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The aim of the study is to analyze the stability of the Energetic Particle Modes (EPM) and Alfven Eigenmodes (AE) in Helitron J and LHD plasma if the electron cyclotron current drive (ECCD) is applied. The analysis is performed using the code FAR3d that solves the reduced MHD equations describing the linear evolution of the poloidal flux and the toroidal component of the vorticity in a full 3D system, coupled with equations of density and parallel velocity moments for the energetic particle (EP) species, including the effect of the acoustic modes. The Landau damping and resonant destabilization effects are added via the closure relation. The simulation results show that the n=1 EPM and n=2 Global AE (GAE) in Heliotron J plasma can be stabilized if the magnetic shear is enhanced at the plasma periphery by an increase (co-ECCD injection) or decrease (ctr-ECCD injection) of the rotational transform at the magnetic axis iota0. In the ctr-ECCD simulations, the EPM/AE growth rate decreases only below a given iota0, similar to the ECCD intensity threshold observed in the experiments. In addition, ctr-ECCD simulations show an enhancement of the continuum damping. The simulations of the LHD discharges with ctr-ECCD injection indicate the stabilization of the n=1 EPM, n=2 Toroidal AE (TAE) and n=3 TAE, caused by an enhancement of the continuum damping in the inner plasma leading to a higher EP beta threshold with respect to the co- and no-ECCD simulations.

Beam-driven Alfvén eigenmodes (AEs) have been studied in the TJ-II low-magnetic-shear flexible heliac (B 0 = 0.95 T, 〈R〉 = 1.5 m, 〈a〉 = 0.22 m), in L-mode hydrogen plasmas with hydrogen NBI and ECR heating (P NBI ⩽ 1.0 MW, E NBI = 32 keV, P ECRH ⩽ 0.6 MW). In low-density plasmas in the range n e ¯ = (0.3–1.5) × 1019 m−3, a large variety of AEs have been observed in the plasma core with the heavy ion beam probe diagnostic and Mirnov coils, in the frequency range 50 kHz f AE ι is varied during the shot (dynamic configuration scan), some AEs exhibit changes in their nonlinear evolution from bursting-amplitude AEs with chirping frequency to steady-frequency AEs, and back. The range of ι intervals within which the AEs studied are chirping or steady-state is determined.

The progress of physical understanding as well as parameter improvement of net-current-free helical plasma is reported for the Large Helical Device since the last Fusion Energy Conference in Daejeon in 2010. The second low-energy neutral beam line was installed, and the central ion temperature has exceeded 7 keV, which was obtained by carbon pellet injection. Transport analysis of the high-T-i plasmas shows that the ion-thermal conductivity and viscosity decreased after the pellet injection although the improvement does not last long. The effort has been focused on the optimization of plasma edge conditions to extend the operation regime towards higher ion temperature and more stable high density and high beta. For this purpose a portion of the open helical divertors are being modified to the baffle-structured closed ones aimed at active control of the edge plasma. It is compared with the open case that the neutral pressure in the closed helical divertor increased by ten times as predicted by modelling. Studies of physics in a three-dimensional geometry are highlighted in the topics related to the response to a resonant magnetic perturbation at the plasma periphery such as edge-localized-mode mitigation and divertor detachment. Novel approaches of non-local and non-diffusive transport have also been advanced.

Alfven 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.

Alfvén eigenmodes (AEs) were studied in neutral beam injection (NBI) heated plasmas in the TJ-II stellarator using a heavy ion beam probe (HIBP) in the core, and by Langmuir (LP) and Mirnov probes (MP) at the edge. AEs were detected over the whole plasma radius by the HIBP with a spatial resolution of about 1 cm. AE-induced oscillations were detected in the plasma density n e, electric potential φ and poloidal magnetic field B pol with frequencies 50 kHz f AE k θ , mode numbers m(m V θ ∼ 30 km s−1 were detected for various branches of AEs, having different radial locations. When the density rose due to NBI fuelling, the AE frequency decreased as predicted by the Alfvén law . During the AE frequency decay the following new AE features were observed: (i) the poloidal wave-vector k θ and mode number m remained constant, (ii) the cross-phases between the oscillations in B pol, n e and electric potential remained constant, having an individual value for each AE branch, (iii) V θ decreased proportional to the AE frequency. The interaction of the AEs with the bulk (thermal) plasma resulted in clearly pronounced quasi-coherent peaks in the electrostatic turbulent particle flux spectra. Various AE branches exhibited different contributions to the particle flux: outward, inward and also zero, depending on the phase relations between the oscillations in E pol and n e, which are specific for each branch. A comparison with MHD mode modelling indicated that some of the more prominent frequency branches can be identified as radially extended helical AEs.

Current drive by lower hybrid waves that have a phase velocity five times the electron thermal velocity has been investigated in the JFT-2M tokamak. Two different modes of tokamak operation, the automatic current regulation (ACR) and the automatic voltage regulation (AVR) modes, were employed to study the plasma control during the current drive. In the ACR mode, which is the ordinary tokamak operation in JFT-2M, the current of the poloidal-field coils which produce the main magnetic flux outside the torus is controlled by thyristor power supplies, while in the AVR mode their voltage is controlled. The results indicate that control of the poloidal field determines the time evolution of the plasma current during current drive whereas operation in the AVR mode suggests the sustainment of a steady-state plasma current through recharging of the Ohmic transformer. A high current drive efficiency was obtained which increased with the plasma current as up to Ip = 150 kA. The dependence of the current drive efficiency on the plasma current is related to the electron temperature and to propagation effects of the lower hybrid waves. The interaction of the excited lower hybrid waves with the electrons was investigated by pulse height analysis of the soft-X-ray emission. It was shown that the measured soft-X-ray energy spectrum is consistent with the theoretical spectrum of the launcher. The characteristic times for the modification of the electron distribution function and for the drop in loop voltage are proportional to the electron density.

Neutral-beam power of up to 1.2 MW injected into the plasma has produced a volume-averaged β of up to 2.6% and a central beta β0 of up to 7%, due to the thermal components in the JFT-2 tokamak. In these beam-dominated discharges, the magnetohydrodynamic behaviour was studied. Four types of internal oscillations were observed: i) enhanced sawtooth oscillations with long repetition time and large sawtooth amplitude; ii) round sawtooth oscillations and/or reduced sawtooth oscillations with short repetition time and small sawtooth amplitude; iii) high-frequency oscillations without sawtooth oscillations, and iv) high-frequency oscillations with sawtooth oscillations. The measured beta values are compared with the critical ones as found from high-n ballooning-mode analysis, and the relationship between MHD behaviour and beta values is also investigated.

Parametric instabilities, which bring about undesirable peripheral heat deposition in lower-hybrid-wave (LHW) heating, are stabilized in the JFT-2 tokamak by an effective electron heating in the boundary plasma. The boundary electron temperature rises significantly by simultaneous injection of neutral beams into the plasma, which may be expected to improve the heating efficiency of LHW in the main plasma

Deuterium and hydrogen energy spectra during ICRF heating as well as nH/nD were measured with the E||B type mass-separated neutral particle energy analyser. High-energy tails of deuterium and hydrogen were observed in the dilute-minority case. From Fokker-Planck calculations of the charge-exchange spectrum and power-balance analysis it is shown that second-harmonic cyclotron damping by the majority deuterium is the dominant mechanism of ion heating in the dilute-minority case.

ICRF heating experiments were carried out in the JFT-2 tokamak with pure high-field-side excitation. Almost pure electron and ion heating regimes were made possible in the experiment so that the metal impurities were reduced. Power balance of the plasma heated by ICRF is revealed by a transport analysis of the experimental results. RF power deposition profiles of each heating regime are obtained. The results of this analysis may serve as an experimental basis for refining the present theoretical treatment of ICRF heating.

The mechanism responsible for the initiation of unipolar arcs on the tokamak first wall is studied by measuring plasma parameters in the scrape-off layer. It is shown that a large electric field in the sheath region produced by runaway electrons is the dominant cause of the arc initiation. The difference in arc frequency in the toroidal direction on the limiter surface is explained by an asymmetry of the ion flux due to ion drift motion.