[en] Two types of plasma termination events, which are MHD instabilities driven by the pressure gradient and plasma current, are studied, respectively. In the Large Helical Device (LHD) experiment, many MHD instabilities are observed. Especially, if the peaked pressure profile was sustained by the pellet injection, a collapse event, so-called the core density collapse (CDC), was happen. In nonlinear MHD simulations, it is expected the CDC is driven by the resistive ballooning mode. On the other hand, in Wendelstein 7-X (W7-X) experiment, collapse events simultaneously happened in the plasma core, if the strongly localized plasma current, which is ECCD for an example, is driven in the plasma core. If the rotational transform, ι, increases by ECCD and crosses the rational of ι = 1, the magnetic island opens on ι = 1 rational surface. However, in nonlinear simulations, MHD event does not happen on ι = 1 rational surface. At the outer region of ι = 1 rational surface, where is the negative shear region corresponding to the positive shear region in tokamaks, the ballooning mode appears and destroys the flux surfaces completely. That is a mechanism to terminate the plasma. (author)

[en] A comprehensive set of neutron diagnostics has been successfully operated in the Large Helical Device (LHD) deuterium discharges, playing an important role in assessing fusion gain (Q), confinement property of energetic particles, and/or radiation safety. So far, maximum neutron emission rate (S_n) of 3.3×10¹⁵ n/s, maximum shot-integrated neutron yield of 5.0×10¹⁵ n/shot, Q_DD of 4×10^-4, and equivalent Q_DT assumed for deuterium-tritium reaction of 0.11 have been recorded. In addition, energetic-particle confinement studies have been significantly enhanced through measurements of S_n, DD neutron emission profile, and secondary triton burnup 14 MeV neutron emission rate. The triton burnup ratio defined as the ratio of DD neutron yield to secondary DT neutron yield has reached 0.45%. (author)

[en] Recent research activities on the SUNIST spherical tokamak are introduced. Experimental researches are focused on MHD behaviors during plasma current (Ip) ramp up /down phases of ohmic discharges, including toroidal Alfven eigenmodes (TAEs) during minor disruption, internal reconnection events (IREs) in Ip ramp-down phase, and tearing mode(TM) instabilities in Ip ramp-up phase. (author)

[en] The measurement of effective ion charge Z_eff in LHD is reconsidered. The plasma equilibrium database, the so-called TSMAP, recently made available is used for mapping of locations on the line-of-sight on the effective minor radius r_eff so that accuracy of the electron temperature and density profiles for evaluating the Bremsstrahlung intensity has been improved. The influence of impurity lines is also estimated from other diagnostic data, and it is demonstrated that unrealistically large Z_eff values derived with the conventional method for low density plasmas are avoidable. (author)

[en] The China Fusion Engineering Test Reactor (CFETR) is the proposed next-generation fusion facility in China, during its two operational phases, a large fraction of externally driven current is foreseen to meet the scientific goals of the two operating scenarios for CFETR. The design of electron cyclotron resonance heating and current drive (ECRH/ECCD) will be carried out with the requirements of physical objectives, as a key design activity of auxiliary heating mix for CFETR. Based on the physical requirements and the technical availability, the frequency is chosen to be 170GHz in order to benefit from the technologies development for ITER, the consideration and the preliminary design of the EC system will be presented in this paper. (author)

[en] Based on the quasi-axisymmetric stellarator(QAS) equilibrium in CFQS, the linear micro-instability, turbulent transport, and zonal flow generation are investigated by means of the electromagnetic gyrokinetic Vlasov simulations. It is elucidated that QAS has more unstable ion temperature gradient(ITG) driven modes with higher growth rate in a wider range of the wavenumber space, compared with that in the case of axisymmetric limit. However, the turbulent transport level is comparable or less in the nonlinear phase. Then, relatively stronger zonal flow generation is identified in QAS for a wide range of the ion temperature gradient, which is a key driving parameter of ITG driven turbulence. (author)

[en] Well understanding of tritium (T) permeation and retention behavior in tungsten (W) requires the knowledge of hydrogen isotopes transport parameters. In the present study, the deuterium (D) permeation behavior in tungsten (W) are studied by gas driven permeation (GDP) and plasma driven permeation (PDP) methods. The D-GDP process in W agrees with the Sieverts’s law that the steady state permeation flux under different temperature are proportional to the square root of pressure. According to the Siverts’ law and Fick’s first law, the D permeability and effective diffusivity in W are obtained from the GDP results. To study the hydrogen isotopes plasma driven permeation (PDP) behavior in plasma facing materials, a linear Radio Frequency (RF) plasma device has been constructed in the radiation controlled area at Shizuoka University. In the PDP experiment, the steady state permeation flux is approximate proportional to the square root of the ion flux indicating that D-PDP process takes place in the RD regime. The D recombination coefficient on upstream surface of W is obtained by the PDP results. (author)

[en] The Chinese First Quasi-axisymmetric Stellarator (CFQS) is a new quasi-axisymmetric experimental device planned for construction at Southwest Jiaotong University (SWJTU), China. This is a joint project of the National Institute for Fusion Science (NIFS) and the SWJTU. The present paper discusses the equilibrium configuration of the CFQS with a major radius of 1.0 m, a toroidal magnetic field strength of 1.0 T, and an aspect ratio of 4.0. As the CFQS is a quasi-axisymmetric stellarator, a tokamak-like substantial bootstrap current is expected. The magnitude of the bootstrap current was estimated by BOOTSJ code. Next, the effects of the bootstrap current on the quasi-axisymmetric property and the neoclassical diffusion coefficient were estimated. Effect of the bootstrap current on the quasi-axisymmetric property is not significant, and a good neoclassical transport property is maintained. (author)

[en] The impact of a low frequency MHD mode crash on triggering the H mode has been studied in detail on the HL-2A tokamak. The mode manifests fishbone characteristics with a procession frequency f approx. =14-19 kHz. The abrupt mode crash evokes substantial energy release from the core to the plasma boundary and hence increases the edge pressure gradient and E_r×B flow shear, which further suppresses turbulence and leads to confinement improvement into the H-mode. Under the same NBI heating (∼ 1 MW), the I-phase plasma transits into H-mode with a rapid mode crash while it returns to the L-mode without the presence of the mode in the I-phase. With increasing heating power by the ECRH added to the NBI, the kink-like mode disappears. The statistical result shows that with the MHD mode crash the heating power for accessing the H mode is significantly lower than that without the mode crash. All these facts reveal that the MHD mode crash in the I-phase plays a critical role in trigging the I→H transition at marginal heating power. In addition, it has been found that with the same NBI power heating, the magnitude of the mode (crash) increases with increasing plasma density, implying larger energy release being needed to access the H-mode for higher density plasmas. The results may have significant implication on ITER operation scenarios with tight margin of L-H threshold power. (author)

[en] Energetic particle driven geodesic acoustic modes (EGAMs) in the Large Helical Device (LHD) plasmas are investigated using MEGA code. MEGA is a hybrid simulation code for energetic particles (EPs) interacting with a magnetohydrodynamic (MHD) fluid. The low frequency EGAMs are reproduced. Also, the energy transfer is analysed and the bulk ion heating during the EGAM activity is observed. The ions obtain energy when the EPs lose energy, and this indicates that an energy channel is established by EGAM. The EGAM channelling is reproduced by simulation for the first time. The heating power of bulk ions is 3.4 kW/m³ which is close to the value 4 kW/m³ evaluated from the experiments. It is found that the sideband resonance is dominant during the energy transfer from EGAM to the bulk ions, and the transit frequencies of resonant bulk ions are half EGAM frequency. (author)