Effect of ECH/ECCD on energetic-particle-driven MHD modes in helical plasmas (original) (raw)
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Stabilization of energetic-ion-driven MHD modes by ECCD in Heliotron J
Nuclear Fusion, 2013
Second harmonic electron cyclotron current drive (ECCD) has been applied in Heliotron J to stabilize magnetohydrodynamic (MHD) modes in Heliotron J. Localized EC current driven at central region modifies the rotational transform profile, , leading to formation of a high magnetic shear. An energetic-ion-driven MHD mode of 80 kHz has been fully stabilized by counter-ECCD, and other modes of 90 kHz and 140 kHz have been stabilized by co-ECCD, indicating that both co-and counter-ECCD are effective at stabilizing the energetic-ion-driven MHD modes. An experiment of scanning the EC driven current shows that there is a threshold in magnetic shear to stabilize the energetic-ion-driven MHD modes.
Plasma and Fusion Research, 2012
The beta-induced Alfvén eigenmode (BAE) like modes during strong interchange mode, whose modenumbers are m/n = 2/1, have been recently observed for the first time in Large Helical Device (LHD). The first harmonic frequencies of these oscillations range from 30 to 70 kHz, much lower than the toroidal-Alfvéneigenmode (TAE) frequency, and are provided with the same order of the low-frequency gap induced by finite beta effects. The magnetic fluctuation spectrogram indicates that the BAEs often occur in pairs, and their modenumbers are m/n = 2/1 and −2/−1. The analysis reveals that the modes propagate poloidally and toroidally in opposite directions, and form standing-wave structures in interchange-mode rest frame. The frequencies of the pair mode are associated with the T e /T i ratio, and the frequency difference of the pair modes is determined by the frequency of interchange mode. The new finding shed light on the underlying physics mechanism for the excitation of the low frequency Alfvénic fluctuation.
Nuclear Fusion
The aim of the present study is to analyze the stability of the pressure gradient driven modes (PM) and Alfvén eigenmodes (AE) in the Large Helical Device (LHD) plasma if the rotational transform profile is modified by the current drive of the tangential neutral beam injectors (NBI). This study forms a basic search for optimized operation scenarios with reduced mode activity. The analysis is performed using the code FAR3d which solves the reduced MHD equations describing the linear evolution of the poloidal flux and the toroidal component of
Characterization of ECH plasma confinement in Heliotron J
The characterization of ECH plasma confinement in Heliotron J was studied with special regard to its magnetic configuration effects. Recent experiments on 70-GHz ECH revealed that the energy confinement characteristics in the normal confinement mode ( T e < 1.5 keV, T i CX < 0.2 keV, n e = (0.2 -3.0 )x10 19 m -3 , W p diam < 3 kJ, and B 0 < 1.5 T ) show the existence of "good" confinement plasmas whose energy confinement time becomes 1.5-2 times larger than the ISS95 scaling. Recently, the spontaneous confinement improvement transition, like that of H-mode, was discovered in Heliotron J during ECH in the two edge iota windows 0.54 < (a)/2 < 0.56, 0.62< (a)/2 <0.63) at rather low threshold line-averaged densities of n e =1.2-1.4x10 19 m -3 . The confinement improvement remains transient on an energy confinement timescale and the enhancement factor (H ISS95 ) of the global energy confinement time with regard to the ISS95 scaling reaches over 1.5 for the lower edge iota window. The relevant transition properties are discussed.
Simulation Study of ECCD in Helical Plasmas
Plasma and Fusion Research, 2011
The electron cyclotron current drive (ECCD) is studied in Heliotron J and LHD plasmas using GNET code in order to study the ECCD physics in helical configurations. The magnetic configuration dependence of ECCD is investigated in the Heliotron J plasma. It is found that the current direction is reversed in high bumpiness configuration compared with the other configurations. The ECCD in LHD is also investigated by changing electron cyclotron heating points fixing the configuration. It is found that the direction of the current reverses when we change the heating point from the ripple top to the ripple bottom.
First dynamic magnetic configuration scans in ECRH plasmas of the TJ-II Heliac
Nuclear Fusion, 2009
The configurational flexibility of the TJ-II Heliac has been upgraded with the commissioning of a mode of operation that allows changing the magnetic configuration dynamically: the currents feeding the different coil sets can be ramped during the discharge, which allows for, for example, moving up or down the offset of the rotational transform profile. The first discharges in this operation mode have been designed to investigate the effect of low order rational values of the rotational transform, ι = ι/2π, in low magnetic shear plasmas created and sustained with electron cyclotron resonance heating. The main magnetic resonances (8/5 and 5/3 in this work) do not deteriorate confinement except occasionally in a short, transient manner that is accompanied by a magnetic event with a clear frequency splitting.
EX/P5-29 Footprint of the Magnetic Configuration in ECH Plasmas of the TJ-II Flexible Heliac
The configurational flexibility of the TJ-II Heliac has been upgraded with the commissioning of a mode of operation that allows changing the magnetic configuration dynamically: the currents feeding the different coil sets can be ramped during the discharge, which allows for, e.g., moving up or down the offset of the rotational transform profile. In these experiments the Ohmic transformer is also activated so as to counteract the induced currents. This capability can be used to investigate the effect of low order rational values of the rotational transform, ι/2π, in transport magnitudes, like the effective diffusivities, without altering considerably the magnetic shear. The experiments in plasmas created and sustained with Electron Cyclotron Resonance Heating show, in agreement with previous experience from the TJ-II, that such low order rational values of ι/2π do not deteriorate the effective heat diffusivity.
Nuclear Fusion, 2007
Intense electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) are employed on the Tokamakà Configuration Variable (TCV) both in second-and third-harmonic X-mode (X2 and X3). The plasma behaviour under such conditions is driven largely by the electron dynamics, motivating extensive studies of the heating and relaxation phenomena governing both the thermal and suprathermal electron populations. In particular, the dynamics of suprathermal electrons are intimately tied to the physics of X2 ECCD. ECRH is also a useful tool for manipulating the electron distribution function in both physical and velocity space. Fundamental studies of the energetic electron dynamics have been performed using periodic, low-duty-cycle bursts of ECRH, with negligible average power injection, and with electron cyclotron emission (ECE). The characteristic times of the dynamical evolution are clearly revealed. Suprathermal electrons have also been shown to affect the absorption of X3 radiation. Thermal electrons play a crucial role in high-density plasmas where indirect ion heating can be achieved through ion-electron collisions. In recent experiments ≈ 1.35 M W of vertically launched X3 ECRH was coupled to a diverted ELMy H-mode plasma. In cases where ≥ 1.1 M W of ECRH power was coupled, the discharge was able to transition into a quasi-stationary ELM-free Hmode regime. These H-modes operated at β N ≈ 2, n e /n G ≈ 0.25 and had high energy confinement, H IP B98(y,2) up to ≈ 1.6. Despite being purely electron heated and having no net particle source these discharges maintained a density peaking factor (n e,o / ≈ 1.6). They also exhibited spontaneous toroidal momentum production in the cocurrent direction. The momentum production is due to a transport process as there is no external momentum input. This process supports little or no radial gradient of the toroidal velocity.
Development of net-current free heliotron plasmas in the Large Helical Device
Nuclear Fusion, 2009
Remarkable progress in the physical parameters of net-current free plasmas has been made in the Large Helical Device (LHD) since the last Fusion Energy Conference in Chengdu, 2006 (O.Motojima et al., Nucl. Fusion 47 (2007) S668). The beta value reached 5 % and a high beta state beyond 4.5% from the diamagnetic measurement has been maintained for longer than 100 times the energy confinement time. The density and temperature regimes also have been extended. The central density has exceeded 1.0×10 21 m-3 due to the formation of an Internal Diffusion Barrier (IDB). Although these parameters have been obtained in separated discharges, each fusion-reactor relevant parameter has elucidated the potential of net-current free heliotron plasmas. Diversified studies in recent LHD experiments are reviewed in this paper.