(EX/P8-1) Oscillatory Zonal Flows Driven by Interaction between Energetic Ions and Fishbone-like Instability in CHS
1) National Institute for Fusion Science, Nagoya, Japan
Abstract. The turbulence-driven zonal flows are one of the most important subjects for realizing a nuclear fusion device. Besides, the behavior of energetic particles is another important issue since it is anticipated that the fusion-generated alpha-particles in a burning plasma should drive MHD instabilities, such as TAEs, Energetic Particle Mode EPMs, and interact the instabilities to lose themselves out from the plasma and to degrade the plasma performance. On the other hand, it has been clarified in the H-mode studies that the energetic particle loss could be the major cause to drive the sheared radial electric field to stabilize the plasma turbulence and create the transport barriers. The internal structural measurements of electrostatic potential and density, using twin HIBPs, have been performed in CHS to elucidate nonlinear evolution of fishbone-like instability (the mode numbers are m/n = 2/1), called here CHS fishbone that is MHD cyclic oscillation (with a period of ∼3 ms) driven by the interaction with energetic ions. This paper presents the finding of a new kind of zonal flows developing during a cycle of the CHS fishbone by the interaction between the energetic ions and MHD instabilities in the electrostatic potential measurements using twin HIBPs. The measurements also allow us to infer the degree of the distortion of the magnetic field flux surface from the m = 2 perturbation of potential, the change of the density gradient that could reflect the energetic ion fraction, and their mutual relationship during a cycle of the CHS fishbone. The observation also provides a quite reasonable view of the nonlinear evolution scenario of beam-driven mode, i.e., the local increase in the energetic ion gradient grows MHD instabilities to lose the energetic ions. Then the decreasing energetic ion gradient may stabilize the instability. The finding of the new kind of zonal flows should be emphasized since the resultant shear of electric field in a future burning plasma could reach a sufficiently large level to reduce the turbulence transport. This leads us to a simple expectation that the alpha-particle loss induced by the magnetic field distortion during alpha-particle driven MHD instabilities in a burning plasma may cause a sudden change of the plasma structure by resultant structured electric field.
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