Drift effects on geodesic acoustic modes

Sgalla, R J F; Smolyakov, Andrei; Elfimov, A. G.; Bashir, M. F.

doi:10.1016/j.physleta.2012.11.025

Cited by 17 publications

(20 citation statements)

References 27 publications

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“…The ion response toṼ E is described by the Chew-Goldberger-Low (CGL) law [17], assuming that the ion perpendicular and parallel pressure are doing work independently. An agreement is reached between the CGL law and the gyrokinetic theory on the thermal GAM frequency [18,19]. It has also been shown that when the mode frequency is much higher than the thermal frequency of the bulk ions (in conventional GAM, q ≫ 1), the CGL law can give a good description of the plasma response [20,21].…”

supporting

confidence: 55%

Energetic Geodesic Acoustic Modes Associated with Two-Stream-like Instabilities in Tokamak Plasmas

Hole

Fitzgerald

2016

Phys. Rev. Lett.

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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_{GAM}, consistent with the onset of the mode right after the turn-on of the beam in DIII-D experiments. The transition of this reactive EGAM to the velocity gradient driven EGAM is also discussed.

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supporting

confidence: 55%

Energetic Geodesic Acoustic Modes Associated with Two-Stream-like Instabilities in Tokamak Plasmas

Hole

Fitzgerald

2016

Phys. Rev. Lett.

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“…m=±1 is thereby ignored in the E × B drift as stated earlier.Finally, using Eq (21). and(22) to eliminateẼ , the parallel component of Eq (12).…”

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confidence: 99%

Linear radial structure of reactive energetic geodesic acoustic modes

Hole

Fitzgerald

2017

Plasma Phys. Control. Fusion

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In this paper we have developed a fluid model to study the radial mode structure of the reactive energetic geodesic acoustic modes (reactive EGAMs), a branch of GAM that becomes unstable in the presence of a cold fast ion beam. We have solved the resulting dispersion relationship, a second order ODE, both analytically in restricted cases and numerically in general. It is found that the reactive EGAM global mode structure is formed with the inclusion of fast ion finite drift orbit effects. In two cases with typical DIII-D parameters but different q profiles, the global EGAM frequency is slightly higher than the local EGAM extremum, located either on axis with a monotonic shear or at mid-radius with a reversed shear. The mode wavelength roughly scales with L 1/2 orbit in the core and L orbit at the edge, though the dependency is more complicated for the reversed shear case when L orbit < 0.06a (L orbit is the fast ion drift orbit width and a the minor radius). Finally, the growth rate of the global mode is boosted by 50% to 100% when switching from co-beam to counter-beam, depending on the fast ion density, which may help to explain the more frequent occurrence of EGAMs with counter-injection in experiments.

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“…Since the early works of Winsor 2 and Mikhailoviskii 21 , several fluid [22][23][24][25] and kinetic 9,[26][27][28][29] models have been developed to improve the theoretical prediction of the GAMs frequencies. Numerous corrections on the dynamics of these modes were included to account for realistic conditions observed in experiments and, among these corrections, those due to electromagnetic effects [30][31][32][33][34] may be essential to predict correctly not only the value of the higher frequency of GAMs, f GAM ∼ T e /m i R 0 (hereafter referred simply as the GAM frequency), but also the parallel current density ( j ).…”

Section: Introductionmentioning

confidence: 99%

“…In Refs. 24,25 , γ i = 5/3, γ e = 1 and ion parallel viscosity (π i ) are considered and, apart from correction of O(q −2 ) and related Landau damping 9,28,40,41 , the kinetic result was recovered, viz.,…”

Section: Introductionmentioning

confidence: 99%

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Drift effects on electromagnetic geodesic acoustic modes

Sgalla

2015

Physics of Plasmas

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A two fluid model with parallel viscosity is employed to derive the dispersion relation for electromagnetic geodesic acoustic modes (GAMs) in the presence of drift (diamagnetic) effects. Concerning the influence of the electron dynamics on the high frequency GAM, it is shown that the frequency of the electromagnetic GAM is independent of the equilibrium parallel current but, in contrast with purely electrostatic GAMs, significantly depends on the electron temperature gradient. The electromagnetic GAM may explain the discrepancy between the f ∼ 40 kHz oscillation observed in TCABR [Yu. K. Kuznetsov et al., Nucl. Fusion 52, 063044 (2012)] and the former prediction for the electrostatic GAM frequency. The radial wave length associated with this oscillation, estimated presently from this analytical model, is λ r ∼ 25 cm, i. e., an order of magnitude higher than the usual value for zonal flows (ZFs).

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Drift effects on geodesic acoustic modes

Cited by 17 publications

References 27 publications

Energetic Geodesic Acoustic Modes Associated with Two-Stream-like Instabilities in Tokamak Plasmas

Energetic Geodesic Acoustic Modes Associated with Two-Stream-like Instabilities in Tokamak Plasmas

Linear radial structure of reactive energetic geodesic acoustic modes

Drift effects on electromagnetic geodesic acoustic modes

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