Evolutions of zonal flows and turbulence in a tokamak edge plasma during electron cyclotron resonance heating

Abstract

Geodesic acoustic mode (GAM) and low-frequency zonal flow (LFZF) are both observed through Langmuir probe arrays during electron cyclotron resonance heating (ECRH) on the HL-2A tokamak edge. The radial distributions of the amplitude and peak frequency of GAM in floating potential fluctuations are investigated through rake probe arrays under different ECRH powers. It is observed that the GAM frequency would decrease and the intensity of carbon line emission would increase as the ECRH power exceeds a certain threshold. The analyses suggest that the impurity ions may play an important role in the GAM frequency at the edge region. It is also found that during the ECRH phase besides the mean flow, both GAM and LFZF are strengthened. The total fluctuation power and the fraction of that power associated with zonal flows both increase with the ECRH power, consistent with a predator–prey model. The auto- and cross-bicoherence analyses show the coupling between GAM and its second harmonic during the ECRH phase. Moreover, the results also suggest that the couplings between GAM and the components with multiple GAM frequency are strengthened. These couplings may be important for GAM saturation during the ECRH phase.

Geodesic acoustic modes in magnetic confinement devices

“…That is, induced collisions tend to decrease the GAM frequency, but can increase the GAM damping at low ν i and decrease the damping at high ν i . These simulation results were consistent with GAM frequency shifts observed in HL-2A during ECR heating induced impurity releases [351].…”

“…Only once the O 8+ impurity component was taken into account (Z eff ∼ 3.5) could the (inner edge) GAM frequency be matched. For comparison with ECR heated edge GAMs in HL-2A a similar impurity compensated formula (also derived from Guo) was used [351]:…”

Investigation of zonal flows by using the collective scattering measurement of density fluctuations

“…frequency method (IFM) using the Doppler reflectometor in the ASDEX-U [15,16] and DIII-D [26,27] tokamaks. Another approach is using the envelope analysis to deduce the zonal flows information embodied in interactions between the zonal flows and turbulence in JFT-2M [11,12], T-10 [13], HL-2A [19,23] and HT-7 [21,24] tokamaks. For understanding the measurement of zonal flows using density fluctuations, the comparison between them should be discussed in detail which is one of the major purposes of this paper.…”

Kinetic theory of geodesic acoustic modes in toroidal plasmas: a brief review

“…Geodesic acoustic modes (GAM) [11,12], as the finite frequency counterpart of zonal flow, have been observed in various machines by different diagnostics [13][14][15][16][17][18][19][20][21][22] in the search of zero frequency zonal flow (ZFZF) [2], with the linear features such as mode frequency, three dimension mode structure, density perturbation and radial propagation identified. An inverse relation of turbulence level and GAM intensity were often observed, suggesting the GAMs are excited nonlinearly by ambient turbulence, as shown by bicoherence analysis [23][24][25].…”