thresupporting
confidence: 93%
See 1 more Smart Citation
“…The smallness of crit Y for the bump-tail distribution implies fast excitation of EGAM after NBI heating is turned on. This result is consistent with observed fast excitation in the DIII-D experiments and is similar to the previous analytic results [12,[22][23]. .…”
Section: B Dependence On Energetic Particle Pressure: Stability Thresupporting
confidence: 93%
“…Since then EGAM has been observed in other tokamaks [7][8][9] as well as in the Large Helical Device (LHD) [10][11]. Extensive theoretical and numerical work has been done on the linear physics of EGAM in the past decade [12][13][14][15][16][17][18][19][20][21][22][23][24][25]. The research on EGAM has been reviewed recently in several papers [26][27][28].…”
Section: Introductionmentioning
confidence: 99%
“…10 and 11. The observed EGAM frequency 6,7 was found to be much smaller than the results of theoretical calculations 14,15 of the x EG -geodesic frequency driven by the beam, plasma parameters, and the angle between the magnetic field and NBI velocity. In the experiments, 6 the EGAM instabilities, driven by the counter NBI, were detected at a half minor radius and deeper, which is covered by the untrapped NBI region, but these types of instabilities are not observed 6 in the trapped ion region of higher radius positions 16 for the injected beam.…”
Section: Introductioncontrasting
confidence: 57%
“…Above some density threshold of the order of n h;cr % 1-2%, two EGAM branches appear. 15,18,27 The higher mode with asymptotic frequency X 1;EGAM % X max2 is strongly dissipative, and the lower EGAM branch is unstable in the region…”
Section: A Discussion Of the Gam-egam Differencementioning
confidence: 99%
“…[10][11][12][13][14] According to the early theoretical models, in discharges with NB injection, the GAM instability can be driven due to an inhomogeneous dependence on the pitch angle distribution [15][16][17] in velocity space. For the parallel balanced injection, 17 the inverse Landau damping is found to be the origin of instability, but a cold beam model 18,19 of the bump may also reproduce the GAM instability that is named 18 as reactive and it begins without threshold due to the absence of dissipation. Generally, the energetic ion model was used in the form of a slowing down distribution…”
Section: Introductionmentioning
confidence: 99%