Abstract
We report properties of a coherent density oscillation observed in the core region and its response to electron cyclotron resonance heating (ECH) in Heliotron J plasma. The measurement was performed using a multi-channel beam emission spectroscopy system. The density oscillation is observed in a radial region between the core and the half radius. The poloidal mode number is found to be 1 (or 2). By modulating the ECH power with 100 Hz, repetition of formation and deformation of a strong electron temperature gradient, which is likely ascribed to be an electron internal transport barrier, is realized. Amplitude and rotation frequency of the coherent density oscillation sitting at the strong electron temperature gradient location are modulated by the ECH, while the poloidal mode structure remains almost unchanged. The change in the rotation velocity in the laboratory frame is derived. Assuming that the change of the rotation velocity is given by the background E × B velocity, a possible time evolution of the radial electric field was deduced.
Highlights
Spontaneous confinement transition in magnetically confined plasmas has been intensively investigated because of its capability for reaching high plasma performance.Numerous studies have clarified that the transport barrier is formed when the confinement transition occurs, in which anomalous cross-field transport is suppressed by inhomogeneous E × B flow structures [1]
We report the observation of a coherent density oscillation observed at the core region in Heliotron J plasmas by use of a multi-channel beam emission spectroscopy (BES) system [13]
The target plasmas are sustained by two neutral beam injections (NBIs) of a total of 600 kW and a electron cyclotron resonance heating (ECH) of 48 kW
Summary
Numerous studies have clarified that the transport barrier is formed when the confinement transition occurs, in which anomalous cross-field transport is suppressed by inhomogeneous E × B flow structures [1]. Tokamaks and heliotrons/stellarators share a typical confinement transition, the so-called electron internal transport barriers (eITBs) [2, 3, 4, 5, 6, 7, 8]. For the case of heliotron/stellarator e-ITB, it is believed that the neoclassical radial electric field bifurcation occurs between the electron-root and the ion-root that forms a strong radial electric field shear [4, 5, 6, 8]. Regarding the e-ITB, low frequency magnetohydrodynamic (MHD) modes are known to impact on the e-ITB dynamics through triggering, quenching, or giving saturations [2, 3, 7]
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