Magnetized plasma is an non-equilibrium matter rich in nonlinear phenomena; its structural formations are dominated by turbulence. This article provides a brief review of experiments and observations of phenomena occurring in turbulent plasmas, with an emphasis on the methodologies for characterizing turbulence, and visualizing the invisible structure created by turbulence and the internal couplings between the elemental waves that constitute turbulence. Zonal flows, streamers, blobs, and other phenomena are investigated by using analytical methods such as Fourier transformation, wavelet analysis, probabilistic density function analysis, bicoherence, wavelet bicoherence. Finally, the contemporary view of plasma turbulence is presented with discussion of unsolved transport issues in fusion plasmas, such as transport barrier formation and nonlocal transport.

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At present it is well understood that the key element in the transition physics is the origin of the strong radial electric field and suppression of the turbulence fluctuation level by a strong poloidal rotation in the E × B fields. As a result, the transport coefficients are strongly reduced at fixed places and transport barriers with steep density and temperature gradients are formed near the separatrix or the last closed flux surface (ETB) or in the core region (ITB). The key element in the transition physics is the origin of the strong radial electric field. The impact of the momentum transport is brought to light.

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This paper reports on the integrated performance achieved in JT-60U toward the steady-state operation foreseen in the ITER and DEMO reactors. Advanced tokamak plasmas with weak shear or reversed shear have been optimized to confront critical issues such as high-beta operation with high confinement, the compatibility of high-density operation with high confinement, and long sustainment with a high non-inductive current drive fraction. As a result, high-integrated performance was achieved in both plasma regimes. For example, high-confinement reversed shear plasmas with a high bootstrap current fraction exceeding the no-wall beta limit have been obtained in the reactor relevant q₉₅∼5.3; high values of β_N∼2.7, HH_98y2∼1.7, n_e/n_GW∼0.87, and f_BS∼0.9 are simultaneously achieved with a reversed q profile with q_min∼2.3.

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There are various physics issues which the LHD can explore in the research field of transport studies in high temperature plasmas, research for MHD stability in high density and high beta plasma, that are crucial for a future device aimed at nuclear fusion. Physics of the confinement improved mode is discussed with a new paradigm of non-linearity, non-diffusivity and non-locality of the transport. Various MHD modes are observed such as the interchange mode and high energy particle driven modes. Magnetic island physics are intensively studied by applying a perturbation field or by controlling the magnetic shear with NBCD. The LHD also gives opportunities to extend research beyond plasma physics. Research concerning atomic processes such as a test of the collisional-radiative model and energy levels of highly charged heavy ions are also investigated using the plasma in LHD.

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We discuss generation processes of two distinct structures, ordered arrays of high density clumps and single-peaked macroscopic structures as observed in relaxation processes starting from non-equilibrium distributions of strongly magnetized pure electron plasmas. The two-dimensional character of the guiding-center system shows equivalence between the distributions of electron density and the vorticity in E×B flow. Observations reveal decisive role of fluctuations in the ambient electrons in assisting the formation of ordered arrays and their destruction that successively leads to next stage of ordered structures. The fluctuating parts extracted via wavelet analyses show characteristic features of spectrum and k-space dynamics in ideal 2D turbulence. Such dynamics interpreted as ideal 2D fluid actually reflect electromagnetic dynamism of charged particle subject to redistribution of potential energy and angular momentum. One extra feature of 2D vortex dynamics of electrons is the contribution of Landau-damping of collective particles' motion which conveys externally applied electromagnetic field to the bulk electrons via resonant electrons as a source of energy and momentum.

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We experimentally investigate chemical erosion of polycrystalline graphite targets coated with boron-doped diamond (BDD) using an induction plasma containing low-energy, high-atomic-hydrogen flux. Chemical erosion is drastically suppressed by diamond coating the graphite target. The chemical sputtering yield for the BDD layer is about two orders of magnitude lower than that for the graphite target. After exposure in low-temperature hydrogen plasmas, however, the surface morphology of the BDD target is significantly modified. The polycrystalline diamond is eroded near the grain boundary, and many pits with diamond-like shapes are observed on the crystal surface. X-ray photoelectron spectroscopy and Raman spectroscopy reveal that the hydrogen atoms penetrate into the BDD target to a depth of at least ∼20 nm.

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Chemical erosion of carbon materials and dust formation in low-temperature and neutral particle-dominated plasmas were investigated using high-pressure inductively coupled plasmas. Experiments were performed with Ar/H₂/N₂ mixture plasma irradiation to graphite targets. The addition of just a few percent of nitrogen gas to hydrogen led to significant suppression of carbon dust formation on the graphite target. From optical emission spectroscopy, CN band spectra were observed strongly in Ar/H₂/N₂ plasmas with a decrease of CH and C₂ band emission intensity. These results showed that CN bond formation, which caused chemical erosion of carbon by producing volatile CN, HCN, and C₂N₂ particles, might have been a key suppression mechanism of the carbon particle aggregation.

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The Monte Carlo Particle and Heavy Ion Transport code (PHITS) was used for diagnosing the focusing and defocusing action of a triplet of permanent magnet quadrupoles on laser-accelerated proton bunches of short duration. The code predicts reasonably well the observed transverse beam profile as measured with a CR-39 track detector demonstrating that PHITS can be used to simulate proton bunch transport by conventional ion optics in the laser-driven case.

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We present a study of ions produced by the perpendicular neutral beam in the high-beta plasma of Large Helical Device (LHD) that uses the Monte-Carlo code based on orbit following in real coordinates with Coulomb collisions. For no neutrals outside the plasma, the results indicate that there are remarkable differences in the distribution functions that depend on whether the loss boundary is set on the vacuum vessel wall or on the last closed flux surface (LCFS). Also, for finite neutral density n_H = 10¹⁸ m⁻³, we find that the minor radial profile of the distribution function slightly differs from that for the infinite neutral density case, which corresponds to the loss boundary being set at the LCFS.

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The spatial distribution of the radiofrequency (RF) magnetic field associated with a high harmonic fast wave (HHFW) was measured using an array of magnetic probes in the plasma inside the University of Tokyo Spherical Tokamak (UTST). Data obtained from 25 probes (19 locations for toroidal polarization and 6 locations for vertical polarization) distributed along the poloidal cross section were analyzed. The RF magnetic field is polarized in the toroidal direction, indicating that the HHFW is excited in the plasma. The RF field is weak on the inboard side. Analysis of the group delay suggests that the waves travel long distances in the plasma, probably because of poor absorption. No indication of parametric decay was observed up to 80 kW of injected RF power.

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A hierarchical entropy balance equation retaining the dynamics in the radial direction is introduced to study non-local turbulent transport and the associated global profile relaxation. It consists of first- and second-order equations that describe the entropy dynamics related to thermodynamics/fluid quantity and the corresponding micro-scale phase space fluctuations, respectively. Specifically, the second-order equation describes not only a local entropy production related to heat and density flux (i.e., zonal flow), but also the spatial convection of perturbed entropy. We investigated the entropy dynamics in ion-temperature-gradient driven turbulence based on a global gyrokinetic Vlasov simulation in slab geometry. Entropy convection plays an important role in the relaxation dynamics dominated by the avalanche process. A self-organized relaxed state is established, in which short-wavelength temperature corrugation, i.e., zonal pressure, is regulated by zonal flow shear.

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An increase in turbulent Reynolds stress preceding an L-H transition in the Compact Helical System (CHS) was observed. A positive increase in the Reynolds stress is associated with a negative jump in the floating potential. The relationship of signs is consistent with the momentum balance equation. Therefore, this observation supports the hypothesis that the Reynolds stress plays an important role in triggering the L-H transition in CHS plasmas.

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The amplitudes of electron temperature fluctuations are evaluated in the edge plasma of TST-2 using a new technique. Langmuir probe current-voltage characteristic curves are conditionally reconstituted in terms of the magnitude of the floating potential. High/low electron temperatures are obtained in low/high floating-potential phases. The relationship between the electron temperature fluctuations and the time-averaged electron temperature gradient is discussed.

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A new 50 GHz microwave interferometer was installed, and non-inductively sustained spherical tokamak plasmas in the TST-2 device were measured. The line integrated electron densities (n_els) on five chords were compared with visible CCD camera image and equilibrium analysis. It is concluded that the high-density region has a C-like shape along the outboard boundary. This shape suggests that the electron density is not constant on the magnetic flux surface.

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A methodology describing the particle motion in a spatially localized high-power laser field in the relativistic regime is presented based on the noncanonical Lie perturbation method, which is comprehensive compared with those based on the conventional averaging method to the equation of motion. We successfully derived the relativistic ponderomotive force in a laser field with a radial intensity gradient and the corresponding particle dynamics up to the first order with respect to the expansion parameter ε ∼ λ/L (= laser wavelength/scale length of the transverse laser amplitude). The longitudinal motion was found to be secular and characterized by a growing oscillation exhibiting a cubic dependence on the laser amplitude.

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Non-neutral electron plasmas were confined in an axisymmetric magnetic mirror field of the mirror ratio R ∼ 3. Axial plasma oscillations (Trivelpiece-Gould modes), diocotron oscillations with the azimuthal mode number m = 2, and solitary waves were investigated experimentally. It was observed that the velocities of an electron soliton and an electron hole decreased at high field side in a magnetic mirror confinement.

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An optics system for microwave imaging reflectometry (MIR) in the Large Helical Device (LHD) was newly developed to optimize the performance of the two-dimensional microwave receiver array. Reflected microwaves from the plasma and the first local oscillator (LO) wave are transmitted to the receiver array via the optics from the front. Finite-difference time-domain (FDTD) calculation was used to design the ellipsoidal or hyperboloidal shapes of the quasi-optical mirrors. It is confirmed that the LO beam in the constructed system covers the receiver antenna aperture area as intended. The S/N ratios of the signals are improved with this optimized optics system from those in the previous system.

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The interplay mechanism between electromagnetic (EM) and electrostatic (ES) turbulence is explored using direct gyrofluid simulations of mixed resistive magnetohydrodynamic (MHD) and ion-scale microinstability. With an elucidation based on a minimal model, we propose a new concept of cross-scale dynamo action induced by microturbulence as the mechanism responsible for the interaction between multiscale ES and EM turbulence. The dynamo appears as a novel magnetic island seesaw oscillation pivoting around the singular surface. This island seesaw mechanism may be applicable to relaxation of mode locking in tokamaks to mitigate major disruptions, suggesting a promising nonlinear approach to plasma control.

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The instabilities observed in many magnetic confinement devices induce radial particle transport. Density and potential fluctuations were measured using a gold neutral beam probe in GAMMA 10. The plasma stored energy repeatedly increases and decreases during ion- cyclotron range of frequency heating. A drift-type fluctuation was observed in both the density and the potential as the plasma stored energy decreased. These results indicated a correlation between the radial particle transport induced by drift-type fluctuations and decreasing plasma stored energy.

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Zooplankton contained in water have been successfully inactivated by irradiation with a pulsed intense relativistic electron beam (PIREB). A treatment chamber is filled with a solution of 3-wt% salt in water containing Artemia larvae as zooplankton samples and is irradiated using the PIREB (2 MeV, 0.4 kA, 140 ns). We found that up to 24% of the Artemia are inactivated by firing 10 shots of PIREB irradiation.

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The trigger for the nonlinear destabilization of the double tearing mode (DTM), referred to as a structure-driven instability leading to explosive growth and subsequent collapse, is investigated. We use the reduced MHD equations that solve the evolution of perturbations from an equilibrium deformed by two-dimensional magnetic islands during the slow evolution of the quasi-steady nonlinear regime. By examining conditions near marginal stability (under which the explosive growth is not triggered), we have identified a new secondary instability that starts growing when the magnetic energy of the primary fluctuations associated with the islands reaches a critical level. The energy source of this instability is different from that of the linear DTM; it originates in the spatial deformation due to the DTM-driven magnetic islands and is responsible for the subsequent nonlinear destabilization. The growth rate of this secondary instability is found to be proportional to the magnetic energy, suggesting that it exhibits modulational characteristics.

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Deepening of floating potential has been observed on the tungsten target plate immersed in high-density helium plasma with hot electron component on the way to nanostructure formation. The physical mechanism is thought to be a reduction of secondary electron emission from such a complex nano fiber-form structure on the tungsten surface.

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Magnetic islands excited by resonant magnetic perturbations (RMPs) in helical plasmas are investigated. A Rutherford-type equation is coupled with the time evolution equation of the radial electric field associated with the neoclassical particle diffusion due to helically rippled magnetic fields. Using the model, bifurcation between the excitation and the annihilation of non-rotating magnetic islands are newly observed, depending on the magnitude of RMPs and the anomalous plasma viscosity. It is found that the transition between these states is triggered by the change in the radial electric field profile in the vicinity of magnetic island.

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Structure formation during the plasma ignition was experimentally investigated under microwave irradiation of magnetite (Fe₃O₄) and graphite composite powder. It was found from microscopic images that there are stepwise structure formations: (i) a luminous layer, (ii) hotspots, (iii) flare like emissions, and (iv) a luminous body above the powder. The 1D-spatially resolved emission spectra show that the light of the luminous layer of (i) has CO bands, and the luminous body of (iv) has iron atomic spectra superimposed on the continuum spectrum. The structure formation indicates a process of plasma-material mixing through chemical reduction of the magnetite powder.

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A real-time control system using a digital signal processor (DSP) was developed to control ion-beam trajectories of a heavy ion beam probe (HIBP) on the Large Helical Device (LHD). The electrostatic potential during temporal evolution of the magnetic field structure by a large plasma current was successfully measured with the help of this system for the first time. It has been demonstrated that the probe beam can be detected even in a plasma having a ∼120 kA plasma current at the toroidal field of 1.3 T using the control system, while the beam is substantially lost by only ∼40 kA current before arriving at the detector plate in the case without control. However, this system is unstable during operations when the probe beam is swept in time across the plasma cross section to obtain the time evolution of the potential profile. It is caused by a nonlinear character of the system due to the finite beam size.

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A dispersion interferometer is immune to mechanical vibrations, which is a great advantage for application to steady-state fusion reactors. This paper describes the performance of a phase-modulated dispersion interferometer with a new phase extraction method using a modulation amplitude ratio.

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By explicitly excluding the Pfirsch-Schlüter diffusion and Spitzer terms from the perturbed distribution functions in δf drift-kinetic Monte Carlo simulations, off-diagonal neoclassical transport coefficients for quasi-symmetric toroidal plasmas can be calculated properly, ensuring the constancy of the geometric factor in the exact axisymmetric limit.

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Fundamental chemical reactions between an atmospheric nitrogen plasma jet and water or a water/ethanol solution have been investigated. When the nitrogen plasma jet was injected directly into pure water, active nitrogen species produced nitric acid and ammonia through fatal reactions with H₂O. The interaction between nitrogen plasma and a liquid mixture of pure water and ethanol produced NH₄⁺ ions effectively. These experimental results clearly show that the active nitrogen species in the pulsed-discharge atmospheric plasma can interact strongly with materials in solution to extract oxygen or hydrogen from them and add atomic nitrogen to them.

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Multi-pass Thomson scattering is an attractive method for measuring electron temperature and density in low density plasmas. The feasibility of multi-pass laser optics consisting of two confocal spherical mirrors has been studied, and analytic expressions for the path have been obtained. These can be used to optimize the design under the given conditions. The effects of aberration, as well as positioning and alignment errors, were found to be critical for obtaining large transit numbers. Beam expansion resulting from aberration, surface roughness of the mirrors and diffraction is considered. Under practical conditions, transit numbers of about 37 (i.e., 19 round trips) are possible.

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We have included the effect of radiation trapping in a collisional-radiative model of neutral helium atoms developed by Goto [M. Goto, JQSRT 76, 331 (2003)], which is used to determine the electron temperature and density in plasmas from visible emission line intensities of atoms. In addition to the electron temperature and density, photo-excitation events from the ground state 1¹S to the 2¹P, 3¹P, and 4¹P states per second per one atom are treated as fitting parameters to reproduce the population density obtained by spectroscopic measurement. The model has been applied to an RF plasma at Shinshu University, Japan. The electron temperature and density and the contribution of radiation trapping to the population density of excited states are evaluated.

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Effects of a non-monotonically evolving external perturbation on a plasma, that is stable against tearing modes, are numerically investigated. It is found that for a magnetic island driven by an external single-cycle magnetic perturbation, the time constants during the phases of growth and decay are different. This difference in time constants causes a finite magnetic island to form even after the external perturbation is removed. Therefore, the saturation width of a magnetic island driven by a successive applications of an external single-cycle perturbation becomes larger than the maximum magnetic island width driven by a single application of that. For a rotating plasma, the background rotation is damped as the magnetic island grows due to an external perturbation [R. Fitzpatrick, Phys. Plasmas 5, 3325(1998)]. Therefore, for a rotating plasma, the driven magnetic island can enter an explosive growth stage due to successive applications of a single-cycle perturbation even with amplitude smaller than the critical value for the onset of the rapid growth in the case of a monotonically increasing or step-function type external perturbation. These features are important in explaining the explosive growth of magnetic islands and the onset of neoclassical tearing mode due to non-monotonically growing MHD phenomena such as sawtooth, fishbones and ELM.

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Surface waves were studied in cold cylindrical plasmas with axially non-uniform density profiles, and the eigenfrequencies and eigenfunctions for the transverse-magnetic modes of pure and hybrid surface waves were obtained numerically for collisional plasmas. The analysis of the wave equation takes into account the singularity caused by plasma resonance at which the wave frequency is equal to the local electron plasma frequency. It is shown that the axial eigenfunction of the pure surface mode peaks at the position of the plasma resonance layer, whereas the axial eigenfunction of the hybrid surface mode has two peaks at the plasma resonance layer and at the interface of the plasma and a quartz plate. Transverse-electric surface modes in axially non-uniform plasmas without plasma resonance are also analyzed.

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In this paper, the Weibel instability, driven by the plasma temperature anisotropy, in the corona of high intense laser fusion plasma is studied. The unperturbed electronic distribution function, f, of the anisotropic corona is supposed to be a bi-Maxwellian. That T_∥ = T_⊥ ± W_O, where W_O = ¼ m_ev_O² is the averaged electron quiver energy in the laser electric field. The first and the second anisotropies of f projected on the Legendre polynomials are calculated as a function of the scaling parameter, W_O / T_⊥. The Weibel instability parameters are explicitly calculated as a function of the scaling parameter. For typical parameters of the laser pulse and the fusion plasma, it has been shown that very unstable Weibel modes, γ ≳ 10¹¹ s⁻¹, can be excited in the corona.

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We study the flute-mode stability of quadrupole-anchored tandem mirror plasmas. The present analysis is based on Newcomb's Lagrangian density with an assumption of small Larmor radius of ions for paraxial approximation. A radial eigenmode equation for flute perturbations is derived without an eikonal approximation in flux coordinates, where effects of E × B plasma rotation due to an ambipolar electric field are considered. The obtained eigenmode equation is applicable to a mode with an arbitrary azimuthal mode number and can describe interchange, E × B rotational, and Kelvin-Helmholtz modes driven by the shear effect of E × B plasma rotation. A quadratic dispersion equation in perturbation frequency ω is derived based on a simplified cylindrical plasma model and is used to discuss the flute stability of quadrupole-anchored GAMMA 10 tandem mirror plasmas, where the anchor beta required for plasma stability is calculated for the m = 1 and 2 modes for different values of central-cell ambipolar potential.

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Oxy-combustion generates a high temperature field (above 3000 K), which is applied to next generation power plants and high temperature industrial technologies because of N₂ free processes. However, the combustion temperature is so high that the furnace wall may be fatally damaged. In addition, it is very difficult to control the heat flux and chemical species' concentrations because of rapid chemical reactions. We have developed a new method for controlling the flame by electromagnetic force on this field. In this paper, we experimentally investigated the power coupling between the premixed oxy-combustion with methane and radio frequency (RF) power through the induction coil. By optimizing the power coupling, we observed that the flame can absorb RF power up to 1.5 kW. Spectroscopic measurements also showed an increase in the emission intensity from OH radicals in the flame, indicating improved combustibility.

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A unified transport model is proposed to study the physical mechanism for the formation of electron internal transport barriers (e-ITB) in helical plasmas and internal diffusion barriers (IDB) observed in the Large Helical Device (LHD). An e-ITB can be predicted with the effect of zonal flows (ZFs) in the low collisional regime when the radial variation in particle turbulent diffusivity is included. The transport analysis in this article shows that particle fueling induces IDB formation when the unified transport model is used in the high collisional regime. After particle fueling, a steep density gradient forms. To examine the density limit for the IDB in helical toroidal plasmas, the effect of radiation loss is included in a set of transport equations.

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The neutral beam injection facility in the National Institute of Advanced Industrial Science and Technology was used to irradiate a polycrystalline tungsten specimen with high energy and high flux helium and hydrogen particles. The incidence energy and flux of the beam shot were 25 keV and 8.8 × 10²² particles/m² s, respectively. The duration of each shot was approximately 30 ms, with 6 min intervals between each shot. Surface temperatures over 1800 K were attained. In the two cases of helium irradiation, total fluence of either 1.5 × 10²² He/m² or 4.0 × 10²² He/m² was selected. In the former case, large sized blisters with diameter of 500 nm were densely observed. While, the latter case, the blisters were disappeared and fine nanobranch structures appeared instead. Cross-sectional observations using a transmission electron microscope (TEM) with the focused ion beam (FIB) technique were performed. According to the TEM image, after irradiation with a beam shot of total fluence 4.0 × 10²² He/m² , there were very dense fine helium bubbles in the tungsten of sizes 1-50 nm. As the helium bubbles grew the density of the tungsten matrix drastically decreased as a result of void swelling. These effects were not seen in hydrogen irradiation case.

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Experimental investigations of the evolution of the dynamics of drift wave turbulence in a linear high-density helicon plasma device are presented. The turbulent density fluctuations in the plasma edge are characterized by large intermittent events caused by radially propagating turbulent structures, which are formed in the radial plasma density region due to increased cross-field transport by a quasi-coherent drift wave mode. Similar to coherent drift wave modes the turbulent structures are correlated with fluctuating parallel currents. The role of fluctuating currents parallel to the ambient magnetic field in the evolution of coherent drift wave modes and drift turbulence is highlighted by investigations of the interaction of drift wave fluctuations with externally driven currents. Frequency pulling of coherent drift wave modes over a frequency range of up to 30% of the natural drift wave frequency is demonstrated. Furthermore, the drive of mode-selective current patterns allow for complete synchronization of drift wave turbulence and consequently leads to a strong reduction of the associated fluctuation-induced transport.

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A lithium beam probe (LiBP) has been developed for the measurement of electron density profiles with highly spatial and temporal resolutions in JT-60U. Using an electron beam heating ion source with a capability of 10 mA extraction, a 5.5 mA beam has been injected to the plasmas. It corresponds to the equivalent neutral beam current of 2 mA. A spectrum width of the beam emission has been small enough to separate Zeeman splitting. By use of the LiBP, time evolutions of pedestal density profiles during type I and grassy edge localized modes (ELMs) have been obtained for the first time. After a type I ELM crash, the drop of the line-integrated density measured by an interferometer delays by 2 ms later than that of the pedestal density. Comparing the line-integrated density to the line integration of the edge density profile measured by the LiBP, it is found that the recovery from the type I ELM crash is correlated with the reduction of core plasma density. As for grassy ELMs, grassy ELMs have smaller density crashes than that of type I ELMs, which is mainly derived from the narrower ELM affected area.

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A new gyrokinetic Vlasov simulation code, GKV-X, is developed for investigating the turbulent transport in magnetic confinement devices with non-axisymmetric configurations. Effects of the magnetic surface shapes in three-dimensional equilibrium obtained from the VMEC code are accurately incorporated. Linear simulations of ion temperature gradient (ITG) instabilities and zonal flows in the Large Helical Device (LHD) [O. Motojima, N. Oyabu, A. Komori et al., Nucl. Fusion 43, 1674 (2003)] configuration are carried out by the GKV-X code as benchmark tests against the GKV code [T.-H. Watanabe and H. Sugama, Nucl. Fusion 46, 24 (2006)]. For high poloidal wavenumbers, the frequency, growth rate, and mode structure of the ITG instability are influenced by the VMEC geometrical data such as the metric tensor components of the Boozer coordinates, while the difference between the zonal flow responses obtained by the GKV and GKV-X codes is found to be small in the core LHD region.

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The Intelligent Shell scheme, where a grid of active coils counteracts in a feedback scheme the measurements provided by an identical grid of sensors, has shown some limitations in the control of the dynamo tearing modes in RFX-mod. The origin of the problem is the aliasing on the measurements coming from the high periodicity sideband harmonics produced by the discrete nature of the active coils. A more efficient feedback on tearing modes is obtained by removing the sidebands from the measurements, thereby counteracting the true tearing Fourier modes. In this scheme, named Clean-Mode-Control, the sidebands are computed in real time from the coils currents using the cylindrical geometry approximation. The Clean-Mode-Control significantly alleviates the wall-locking of tearing modes in RFX-mod, giving the possibility of operating at a plasma current (1.5 MA) never reached before in a RFP machine. These features are well explained by a MHD model describing the tearing mode dynamic under the viscous torque due to the fluid motion and the electromagnetic torques produced by the feedback, the conductive structures surrounding the plasma and the non-linear interaction between the different modes [P. Zanca, Plasma Phys. Control. Fusion 51, No. 1, 015006 (2009)]. Here some new results obtained with this model are discussed. In particular we will show that the edge radial field control improves by reducing the ratio between the delay introduced by the digital acquisition of the measurements and the time constant of the shell that contains the plasma. In this formulation the active coils are assumed to be located outside the shell.

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An open system one-dimensional electrostatic particle code that adopts the new constant current generator model has been developed. Using this new model, we have been able to study various phenomena at large electron drift velocities, where the original model is not applicable. In this new model, the contribution of the ion flux, which is not considered in the original model, is added into the electric current. By examining the code, we find that the result in a case where the electron drift velocity is smaller than the electron thermal velocity is similar to that of a previous study, which showed double layer creation as a result of ion-acoustic instability. We also present the results of simulations of an electron drift velocity comparable to or larger than the electron thermal velocity.

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Instabilities in low beta l = 2 heliotron plasmas with peaked toroidal current density profiles are investigated using resistive reduced magnetohydrodynamic equations. Such heliotron plasmas can have a nonmonotonic rotational transform ῑ profile with two ῑ = 2/3 rational surfaces. When the distance between the resonant surfaces is large, resistive instabilities can be found. Current-driven ideal modes with larger growth rates appear when the minimum of the rotational transform becomes just above the rational number 2/3 and there is no resonant surface. The existence of this non-resonant mode is explained by the expression for the current-driven term of the plasma potential energy.

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In fast wave current drive, fast waves accelerate resonant electrons in the direction parallel to the static magnetic field, causing parallel velocity to increase. The trajectory of a trapped resonant electron is calculated by computer code in which fast wave induced diffusion in velocity space is accounted for by a quasi-linear operator. Simulations show that the orbit of a trapped resonant electron transits from trapped to passing in some cases, reducing the effect of trapped electrons on current drive and improving current-drive efficiency. We determined the conditions under which this type of transition occurs.

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Fluid models are not generally applicable to fusion edge plasmas without external provision of kinetic factors: closure parameters and boundary conditions inside the sheath region. We explain the PARASOL-1D simulation, a particle-in-cell code with a binary collision Monte-Carlo model, and use it to determine four kinetic factors commonly needed in fluid codes. These are the electron and ion heat flux limiting factors, α_e and α_i, the ion adiabatic index, γ_A, and the electron and ion temperature anisotropy, T_∥/T_⊥. We survey these factors over a wide range of collisionalities and find that, as predicted, the conductive heat flux is accurately described by the Spitzer-Härm expression in the collisional limit and asymptotes to a constant value in the collisionless limit. However, unique behavior occurs in the weakly collisional regime when the ratio of the mean free path to connection length is 0.1 < λ_mfp/L_∥< 10, when the SOL is between the conduction- and sheath-limited regimes. We find that α_e can peak, becoming larger than the collisionless limit, γ_A is less than unity, and only the ions are anisotropic. The effects of electron energy radiation and Langevin heating are explored. Finally, the strong deviations of the energy distribution function from Maxwellian in the weakly collisional and collisionless regimes are explained.

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A new Monte Carlo code based on particle tracing using real coordinates has been developed to properly treat the re-entering particles that repeatedly pass in and out of the last closed flux surface (LCFS). The particle loss due to the charge-exchange reaction has also been taken into account in this code. We apply this new code to the analysis of high-energy particles produced by tangential neutral beams (NBs) of the large helical device (LHD). It is confirmed that reasonable solutions of distribution functions are obtained for particles produced by the tangential-NBs. It is also confirmed that the effect of the particle orbit and the charge-exchange loss on the distribution function is properly included. The shapes of the distribution functions of particles, produced by the tangential-NBs in two temperature cases (1 keV and 0.1 keV), are the same. It is found that the re-entering particles play an important role in the analyses of the distribution function of particles produced by the NBs.

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The behavior of hydrogen neutral particles in and around the ergodic layer of Large Helical Device plasmas has been investigated through Hα emission spectral line profiles using plasma polarization spectroscopy (PPS). The PPS technique enables us to quantitatively evaluate emission locations, atomic temperatures and velocity components along the line-of-sight (LOS) for both inner and outer peripheral regions. The emission locations and the LOS components of atomic velocities are determined by varying the magnetic field axis R_ax from 3.60 m to 4.00 m, shot by shot. The high intensity region of Hα emissions is localized in the inner ergodic layer for the inward configuration. With an increase in R_ax, the high intensity region of Hα emission moves outward.

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Data mining techniques, which automatically extract useful knowledge from large datasets, are applied to multichannel magnetic probe signals of several helical plasmas in order to identify and classify MHD instabilities in helical plasmas. This method is useful to find new MHD instabilities as well as previously identified ones. Moreover, registering the results obtained from data mining in a database allows us to investigate the characteristics of MHD instabilities with parameter studies. We introduce the data mining technique consisted of pre-processing, clustering and visualizations using results from helical plasmas in H-1 and Heliotron J. We were successfully able to classify the MHD instabilities using the criterion of phase differences of each magnetic probe and identify them as energetic-ion-driven MHD instabilities using parameter study in Heliotron J plasmas.

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In the imaging bolometer a thin metal foil converts plasma radiated power to infrared radiation measured by an infrared camera. Calibration of the foil provides information on its sensitivity, which is helpful in selecting the best foil material. In this study thermal properties of submicron Au and Pt foils are investigated by heating the foils with a chopped HeNe laser beam (∼20 mW) and observing the temperature change, ΔT, and thermal time constant, τ, of the foil temperature. Assuming that the foil cooling is dominated by diffusion, we can compare the relative sensitivities of the foils by comparing the ratio of the thermal diffusivity to the thermal conductivity of the foil, κ/k, to the ratio ΔT/τ. The results indicate that Pt is more than 9 times more sensitive than Au even though standard thermal properties indicate that Au should be slightly (14%) more sensitive than Pt. This inconsistency indicates that the IR radiation is dominant over diffusion in the foil cooling. In that case the sensitivity should be evaluated by 1/k ∼ ΔT, which indicates that Pt is 8 times more sensitive than Au, while the ratio of thermal conductivities indicates that it should be only 4 times more sensitive.

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This paper describes the dependence of the rotation frequency of an m/n = 2/1 neoclassical tearing mode (NTM) on its magnetic island width in JT-60U (m and n are the poloidal and toroidal mode numbers, respectively). Throughout the experiments, the island width is actively controlled by changing the location of the electron cyclotron current drive (ECCD), based on the fact that aligned ECCD and misaligned ECCD make the NTM islands shrink and grow, respectively. The NTM frequency was found to gradually decrease with the increasing island width. While the frequency is 5.5 kHz for zero island width, it is 3 kHz for the full island width of about 15% of the plasma minor radius. Also, the mode frequency suddenly decreases (typically by 80%) shortly after the misaligned ECCD, during which the increment in the island width is only less than 10%. A sudden increase in the mode frequency is observed shortly after the ECCD turnoff, which is nearly the reverse process of the sudden decrease, but with a hysteresis in the island width at which downward and upward frequency changes occur. The downward frequency change occurs when the frequency decreases to about half of that without the NTM.

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