Alfvén eigenmode (AE) activity driven by NBI-produced fast ions is observed in TJ-II plasmas. A two-step response of the measured AEs to electron cyclotron heating (ECH) power is seen. In a first step, the continuous character of the unstable AEs changes to a chirping character of the marginally unstable AEs when moderate values of ECH power are applied to the NBI-only-heated plasma. In a second step, a significant reduction of the AE amplitude is observed when the ECH power is doubled. This stabilizing effect has been experimentally confirmed both on a shot-by-shot basis and along a single discharge by means of ECH modulation. The observed stabilizing effect is stronger with on-axis ECH than with off-axis ECH power injection.
Experiments of suppressing fast-ion-driven MHD instabilities such as energetic particle modes (EPMs) and global Alfvén eigenmodes (GAEs) have been made by using a second harmonic X-mode electron cyclotron heating (ECH) and current drive (ECCD) in the helical-axis heliotron device, Heliotron J. ECCD experiments show that the GAE destabilized by fast ions of neutral beam injection (NBI) with the observed frequency around 140 kHz are fully stabilized, and the EPMs with the observed frequency around 90 kHz are suppressed when the EC-driven plasma current flowing in the counter direction reaches approximately 0.7 kA. The low magnetic shear under the vacuum condition is modified into positive magnetic shear when counter-ECCD is applied, and the amplitude of GAEs and EPMs decreases with an increase of the EC-driven plasma current. These results indicate that magnetic shear is a key role in controlling GAEs as well as EPMs. The comparison of the calculation of shear Alfvén spectra with experimental results shows that the increasing continuum damping rate with an increase in local magnetic shear by EC-driven current is important for both EPMs and GAEs. Moreover, the increase in plasma current lead to the inward movement of GAEs. This effect would also contribute to suppression of GAEs because the continuum damping rate increases more and more toward core. Steady ECH is also found experimentally to be effective to control the amplitude of both GAEs and EPMs. The amplitude of EPMs, and especially for GAEs decreases with an increase in the ECH power under fixed density conditions.
A zonal magnetic field is found in a toroidal plasma. The magnetic field has a symmetric bandlike structure, which is uniform in the toroidal and poloidal directions and varies radially with a finite wavelength of mesoscale, which is analogous to zonal flows. A time-dependent bicoherence analysis reveals that the magnetic field should be generated by the background plasma turbulence. The discovery is classified as a new kind of phenomenon of structured magnetic field generation, giving insight into phenomena such as dipole field generation in rotational planets. Nowadays, a number of phenomena analogous to geomagnetism are known to occur ubiquitously in nature: e.g., the planetary dynamo, sun spots, the galactic dynamo, etc. A widely accepted hypothesis is that turbulence should be a main player for the phenomena of magnetic field generation [1][2][3]. Along with accumulating support for this hypothesis from direct nonlinear simulations [4,5], laboratory experiments have made progress in observing the phenomena associated with a dynamo in turbulent materials [6 -13]. For instance, the fundamental processes of a dynamo, such as a self-exciting magnetic field [11] and the generation of a magnetic field by turbulent flows [13], have been demonstrated in experiments using turbulent conducting flows of liquid metals.In toroidal plasma, which aims at realizing a Sun on the Earth, strong turbulence sustained by a steep pressure gradient organizes the thermal structure through selfregulating transport. Recently, a structured electric field driven by microscopic turbulence was identified using heavy ion beam probes (HIBPs) in a toroidal plasma named the compact helical system (CHS) [14]. In a magnetically confined plasma, the electric field is equivalent to the perpendicular flows to the confinement magnetic field through E B drift. The structured electric field dubbed zonal flow or zonal electric field [15] has a coaxial symmetry around the magnetic axis, and it fluctuates with a radial wavelength with a constant phase on a magnetic surface. This field is classified as a mesoscale, since the scale of the radial wavelength is larger than the characteristic scale of the turbulence but smaller than the system size.Current theory suggests the possibility that turbulence should generate a zonal magnetic field in addition to a zonal flow [15][16][17][18]. Similarly to the zonal flow, the zonal magnetic field fluctuates homogeneously on the magnetic surface and alternates direction as a function of plasma radius. This field therefore remains finite even upon averaging over a magnetic surface to be regarded as a mean field. The HIBP has an ability to sense a magnetic field fluctuation simultaneously with an electric field one. Trials to search a zonal field, utilizing the advantage of the diagnostics, have been performed in CHS. In this Letter, we present the discovery of a zonal magnetic field-a new kind of structured magnetic field generated from background turbulence. The present experimental results provide the first clear ...
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