This paper reviews the results of an experimental study undertaken in Heliotron J over the past few years to explore the physics design base for a new concept of a helical-axis heliotron. Measurements of electron cyclotron resonance (ECR)/neutral beam injection (NBI)/ion cyclotron range of frequencies (ICRF) heating plasmas have been made for understanding global energy confinement in connection with the international stellarator scaling law (ISS04), spontaneous confinement improvement (L-H transition), confinement improvement based on supersonic molecular beam injection (SMBI), magnetohydrodynamic (MHD) activity, edge plasma characteristics, including rotation of a filamentary turbulence structure, and plasma current control, including the electron cyclotron current drive (ECCD), the energetic-particle driven Alfvén eigenmodes, and related fast ion dynamics. The results are discussed in terms of the rotational transform ι/2π and the bumpiness ε_b (or the effective helical ripple ε_eff). Control of these two parameters was experimentally demonstrated to be the key issue in determining the optimum performance of Heliotron J. The result confirms that the helical-axis heliotron provides a unique and high potential for exploiting an alternative and advanced path to future helical systems.

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A variety of density profiles observed in the Large Helical Device (LHD) have suggested an interesting core impurity transport. The edge impurity transport in the ergodic layer formed by stochastic magnetic field lines with long connection length (10-2000 m) can also exhibit interesting phenomena in the competition of perpendicular and parallel transport. The LHD discharge is highly robust against impurity buildup, and operation is possible essentially up to the global power balance limit because current-driven instability does not principally exist. The LHD plasma has therefore provided information on many interesting physics issues closely related to impurities. Recent results of impurity-related physics experiments in the LHD are briefly reviewed. The specific contents presented here are (1) core impurity behavior with perpendicular transport, (2) edge impurity behavior with parallel transport, (3) high-Z discharges with high ion temperature, (4) impurity pellet injection with improved plasma performance, (5) impurity pellet ablation in the presence of energetic ions with high heat flux and (6) observation of magnetic dipole forbidden transitions for high-Z elements. A result from the Compact Helical System (CHS) is used only in the impurity pellet ablation study, because detailed data have not yet been obtained from the LHD. Finally, the results are summarized and future directions in these topics are noted.

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The interaction of mean plasma flows (driven by equilibrium E×B forces) and oscillating plasma flows - such as zonal flows (ZFs) and geodesic acoustic modes (GAMs) - (radially localized E × B plasma flows generated by non-linear turbulence interactions) is investigated in the ASDEX Upgrade tokamak using Doppler reflectometry. ZFs and GAMs play an important role in magnetic confinement devices by enhancing the velocity shearing of turbulent eddies which reduces turbulent cross-field transport. Although the GAM is universally observed across the plasma edge gradient region of ohmic and additionally heated low confinement L-mode regimes, it is not seen in the high confinement H-mode. Using slow power ramping of the additional ECRH and NBI heating systems the behaviour of the edge GAM, and its interaction with the mean flow shear, i.e. the radial electric field E_r profile, across the L to H-mode transition are studied. As the edge E_r well deepens with improving confinement, the associated negative E_r shear region narrows and the radial extent of the GAM shrinks. The main feature, however, is the movement of spectral power out of the coherent GAM into large amplitude, low frequency random flow perturbations. It is speculated that these flow perturbations may play a role in the triggering of the confinement transition.

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Multi-hierarchy physics in magnetic reconnection is investigated using two kinds of numerical simulation models: the electromagnetic particle simulation model for an open system (PASMO) and the magnetohydrodynamic and particle-in-cell (MHD-PIC) interlocked model for the multi-hierarchy simulation of magnetic reconnection. A series of simulation studies using the PASMO code have disclosed that ion and electron dissipation regions form inside a kinetic regime in which ion and electron meandering orbit effects are crucial in triggering collisionless reconnection (CR). Anomalous resistivity leading to CR is also generated through the excitation of ion-ion kink instability in an ion-scale current sheet. We confirmed that the MHD-PIC interlocked model, based on the domain decomposition and multi-time-step methods, can describe collisionless driven reconnection in an open system as a multi-hierarchy phenomenon with high accuracy.

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We consider the emission of visible light by hot metal surfaces having uniform and non-uniform temperature distributions and by small droplets of liquid metal. The calculations employ a nonlocal transport theory for light emission, using the Kubo formula to relate microscopic current fluctuations to the dielectric function of the material. We describe a related algorithm for calculating radiation emission in particle simulation of hot fusion plasmas.

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Integrated high performance was achieved in a wall-stabilized reversed shear plasma, and the high values of H_H98(y,2) = 1.7, β_N = 2.7, f_GW = 0.87, f_BS = 0.9, and f_NI = 0.95 were simultaneously realized at the ITER/DEMO-relevant value of q₉₅ = 5.3. The β_N value achieved was above the no-wall β_N limit of 1.9 and close to the ideal-wall β_N limit of 2.9 in that discharge. The β_N was limited by the appearance of the Resistive Wall Mode (RWM) when the toroidal rotation velocity at the q = 3 surface decreased to below the critical velocity. Another discharge realized a fully relaxed current profile at the high value of f_BS = 0.5 in a fully non-inductive Current Drive (CD) condition at q₉₅ = 5.8 through use of a lower-hybrid CD and Neutral Beam (NB) CDs with bootstrap current. Integration of operation scenarios to ensure steady-state high performance is a major mission with the JT-60SA. A DEMO-equivalent value of β_N = 4.3 is being aimed at utilizing highly shaped configurations, closely-placed conducting walls, low ripple rates, and RWM control coils. Steady-state q profiles can be maintained using an off-axis CD resulting from a negative-ion based NB and bootstrap current for 60-100 s. An operation scenario that supports building up the I_p with less consumption of the magnetic flux is a key issue in the realization of a compact DEMO SlimCS. Fully non-inductive buildup of I_p and recharging of the Ohmic coil current were demonstrated utilizing the JT-60U. Development of an overall operation scenario from the buildup of the I_p to steady sustainment of a high performance plasma is an important challenge with the JT-60SA.

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Various phenomena of plasma-wall interactions during long duration discharges in TRIAM-1M are investigated from macroscopic and microscopic viewpoints. It is found that the density dependence of the hydrogen neutral flux decay length is not very sensitive to the density (i.e. n_e^−0.2 ). Neutral transport through the scrape-off layer is important for structural formation of hydrogen recycling. The hydrogen retention in the co-deposited layer, which was obtained using a material probe during long duration discharges, is consistent with the global wall pumping rate estimated from particle balance analysis. The wall temperature and co-deposition play important roles on hydrogen re-emission and hydrogen absorption, respectively. Oxygen impurities should affect the erosion and deposition properties of the plasma-facing wall as well as the hydrogen retention property of the co-deposited layer. The co-deposition of hydrogen with molybdenum and the oxygen impurities both play important roles in the hydrogen recycling.

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In the tandem mirror GAMMA 10, plasma confinement is achieved by not only a magnetic mirror configuration but also high potentials in both end regions. Before plug electron cyclotron resonance heating (P-ECH) was applied, drift-type fluctuations were observed in potential and density measurements. Potential and density fluctuation suppression during potential formation by the application of P-ECH was clearly observed using a gold neutral beam probe (GNBP) system. We study the relationship between the suppression levels of potential fluctuations and the effects of the produced potentials by varying the applied ECH power. Moreover, the particle-flux-related values obtained by the phase difference between the potential and density fluctuations are measured using the GNBP. We can clearly show that radial anomalous transport induces radial particle transport, which decreases the plasma stored energy.

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This paper gives an overview of recent results on the application of electron Bernstein waves for plasma heating and diagnostic purposes at the WEGA stellarator. By applying a two-step mode conversion process it was possible to reach over-dense argon and helium plasmas with central densities of above 0.97 × 10¹⁹ m⁻³ at a magnetic field strength of 0.5 T fully sustained by electrostatic Bernstein waves. An unexpected feature observed during the experiments was the detection of a strong increase of the radiation temperature associated with the existence of a super-thermal electron component in the keV-range during this heating period.

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Properties of various kinds of plasmas are determined by atomic radiation and collision processes occurring between plasma particles, therefore, the detailed knowledge of the effective cross sections and rate coefficients for these processes is highly required. In this work, charge-changing processes, mainly for heavy-particle collisions, including impurity ions, are considered using available computer codes, and the background physics is discussed. Besides general features, the role of metastable states of plasma atoms and ions, which are often observed in most of plasmas and play a key role in plasma formation, is considered. The non-linear effects observed for heavy particle collisions, such as the density effect and multiple-electron ionization, are also considered. In addition, the isotope effects are briefly discussed related with electron capture in collisions of alpha particles with hydrogen and its isotopes (D and T).

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Turbulence-induced high density filaments, blobs, might represent the new paradigm for radial convective transport in the Scrape-Off-Layer of magnetic fusion devices. Models for individual blob dynamics, considering their generation on the Low Field Side (LFS) of the torus have been put forward. We investigate the existence of blobs in the peripheral region of the High Field Side (HFS) of the FT-2 tokamak. Langmuir probe measurements are used to detect and characterize blobs. The results were obtained in plasmas with enhanced Lower Hybrid Heating and data was acquired with a new fast data acquisition system with 50 MHz sampling rate. While the majority of the blobs are observed to move towards the wall as expected, some are observed to move inwards towards the core. This effect is not understood in the light of existing models for the LFS where only the outward direction is expected. Further characterization of plasma blobs should therefore involve both HFS and LFS. This could be done at ASDEX Upgrade where reflectometry systems are capable of measuring simultaneously the density turbulence on both HFS and LFS. The dependence between radial velocity, size and density of the blobs is also investigated.

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Measurements with a three dimensional set of Langmuir probe arrays have unambiguously demonstrated the coexistence of intensive low frequency zonal flows (LFZFs), geodesic acoustic modes (GAMs), low frequency fluctuations (LFFs) and high frequency ambient turbulence (HFAT) in the edge of HL-2A tokamak plasmas, by verifying the temporal and spatial characteristics of the electrostatic fluctuations. The intensity of the LFZFs is observed to increase and decrease with increases of ECRH power (∼ 300-700 kW) and safety factor q (∼ 3.5-6.2), respectively, and the intensity of the GAMs increases with the ECRH power as well as q. The radial wave number-frequency spectra of the LFZF show that the LFZF packets propagate outwards and inwards as equally likely events, while the GAM packets propagate predominantly outwards. The three wave coupling of the zonal flows, including the GAMs, and the LFFs with the HFAT is investigated in detail.

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The renormalized nonlinear analysis of the temporal evolution of drift-type modes in plasma shear flows is developed. The theory accounts for the effect of the tubulent motions of plasma on the saturation of the resistive drift instability. The nonlinear balance equation, which determines the saturation level of the resistive drift instability in shear flow is obtained. It was prowed that the “nonlinear effect of the enhanced decorrelation by shear flow” has nothing in common with process of the saturation. The same conclusion is applicable to all fluid models of plasma, obtained in drift approximation, in which all nonlinearities, other than E×B are ignored. The linear non-modal kinetic theory to the Vlasov-Poisson system is developed. This theory reveals the velocity shear in a non-modal time-dependent effect of the finite Larmor radius.

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Nonlinear mode couplings in plasma turbulence generate mesoscale global structures, such as zonal flows and streamers. A streamer is a poloidally localized structure that is radially elongated. In the Large Mirror Device-Upgrade, a streamer structure was observed in the ion saturation-current fluctuation measured with a 64-channel poloidal Langmuir probe array. The bunching of waves was poloidally localized and rotated slowly in the ion diamagnetic direction. The frequency and poloidal mode number of its envelope corresponded to one particular mode, which was strongly coupled with many other modes. By calculating the radial biphase profile of three pronounced modes, including the particular mode, the streamer structure was confirmed to be radially elongated.

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Magnetic reconnection without a guide field in low-background-density plasmas has been studied by means of two-dimensional particle-in-cell simulations, and results in hydrogen and electron-positron (pair) plasmas have been compared. Reconnection is impulsive, and maximum reconnection rates of the order of one (measured in units of the Alfvén speed) have been observed in both types of plasmas when the background density in the Harris sheets is 1% of the current sheet density. This impulsive, strong reconnection electric field is important for particle acceleration. As the system evolves in time, the electron diffusion region extends in both inflow and outflow directions. Because of the broadening of the diffusion region, the aspect ratio of the diffusion region remains small, so that fast reconnection is sustained. In pair plasmas, the inertial term in the generalized Ohm's law becomes the most dominant term to balance the reconnection electric field before the maximum reconnection rate is attained, which contrasts with hydrogen plasmas where the most dominant term is the pressure tensor term.

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A series of ELM control experiments have been performed on JET aiming at a better understanding of the plasma response to applied magnetic perturbations. The dynamics of the edge profiles with applied n = 1 fields have been studied in the type-I ELM H-mode plasmas. Typically, the pedestal density decreased by about 20 % when the n = 1 perturbation field was applied (So called pump-out effect). However, there is no influence of the n = 1 fields on the recovery rate of the pedestal pressure, but the ELM crash occurs earlier and at a lower level of the pedestal pressure and pressure gradient. The compensation of the density pump-out has been demonstrated using either gas fuelling or pellets injection in low triangularity H-mode plasmas. Strong toroidal rotation braking by more than 60 % has been observed, and found to be independent on the safety factor. The calculated Neoclassical Toroidal Viscosity (NTV) torque profile in the ν regime including the boundary effect is in agreement with the observed torque profile induced by the n = 1 fields on JET. No complete ELM suppression was observed by application of either n = 1 or n = 2 fields with a Chirikov parameter above one at √ Ψ ≥ 0.925, which is one of the important criteria for the design of ITER ELM control coils.

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Energetic ion driven Alfvén Eigenmodes (AEs) in the NBI-heated plasma at the TJ-II heliac were studied by Heavy Ion Beam Probing (HIBP) in the core, and by Langmuir and Mirnov probes (LP and MP) at the edge. HIBP observed the locally (∼ 1 cm) resolved AE at radii -0.5 < ρ < 0.9. The set of AE branches with low poloidal numbers (m < 8) was detected by MP. The most plausible candidates are global, helical and toroidal AEs. AEs on the density, electric potential and poloidal magnetic field oscillations were detected by HIBP at frequencies 50 kHz < f_AE < 300 kHz with a high resolution (< 5 kHz). The amplitude of the AE potential oscillations δφ_AE ∼ 10 V was estimated. The MP and HIBP data have a high coherency at f_AE. When the density rises, AE frequency is decreasing, f_AE ∼ n_e^−1/2, but the cross-phase between the density and potential remains permanent. Poloidally resolved potential measurements by HIBP and LP shows high coherency and finite cross-phase at f_AE, resulting in finite electric field δE_pol. Depending on the cross-phase between δn_e and δE_pol, AEs may bring small or significant contribution to the turbulent particle flux Γ_E×B for the observed k_θ < 3 cm⁻¹.

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Chemical sputtering on a graphite (0 0 0 1) surface due to hydrogen atom bombardment was investigated by a molecular dynamics simulation. To understand the temperature dependence of the sputtering yield of CH_x and C₂H_x, two thermostat methods used to control material temperature, the Langevin and Berendsen thermostats, were compared. The simulated sputtering yield of CH₄ with a peak at 600 K under the Langevin thermostat agreed with experimental observations. The present simulation showed that the sputtering yield depends on the thermal relaxation time, which is the time required for the temperature to reduce and is the second key parameter for both thermostat methods. In addition, it was demonstrated that the Berendsen thermostat, which controls temperature globally, provides larger sputtering yields of CH_x and C₂H_y than the Langevin thermostat, which controls temperature locally.

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We measured extreme ultraviolet spectra of plasmas in the Large Helical Device after an Fe pellet was injected and analyzed the Fe XXI spectral lines λ 12.12 nm (2s²2p^{2 3}P₂-2s2p^{3 3}P₂) and λ 12.875 nm (2s²2p^{2 3}P₀-2s2p^{3 3}D₁). By constructing a collisional-radiative model for Fe XXI, we obtained the electron density dependence of the line intensity ratio, which is the result of the excitation process among the fine-structure levels of the ground state, 2s²2p^{2 3}P_J by electron and proton impact. Using the obtained electron density dependence, we estimated the electron density from the measured intensity ratio. By comparing the estimated density with the electron density distribution measured by a far infrared interferometer, we could estimate the location of the Fe XXI emitting region. We found that Fe XXI appeared first around the density peak formed by the pellet and later expanded toward a low-density region. The corresponding electron temperature of the location is lower than that for ionization equilibrium, and the Fe XXI ions are expected to be in a non-equilibrium ionization state.

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The helical pitch parameter (γ) dependency of equilibrium and the stability of the high beta plasma in the LHD type magnetic configuration is studied numerically. It is confirmed that the small γ configurations are favorable for the LHD-type fusion reactors in the point of robustness of high beta equilibrium, compatibility of easy ignition and high output power of core plasma, in addition to a sufficient space for blankets.

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The role of the effective helical ripple in the helical plasma confinement is analyzed using the transport code via the effect of zonal flows. One-dimensional coupled transport equations are calculated in the different cases of the effective helical ripple. The reduction of the effective helical ripple ratio lowers the criterion for the excitation of zonal flows in helical plasmas. It is demonstrated that the reduction of the turbulent transport can be obtained in the case of the smaller neoclassical transport when zonal flows are excited.

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We have solved the two-dimensional time-dependent Schödinger equation for a particle with and without the interparticle potential in a fusion plasma. It was shown that spatial extent of a free particle grows monotonically in time. Such expansion leads to a spatial extent or size of a proton of the order of the average interparticle separation Δl ≡ n^−1/3 ∼ 2 × 10⁻⁷ m in a time interval of 10⁶ × Δl/v_th ∼ 10⁻⁷ sec for a plasma with a density n ∼ 10²⁰ m⁻³ and a temperature T = mv_th²/2 ∼ 10 keV. It was also shown that, under a Coulomb potential, the wavefunction of a charged particle first shrink and expand in time. In the expansion phase, at times t ≥ 10⁻¹⁰ sec, the size of particle in the presence of a Coulomb potential is much larger than that in the absence of it.

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We have constructed a semiclassical collisional diffusion model. In this model, a field particle is treated as either a point charge or a spatially distributed charge. The test particle is treated as a distributed point charge with Gaussian distribution. It was shown that the collisional changes in velocity in our model is of the same order as the classical theory for a typical proton in a fusion plasma of T = 10 keV and n = 10²⁰ m⁻³. It was also shown that the spatial extent of the distribution, or the quantum-mechanical uncertainty in position, for the test particle obtained in our model grows in time, and becomes of the order of the average interparticle separation Δl ≡ n^−1/3 during a time interval τ_r ∼ × 10⁷ Δl/g_th, where g_th = √ 2T/m is the thermal speed, with m being the mass of the particle under consideration. The time interval is 3-4 order of magnitudes smaller than the collision time. This suggests that particle transport cannot be understood in the framework of classical mechanics, and that the quantum-mechanical distribution of individual particles in plasmas may cause the anomalous diffusion.

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The algebraic model (ALG) proposed by the authors has sufficiently high accuracy in calculating the motion of a test particle with all the field particles at rest. When all the field particles are moving, however, the ALG has relatively poor prediction ability on the motion of the test particle initially at rest. Nonetheless, the ALG approximation gives a good results for the statistical quantities, such as variance of velocity changes or the scattering cross section, for a sufficiently large number of Monte Carlo trials. We have implemented a graphics processing unit (GPU) using NVIDIA's CUDA architecture into the ALG scheme for Coulomb multibody problems. For N=28-body problem, the ALG calculations on the GPU is several times faster than on a typical CPU. The achieved speedup ratios on an NVIDIA GTX-285 are 10.5 and 2500 against the ALG-CPU and the DIM-CPU, respectively both on an Intel Celeron @3.06 GHz.

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Pellet injection to D-T burning plasmas was analyzed using an integrated equilibrium-transport simulation code, TOTAL, for both tokamak and helical reactors. The condition of pellet injection required for the formation of an internal transport barrier and the dependence of plasma parameters in the steady self-burning state on the D/T ratio in the pellet were investigated. A simulation with different D/T ratios in the pellet revealed that the operational region of temperature and density varies with the D/T ratio. When a deuterium-rich pellet was employed, the amount of tritium to be used in the reactor was approximately 30%-40% less than that when the D/T ratio was 1.

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Toroidal nonneutral plasmas confined on helical magnetic surfaces with a helical axis are experimentally studied for the first time. Variations in space potential φ_s and electron density n_e on closed helical magnetic surfaces are clearly observed similar to those of helical plasmas with plane axes in past experiments. The measured data in this experiment are compared with numerical results obtained from Poisson's equation applied to poloidal cross sections of Heliotron J.

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Improvement of ion heat transport was observed in the core plasma heated by neutral beam injection (NBI) in the Large Helical Device (LHD). A peaked ion temperature profile with a steep gradient (an ion internal transport barrier, ion ITB) formed in the plasma. To investigate the formation conditions of the ion ITB, a simple definition of ITB based on the reversal of the temperature gradient between the core and the edge is proposed. An ion ITB formed in the low-collisionality regime of 1/ν with low density and high ion heating power in the LHD.

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D-³He reactor has a great advantage of neutron-poor operation, but it requires higher-beta value and higher bootstrap current fraction than D-T reactor. In this paper, we investigated self-consistent plasma pressure and current profiles in the case of a positive magnetic shear mode (parabolic temperature profile) and a negative magnetic shear mode (ITB temperature profile) using TOTAL-T code including Cytran module. In a negative magnetic shear mode, toroidal beta value β_T ∼ 52.4 % and bootstrap current fraction f_bs ∼ 0.93 was obtained in a plasma with aspect ratio 1.77, plasma major radius 6.2 m, toroidal field 4.2 T and plasma current 49.3 MA.

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Amplified poloidal flux of spheromak has been observed in the repetitive discharge (50 kHz) of magnetized coaxial plasma gun (MCPG). In this experiment, ejected spheromak is confined within magnetic mirror configuration, and generated flux and emission has been measured. In the case of repetitive discharge, increased poloidal flux and its decay time compared to quasi-DC discharge has been observed. Also increased emission light and its decay time has been observed. These results indicates increasing of energy confinement time. In addition, global stabilization of behavior of spheromaks when it is transporting in discharge tube has been observed in repetitive discharge case.

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In the TST-2 spherical tokamak, non-inductive start-up experiment using electron cyclotron heating (ECH) at 2.45 GHz are performed. Hydrogen and deuterium discharges were compared after sufficient wall cleaning by Ohmic discharges to ensure discharge reproducibility. After the wall cleaning, wall recycling seemed to be reduced, because additional gas puffing was necessary to sustain a discharge. Even in that case, hydrogen emission measurement revealed the existence of finite wall recycling source. A current jump occurred earlier in the deuterium plasma than in the hydrogen plasma. Except for the timing of the jump, the discharge time evolution and various values such as the plasma current were almost the same for the deuterium and hydrogen plasmas.

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In order to reduce high heat load on divertor plate in fusion reactors, we investigated radiative mantle formation scenarios by impurity seeding into scrape off layer (SOL) in ITER using the TOTAL simulation code. The low-Z impurity, like He, could not form a radiative mantle and have almost no contribution to the reduction of divertor heat load. On the contrary, the medium-Z impurity, like Kr, can form radiative mantle definitely and can radiate about 84% (core:33% / mantle:51%) of input power inside the last closed flux surface (LCFS) without any serious changes in density and temperature profile, and without inducing back transition from H to L mode. It can reduce divertor heat load about 60% compared with the case of no impurity injection in ITER.

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Since a high performance operation is needed in the future fusion reactor, it is expected that the operation with Internal Transport Barrier (ITB) is utilized to improve core plasma confinement in the reversed magnetic shear. In this study, firstly, the simulation results are compared with the experimental result, and the validity of the model is checked. Secondly, the effects of the magnetic shear and the pellet injection on ITB formation are simulated in tokamak reactor using the Toroidal Transport Analysis Linkage code TOTAL. In this simulation, the ITB formation is confirmed in the condition that magnetic shear is nearly zero and the density gradient exists, and it found that the reversed magnetic shear configuration is effective in the operation with ITB.

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On tokamak plasmas like ITER, it is necessary to stabilize neoclassical tearing mode (NTM) because the NTM reduces plasma temperature and fusion power output. For the analysis of stabilizing NTM in fusion plasmas, the electron cyclotron current drive (ECCD) and the non-resonant external helical field (NRHF) application are simulated using the 1.5-dimensional equilibrium/transport simulation code (TOTAL code). The 3/2 NTM is stabilized by only external helical field, but the 2/1 mode is not stabilized by only external helical field in the present model. The stabilization time becomes shorter by the combination of ECCD and NRHF than that by ECCD alone.

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A tokamak-helical hybrid device TOKASTAR-2 with outboard helical field coils located outside the toroidal field coil system was constructed. Initial plasmas are produced by the electron cyclotron heating scheme, and the plasma electron density profile is measured using electrostatic double Langmuir probes. The peak location of measured electron density profile in the radial direction agrees with the first electron cyclotron resonance layer at initial beak-down period and at final disappearance period of the plasma discharge. The suppression of bursting electron density oscillations and the shift of plasma density profile are observed in the case of applying outboard helical magnetic field.

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TOKASTAR, a hybrid magnetic configuration between tokamak and stellarator, has been proposed for the study of magnetic confinement optimization. The construction of the small device named TOKASTAR-2 was started in April 2008, and completed in March 2009. As a preliminary experiment, spatial distribution of electron temperature (T_e) and electron density (n_e) are measured in a simple toroidal magnetic field configuration. Initial ECH plasmas with T_e ∼ 10 eV, n_e ∼ 10¹⁶ m⁻³ are confirmed. In the preliminary experiment of ohmic heating plasma production, plasma current (I_p) of 25 A is observed. The plasma equilibrium is calculated by the TOSCA code, and the magnetic flux surface and optimal vertical field coil currents are clarified for the future experiment.

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The axisymmetric electrostatic eigenmodes of spheroidal pure electron plasmas have been studied experimentally. It is confirmed that the observed spheroidal plasma attains a theoretically expected equilibrium density distribution, with the exception of a low-density distribution surrounding the plasma. When the eigenmode frequency observed for the plasma is compared with the frequency predicted by the dispersion relation derived under the assumption of ideal conditions wherein the temperature is zero and the boundary is located at an infinite distance from the plasma, it is observed that the absolute value of the observed frequency is systematically higher than the theoretical prediction. Experimental examinations and numerical calculations indicate that the finite temperature effect alone cannot account for the upward shift of the eigenmode frequency, which is significantly affected by image charges induced on the conducting boundary.

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A unique characteristic of the quasi-poloidal/isodynamic transport optimization strategy is that it can lead to stellarators that deviate from the usual “doughnut” shape; i.e., they can have extended relatively straight cylindrical sections of plasma (connected by corner regions). This offers a number of potential design advantages, including simplified coil geometries, novel divertor approaches, low bootstrap current (less potential for ELMs and disruptions), more acceptable wall heat fluxes, and demountable blankets for reactors. The STELLOPT approach has been used to develop optimized configurations of this type for two and four field periods with aspect ratio <R>/<a> in the range of 8 to 16.

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It is widely accepted that the radial electric field strongly affects plasma confinement through the transport process. We have analysed the time evolution of the radial electric field and the radial transport in plasmas in a helical magnetic configuration by extending the TASK/TX code [M. Honda and A. Fukuyama, J. Comput. Phys. 227, 2808 (2008)]. TASK/TX is a one-dimensional dynamic transport code originally developed for axisymmetric plasmas. A set of flux-surface averaged fluid equations is solved simultaneously, rather than a set of diffusion equations based on the flux-gradient relations. TASK/TX consists of one-dimensional two fluids (electron and ion) equations, Maxwell's equations and diffusion equations of neutrals. To apply TASK/TX to helical plasmas, we have included two additional effects; helical neoclassical viscosity force and diffusion due to magnetic braiding. Plasma transport simulation is carried out mainly using the LHD parameters. We obtained negative radial electric field (ion root) with ion heating and positive radial electric field (electron root) with electron heating. Effects of magnetic braiding are also studied.

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Island effects on the plasma periphery due to the external perturbation fields in the Tohoku University Heliac (TU-Heliac) were surveyed. A fixed m = 3 magnetic island was produced by two pairs of external cusp field coils. The electron density decayed from the outer edge of the island after perturbation field application. Radial profiles of the electron temperature and plasma space potential in the island region revealed the magnetic island structure. The radial electric field at the inner edge of the island increased after perturbation field application. The positions of local maxima in the plasma space potential profile were in good agreement with the position of the n/m = 5/3 rational surface. The potential profile in the island grew with the perturbation field strength. The full width at half maximum of the potential profile depended on the square root of the perturbation field coil current.

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The confinement properties of perpendicular fast ions, which have kinetic energies mostly perpendicular to magnetic field lines, are investigated in the Large Helical Device (LHD) using short pulse injections of neutral beams (NB). Fast-ion lifetimes are evaluated using the fast neutral decay times after NB turn-off, where the neutrals are measured by an array of silicon-diode fast neutral analyzers. We found that the lifetime normalized by the 90-degree pitch-angle scattering time can be used as a scale of fast-ion confinement properties in magnetically confined fusion devices. The good confinement properties of the LHD's inwardly shifted configuration were demonstrated using the evaluated normalized lifetimes.

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The steady state in the GAMMA10 tandem mirror, in which radial transport due to flute mode turbulence is balanced by plasma heating and production, is investigated. The basic equations used here exclude the Alfvén and acoustic modes, which are unnecessary for transport due to the flute modes. The basic equations can be applied to the magnetic divertor if the system is assumed to be axisymmetric. High-pressure effects in non-axisymmetric magnetic fields in GAMMA10 are taken into the magnetic specific volume. Numerical calculation is performed to investigate the transport process due to flute mode fluctuations. Mild plasma heating with production leads the system to a steady state, whereas strong heating with production leads the system into sawtooth-like oscillation due to strong flute mode turbulence.

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The turbulence fluctuation spectra of linear magnetized plasmas are measured under various neutral pressure conditions. Strong turbulent waves and symmetric waves are observed in low and high neutral pressure conditions. In this study, we compare the radial structure of the probability density function (PDF) of these fluctuations. The characteristics of the PDFs are related to those of the radial and poloidal turbulent structures.

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The two-dimensional (2-D) image of high temperature plasmas in the GAMMA 10 tandem mirror has been measured at the central-cell by using a high-speed camera. Temporal behavior of visible image in the central-cell was observed with high-speed camera in the experiment of antenna optimization of central-ECRH system. Observed results showed a significant dependence on the position of the central-ECRH antenna. We discuss the characteristics of the fluctuation in the light emission intensity and the correlation with the radial distribution of electron density in the period of ECRH.

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Electron cyclotron resonance heating (ECRH) is superimposed on a linear cylindrical helicon plasma in the Large Mirror Device-Upgrade (LMD-U) to study the fluctuation characteristics of high density helicon plasma with/without ECRH injection. The radial profiles of electron density and electron temperature rose by < 10 %, and the drift wave frequency decreased with the ECRH injection. Bicoherence analysis reveals that the nonlinear interaction between drift waves and broad frequency band components exists with or without ECRH injection.

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The structure of fluctuations in the Large Mirror Device-Upgrade (LMD-U) depends on the neutral density. In this study, we focus on the high neutral density plasma in which the potential and density fluctuation waveforms are nontrigonometric functions similar to sawtooth waves, and peaks in the power spectrum lie along a straight line when plotted as frequency versus wavenumber. Two-dimentional auto and cross bispectral analyses of I_is and/or V_f, which consider both the matching conditions for both the azimuthal wavenumber and the frequency, have been performed; significant couplings are observed at particular peak frequencies. Similarities and differences between the auto and cross bicoherence of I_is and/or V_f are discussed.

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We studied the basic features of energy transfer in fully developed, freely decaying, isotropic, and homogeneous turbulence in magnetohydrodynamic (MHD) and Hall MHD (HMHD) media using orthonormal divergence-free wavelet analysis. The analysis supports the idea that energy transfer occurs locally; i.e., intense energy transfer occurs between modes that have very close spatial scales. The wavelet counterpart of triad interaction analysis in Fourier analysis shows that local interaction, i.e., combinations of three wavelet modes that have very close spatial scales, dominates the energy exchange between the velocity and magnetic fields. Energy transfer due to the Hall effect has opposite tendencies at larger and smaller scales, which is consistent with the results of Mininni et al. [P.D. Mininni et al., J. Plasma Phys. 73, 377 (2007)]. At larger scales, it causes a moderate inverse cascade. In contrast, it causes an intense forward cascade at smaller scales.

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A new technique is used to estimate the amplitude of temperature fluctuation in the edge plasma on TST-2. Langmuir probe current-voltage characteristic curves are conditionally reconstituted in terms of the intensity of the floating potential. High/low electron temperatures are obtained in low/high floating potential phases. As a result, normalized temperature fluctuation levels are found to be about 19 % or larger.

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An entropy balance equation that maintains the dynamics in the radial direction is derived to study the role of zonal flows in turbulent transport. The equation describes the dynamics not only of the entropy density production related to zonal flow and local heat flux, but also of entropy density convection. We investigated these spatio-temporal dynamics of entropy in ion temperature gradient driven turbulence based on a global gyrokinetic Vlasov simulations in a slab geometry. The contribution of zonal flow to entropy production is observed to be small in the strong instability case, whereas the zonal flow energy is sufficiently large. We also found that the entropy density with n = 2 in the Hermite polynomial expansion in velocity space plays an important role in convecting the entropy density, causing turbulent spreading.

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We present a study of the role of zonal flows in relaxation and transport in a reduced model of collisionless ITG turbulence. A fundamentally new constituent in the relaxation dynamics is revealed, namely that ion and electron guiding center motion togather necessitate a radial flux of polarization charge, which in turn exerts a dynamical friction on phase space density evolution. This effect then enters the evolution of δf² and the transport dynamics, as described by a Lenard-Balescu type equation. The underlying physics is similar to that which follows from conservation of potential vorticity, albeit now for a phase space fluid, and is not simple shearing or wave packet modulation. Consequences for zonal flow momentum balance are discussed.

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Parallel ion flow velocity along a magnetic field has been measured using a laser induced fluorescence (LIF) method in an electron cyclotron resonance (ECR) argon plasma with a weakly-diverging magnetic field. To measure parallel flow velocity in a cylindrical plasma using the LIF method, the laser beam should be injected along device axis; however, the reflection of the incident beam causes interference between the LIF emission of the incident and reflected beams. Here we present a method of quasi-parallel laser injection at a small angle, which utilizes the reflected beam as well as the incident beam to obtain the parallel ion flow velocity. Using this method, we observed an increase in parallel ion flow velocity along the magnetic field. The acceleration mechanism is briefly discussed on the basis of the ion fluid model.

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