A simple model of radio frequency wave (RF) assisted breakdown based on Townsend avalanche theory is proposed for the purpose of evaluating the effect of RF on Townsend avalanche breakdown. According to this model, the required minimum electric field for RF-assisted breakdown can be decreased down to half of that for breakdown by the induction electric field alone. The electric field of RF reaches a minimum when the frequency of the RF is equal to the electron cyclotron frequency.

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A new explanation regarding the toroidal spin-up of a field-reversed configuration (FRC) is provided. A physical picture showing that the poloidal flux can convert directly to kinetic angular momentum is described. Through the use of an ion orbit calculation in resistively decaying FRC plasma, toroidal rotation at both the separatrix and the field-null is found to occur.

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The alleviation effect of transient heat load on helium holes/bubbles has been demonstrated using ruby laser pulses for the transient heat source in the divertor simulator NAGDIS-II. It is shown that the holes and bubbles disappeared following the irradiation of the ruby laser pulses, while the surface roughness was significantly enhanced by the irradiation of the Nd:YAG laser pulses. Based on the numerically calculated temperature in the specimen, the physical mechanism causing this difference is discussed particularly in terms of pulse width (0.6 ms for ruby laser without a Q-switch and 5 ns for Nd:YAG laser with a Q-switch).

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Two sets of material probes were installed in LHD in the vicinity of the Local Island Divertor (LID) head made of carbon fiber composite. One set of the material probes faced the LID head, while the other set was placed in a shallow line of sight to the head. Carbon films on the former and the later sets of probes are expected to be formed by the deposition of physically sputtered carbon atoms and chemically sputtered hydrocarbon, respectively, during the LID discharges. Surface morphology and thickness of the films, and the amount of retained hydrogen in the films were investigated, and hydrogen concentration in the probes was estimated. The hydrogen concentration in the carbon films on the probes placed in a shallow line of sight of the head was significantly large with the atomic ratio of H/C = 0.6-1.1.

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We propose a forward-looking ground penetrating radar using laser produced-plasma channels. The plasma channels work as a microwave guide to and from the buried objects. In order to confirm this method's feasibility, we investigated the propagation properties of plasma channels.

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We studied the influence of electrostatic and magnetic fields at the rear surface of solid targets on the hot electron emission generated by ultra-intense laser pulses. The number of emitted electrons increases in experiments when a pre-plasma is created on the target rear surface. The formation of the electrostatic potential is clearly retarded in particle-in-cell simulations resulting in the elongation of time window for hot electron emission. The increase in the number of electrons is consistent with the Alfvén current within the elongated time window.

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The encapsulation of C₆₀ molecules inside double-walled carbon nanotubes (DWNTs) is realized by means of a plasma ion-irradiation method. Transmission electron microscopy observations confirm that DWNTs have been filled with C₆₀ fullerenes having amorphous morphology. More importantly, electrical transport measurements indicate that C₆₀-encapsulated metallic DWNTs exhibit a novel negative differential resistance behavior.

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A new and simple control method of the unstable operating point in the Force Free Helical Reactor (FFHR) is proposed for the ignited operation with high-density plasma. Proportional-integration-derivative (PID) control of the fueling has been used to obtain the desired fusion power with the fusion power error of e(P_f) = (P_fo − P_f) in the stable operating point. We have discovered that in the unstable regime the error of the fusion power with an opposite sign of e(P_f) = −(P_fo − P_f) can stabilize the unstable operating point. Around the unstable operating point, excess fusion power supplies fueling and then increases the density and decreases the temperature. Less fusion power in the sub-ignited regime reduces the fueling, decreases the density, and increases the temperature. The operating point approaches the final unstable operating point as oscillation is damped away.

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In order to detect rf modulation of visible light emitted from plasma, a high-speed photodiode measurement system was developed. The system is located outside the vacuum vessel of the TST-2 spherical tokamak and measures visible light emissions through a quartz window. A dedicated amplifier for the photodiode was made. Care was taken to reduce the rf pickup noise. The frequency spectrum of the light signal detected during high harmonic fast wave (HHFW) heating showed modulation by HHFW. This is the first measurement of visible light modulation induced by HHFW, and shows promise for measurement of the rf electric field in the plasma core.

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Multi-mode oscillation was observed in a 300 GHz fully CW gyrotron. It has been developed and installed in the Research Center for Development of Far-Infrared Region, University of Fukui as a power source of a submillimeter-wave material processing system. This gyrotron delivers 1.75 kW/CW at maximum. The radiation pattern is a Gaussian beam when the magnetic field strength B_c at the cavity is properly adjusted. However, within a range of B_c values, simultaneous oscillation of competing modes is observed, manifesting in radiation of the output power in multiple directions.

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X-ray spectra from highly charged tungsten ions have been observed in the interaction with an electron beam at energies ranging from 14 to 1.5 keV by using an electron beam ion trap (EBIT). In this energy range, resonant x-ray transitions at around 9 keV predominantly take place through dielectronic recombination processes. The measured spectra were compared with the theoretical calculation with the Hebrew University Lawrence Livermore Atomic Code (HULLAC).

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In the course of an experimental study on collisional processes of highly charged ions (HCIs), we developed a procedure to produce and extract HCIs with a very high charge-state for various kinds of heavy elements at the Tokyo EBIT (Electron Beam Ion Trap). The charge-state spectra of the extracted HCIs for some elements were measured using dierent EBIT operation conditions.

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The population density of metastable helium atoms is experimentally investigated in a hollow cathode discharge. The metastable population density measured using laser absorption spectroscopy is proportional to the square root of electron density under conditions of constant electron temperature and a constant neutral density. Calculation based on a collisional-radiative model and diffusion loss indicates that metastable population density is linearly proportional to electron density.

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The electron cyclotron current drive (ECCD) is investigated in Heliotron J for two new magnetic configurations with ECRH heating located at the ripple top and the ripple bottom, respectively, using different incoming wave polarizations. The results are compared with the previous ECCD experiments, performed in the low, medium (standard) and high bumpiness configurations, where a sign reversal of the EC current is observed between the ripple top and the ripple bottom heating conditions. It is shown here that a similar behaviour is obtained in the new configurations. Positive currents are obtained when the ECR heating is located at the magnetic field minimum, whereas negative currents, but not higher in magnitude than the ones obtained in the previous experiments, appear with ripple top heating. The dependence of the ECCD on the polarization of the injected waves agrees with the expected behaviour.

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In a high-density plasma produced by helicon waves in a linear device, a change from the coherent to broad-band density fluctuations was observed on increasing the magnetic field. The coherent mode exhibited the drift wave features, and a steady oscillation state was observed. When the magnetic field was increased, the fluctuation level increased, and the higher azimuthal Fourier components were excited. When the spectrum became broad at the higher magnetic field, the behavior of the azimuthal structure became complex and changed rapidly with time. The analysis performed using the movable probe array (48ch.-probe) decomposed the short-lived structure into poloidal mode-frequency space and indicated mode-mode coupling in the broad-band spectrum.

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The energetic electrons and ions generated by the interaction of an intense, ultrashort laser pulse with a finite scale-length dense plasma were investigated for various laser wavelengths using particle-in-cell simulation. The hot-electron temperature for the density scale-length L = 2.5 μm is not governed by the Iλ²-scaling laws, where I is the laser intensity and λ is the laser wavelength. The maximum energy of the energetic ions is not only proportional to the hot-electron temperature but depends on the electron density.

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A large diameter diaphragmless shock tube has been recently developed and designed to perform detailed studies of CO₂-N₂ gas-dynamic laser (GDL). This large diameter diaphragmless shock tube offers various advantages over the conventional shock tubes (diaphragm-type) as longer test times, higher degree of reproducibility of shock-tube data, and especially low-impurity operation condition. The latter advantage is experimentally demonstrated herein, which is very critical issue in the CO₂-N₂ GDL studies. A supersonic nozzle section was mounted at the end wall of the shock tube and instrumented for simultaneous measurement of laser output power and energy. The GDL action in a CO₂-N₂ mixture under low impurity condition has been obtained by using the large diameter diaphragmless shock tube for the first time.

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Competition behavior between the drift instability and the flute instability has been investigated experimentally in a bounded linear electron cyclotron resonance (ECR) plasma. Time evolutions of the fluctuation in ion saturation currents clearly show the successive appearance of the two instabilities. The drift instability is excited first, then the flute instability appears along with the suppression of the drift instability, and finally, both fluctuations are stabilized. The cyclic competition process in these instabilities continues periodically, and is closely related to the radial density profile.

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In the National Spherical Torus Experiment (NSTX), a peak plasma current up to 390 kA has been successfully generated by the Coaxial Helicity Injection (CHI) current drive method. The plasma rotation (∼ 20 km/s) driven in the E × B toroidal direction by CHI has been clearly identified by an ion Doppler spectroscopic measurement. The n = 1 mode has been also observed to rotate in the same direction. This rotating kink behavior observed for the first time in NSTX is consistent with the electron locking model developed in the Helicity Injected Torus-II (HIT-II) experiments to explain the mechanism of CHI current drive.

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An experiment on a new type of z-pinch with divergent gas-puff was conducted for the realization of a point radiation source with high efficiency. X-ray radiation of energy 150 - 200 keV that far exceeded the power-supply voltage was observed. The x-rays were radiated from the center of the anode surface, and the point radiation source was achieved.

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Single pass absorption of second-harmonic EC waves propagating obliquely to the magnetic field has been studied in Heliotron J. Two orthogonal signals of transmitted high power EC waves are measured to estimate the single pass absorption rate. The experimental results agree well with ray tracing calculation results based on a linear absorption theory under the condition that the refraction effect is not strong, n_e/n_c < 0.3 (n_c is a cut-off density of second-harmonic X-mode), that is, the waves reach the diagnostic port.

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The Ring Trap-1 (RT-1) experiment succeeded in producing a long-lived (of the order 10² s), stable, non-neutral (pure electron) plasma. Electrons are confined by a magnetospheric dipole field. To eliminate a loss channel of the plasmas caused by support structures, a superconducting coil was magnetically levitated. This coil levitation drastically improved the confinement properties of the electron plasma compared to previous Prototype-Ring Trap (Proto-RT) experiments.

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A very high beta value was obtained during pulsed poloidal current drive (PPCD) operation in a reversed-field pinch (RFP) plasma during a TPE-RX experiment. Poloidal beta is almost equal to total beta in the RFP, and is as high as 30% near the end of PPCD operation. During the PPCD operation, the magnetic fluctuation associated with the dynamo effect is reduced, and improved confinement is realized, which results in this high beta value.

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Measurement of drift wave turbulence using multi-probe systems was performed in the Large Mirror Device-Upgrade linear plasma. We compared a drift wave's two-dimensional (poloidal wave number and frequency) power spectrum with its calculated linear dispersion relation. As a result, a few wave modes that satisfy the linear dispersion relation were observed. Moreover, cascades to non-mode peaks and broadband components that do not satisfy the dispersion relation were identified.

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Multi MeV proton beam is generated via target normal sheath acceleration when the target is irradiated with an ultra-intense laser pulse. In addition, a unique structure, “zonal pattern”, of energetic protons is observed in the perpendicular directions of the target edges using triangular targets. The sheath field production on the target edges may be responsible for this zonal pattern. Two dimensional particle-in-cell simulations show that the electrostatic field initially produced at around the cross point at the laser axis and the rear surface expands on the target surface in time. The field enhancement occurs at the target edges when the sheath field reaches there. The enhanced field can accelerate protons in a zonal pattern.

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Magnetic dipole (M1) transitions from highly charged heavy impurities have been surveyed in visible and near-UV wavelength ranges longer than 2500 Å using a 1.33 m Czerny-Turner spectrometer in the Large Helical Device (LHD) for use in future visible impurity spectroscopy of D-T burning plasmas. The M1 transitions of KrXXII (Kr²¹⁺: P-like) 3s²3p^{3 2}D_3/2-²D_5/2 3463.75 ± 0.05 Å, KrXXIII (Kr²²⁺: Si-like) 3s²3p^{2 3}P₁-³P₂ 3841.07 ± 0.03 Å, MoXXIX (Mo²⁸⁺: Si-like) 3s²3p^{2 3}P₁-³P₂ 2842.10 ± 0.05 Å, XeXXXIII (Xe³²⁺: Ti-like) 3d^{4 5}D₃-⁵D₂ 4139.01 ± 0.02 Å have been successfully observed using an external puff of Kr and Xe and an impurity pellet injection of Mo. As a result, the identication of the Ti-like XeXXXIII M1 transition, as observation for the first time in laboratory fusion plasmas, strongly suggests that the visible impurity spectroscopy of tungsten ions using Ti-like WLIII (W⁵²⁺: 3626 Å) instead of the conventionally used EUV spectroscopy is possible in future D-T burning plasmas.

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In our recent experiment, it was found that intense negative ion beams could be extracted from high-density and high-electron temperature plasmas under a Cesium seeded condition. This indicates that such plasmas contribute to the surface production of negative ions, and that the negative ion production exceeds the destruction process via electron detachment. Thus, it has been suggested that highly ionized and dissociated plasmas are suitable for the surface production of negative ions. This paper reports the results of specific experiments to confirm the negative ion production in plasmas with high-electron temperature. In these experiments, high electron temperature plasmas were produced by (1) placing filaments (acting as cathodes) near the extraction area, and (2) reducing the magnetic filter strength, both to allow fast electrons access to the beam extraction area. The results support the production of high-density H^- ions even in plasmas with high-electron temperature. Also included is a discussion on the surface production mechanism of negative ions under a Cs-seeded condition.

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Johnsen-Rahbek electrostatic chuck (ESC) is installed on the cathode side of a capacitive RF discharge, and the ESC voltage-current (V-I) characteristic is measured under various conditions. First, the reference V-I curve is obtained for a grounded aluminum (Al) wafer without discharge. The observed nonlinear characteristic is attributed to the field emission of electrons at irregular contacting surfaces. When the discharge is turned on with an electrically floating wafer, the V-I curve shifts from the reference curve toward the negative direction along the chuck voltage axis. The amount of shifted chuck voltage coincides with the self-bias DC voltage induced on the wafer. This plasma effect on the V-I characteristics can be explained well in terms of the effective chuck voltage, taking into account the self-bias. On the other hand, the replacement of the Al wafer with a silicon (Si) wafer leads to a considerable reduction in the chuck current. When a thin Al foil is inserted between the Si wafer and the aluminum nitride (AlN) spacer layer, the chuck current recovers upto the reference value, suggesting that the Johnsen-Rahbek effect is extremely sensitive to the electrical and mechanical properties of the contacting interface.

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A high density internal diffusion barrier has been produced in the intrinsic helical divertor configuration in LHD by optimizing the pellet fueling scenario and magnetic configuration. The internal diffusion barrier easily appears in the outer shifted magnetic configuration in which magneto-hydrodynamic stability properties are considered to be favorable. The attainable central plasma density becomes higher as the magnetic axis shifts outward and the central density exceed 5 × 10²⁰ m^-3. Central pressure exceeds 130 kPa and, therefore, very large Shafranov shift is observed, even at high magnetic field (B_t > 2.54 T).

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We investigate the linear instability of electromagnetic modes caused by the interaction of a relativistic electron beam with a dense plasma, taking into account the relativistic thermal spread of the beam. To obtain a linear dispersion relation, the relativistic factor γ of the beam is expanded within the first order of the beam thermal spread. We show that growth rate becomes large when the relativistic thermal spread of the beam is taken into account. In addition, we discuss the effects of the relativistic beam thermal spread on growth rate and the wave number vector that yields the maximum growth.

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Diamond-like carbon (DLC) coating was tested as a candidate solid lubricant for transmission gears of the maintenance equipment of the blanket of the ITER instead of an oil lubricant. The wear tests using the pin-on-disk method were performed on disks with SCM440 and SNCM420 as the base materials and coated with soft, layered, and hard DLCs. All cases satisfied the required allowable contact stress (2 GPa) and lifetime (10⁴ cycles), and therefore the feasibility of the DLC coating was validated. Among the three types of DLCs, the soft DLC showed the best performance.

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The concept of isothermal confinement is presented. The idea is a revival of the early magnetic fusion concepts with new insight. The plasma core is confined magnetically and is surrounded by a quasi-vacuum region. The temperature of the core is uniform and the turbulence associated with the temperature gradient is absent. The quasi-vacuum region is unstable against the pressure gradient and the turbulent transport rate is much larger than that of the core. Two modes of operation, pulsed and steady state, are considered. Recent experimental results in LHD and CDX-U appear to support the concept.

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Tritium production rates were measured using a DT neutron source for solid breeder blanket mockups under conditions without and with a neutron reflector at Fusion Neutronics Source facility in Japan Atomic Energy Agency in our previous studies, and the experimental results were compared with the calculated ones. Uncertainties of the calculation results for the experimental condition with the reflector were larger than those without one. We have studied influence of reflector on calculation accuracy for tritium production rate in the present study. From the Monte Carlo calculation results evaluating the tracked path of each neutron, it can be clarified that the ratios of the tritium production due to neutrons scattered by the reflector to that due to all neutrons are 0.24 ∼ 0.57. The divergence of the ratio of the calculation result to the experimental one on tritium production from unity increases with the ratio of the tritium production due to neutrons scattered back from the reflector. It can be concluded that this increase is due to neutrons scattered by the reflector, and the calculation accuracy is enhanced by improving the calculation for back-scattered neutrons.

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A comprehensive research concept, known as SPIRIT, is described for the investigation of the formation, stability, and sustainment of oblate field reversed configurations (FRCs). This concept, whose name stands for Self-organized Plasma with Induction, Reconnection, and Injection Techniques (SPIRIT), allows for the study of FRC stability properties on time scales much longer than the energy confinement time. Counter-helicity merging of inductively formed spheromaks is utilized to form large-flux FRCs. These FRCs are sustained by neutral beam injection with the initial aid of compact ohmic solenoids. Stability to n = 1 tilt/shift modes is provided by plasma shaping and conducting shells. Stability to n ≥ 2 co-interchange modes is achieved by a distribution of high-energy non-thermal ions provided by the neutral beam. The combination of plasma shaping, conducting shells, current sustainment, and the non-thermal beam component are expected to lead to a configuration with stability to all global MHD modes, a regime recently discovered through hybrid-MHD simulation using the HYM code. An experimental test of the concept, utilizing the existing Magnetic Reconnection Experiment (MRX) facility, is described. Initial experiments in MRX have confirmed the viability of the SPIRIT concept, and calculations indicate that the confinement of high-energy ions in MRX should be sufficient to test the SPIRIT concept.

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Recent experimental studies are presented regarding ELMy H-mode plasma having Type-I Edge Localized Modes (ELMs) in the JT-60U. Toroidal rotation scan experiments in inward-shifted, small-volume plasma show that the ELM energy loss decreases as the toroidal rotation at the plasma edge increases in the direction counter to the plasma current. Specifically for a large volume plasma having a toroidal field ripple of ∼2% at the plasma edge, small Type-I ELMs are observed whose ELM energy loss normalized by the pedestal stored energy is smaller than that of an acceptable ELM size in the ITER. However, the pedestal pressure tends to decrease when plasma volume increases. No remarkable effect of reduced toridal field ripple due to the installation of Ferritic Steel Tile (FSTs) inside the vacuum vessel on the JT-60U on ELMs for large volume plasma is seen in the ELM energy loss normalized by the pedestal stored energy. These new findings suggest that the toroidal field ripple itself may not directly affect the normalized ELM energy loss, and that toroidal rotation at the plasma edge as well as plasma configuration might play important roles in the prediction of ELM size in future devices.

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A numerical method for the stability analysis of ideal MHD modes is devised by using a physical model based on the two-dimensional Newcomb equation in combination with the conventional ideal MHD model. The MARG2D code based on this numerical method is able to analyze the stability of ideal MHD modes with a wide range of toroidal mode numbers. The validity of the MARG2D code has been confirmed through benchmarking tests using the DCON code for the low toroidal mode number MHD mode analysis, and tests using the ELITE code for intermediate to high toroidal mode number mode analysis. By using the MARG2D code, the MHD stability property of JT-60SA, the complemental device of ITER, is investigated with a focus on the effect of the plasma shape.

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X-ray polarization spectroscopy is proposed as an advanced diagnostic tool to measure the anisotropic velocity distribution of hot electrons generated by an ultra-high intensity laser pulse. The relationship between the anisotropy of the hot electron velocity distribution and the polarization degree was investigated for various tracer materials. It is shown that the polarization degrees of Heα lines are a function of electron energy normalized by the excitation threshold. Depolarization is caused by isotropic bulk electrons in high temperature plasma. To diagnose the anisotropy of the hot electron velocity distribution without depolarization, it is essential to select a tracer material whose energy of Heα line is 10 times higher than the bulk electron temperature.

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Intrinsic impurities have been much reduced in toroidal fusion devices through the development of several wall-conditioning techniques as well as by the use of carbon materials in the first wall and divertor plates. Impurity elements useful for passive plasma spectroscopy have been then extremely limited. At present, only carbon is a subject for spectroscopic diagnostics in most discharges except for fuel atoms. The use of rare gas as a brighter light source is a method to overcome the present difficulty in passive spectroscopy. Recently, rare gases have also been used for edge cooling to reduce the divertor heat flux. Therefore, high-resolution spectra (Δλ ∼ 0.2 Å) from neon and argon in a 250 to 2300 Å wavelength range have been measured using a 3 m normal incidence spectrometer in Large Helical Device (LHD) and the measured spectra were precisely analyzed. The VUV spectra of carbon, neon and argon are presented for spectroscopic use and their wavelengths are tabulated with their relative intensities. The spectral profiles of almost all the spectral lines measured here are formed by the Doppler broadening and self-absorption processes. The Doppler broadening of neon and argon spectra are plotted against the ionization energies and Doppler spectra from carbon lines are presented. The self-absorption spectra of the hydrogen Lyman-α line, which are found in the LHD high-density discharge, are also presented and the neutral density is analytically estimated.

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Axisymmetric magneto-rotational instability (MRI) is studied in comparison with interchange instability (IntI) in a rotating cylindrical plasma. MRI is driven by the shear of plasma rotation, and the IntI by the density gradient with effective gravity due to the plasma rotation. The eigenmode equation for the MRI has the same form as that for the IntI. The local stability criterion is also summarized in a similar statement as “the spatial gradient of centrifugal force greater than the square of Alfvén frequency causes instability.” However, the MRI is essentially different from the IntI because of the non-Hermitian property. The Keplerian rotation generates irregular singularity at the center of the disk, which yields a continuum of eigenvalues with non-orthogonal and square-integrable eigenfunctions.

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Electron acceleration processes taking place in the interaction of ultra-intense laser pulses with cone targets are studied by using two-dimensional Particle-in-Cell (PIC) simulations to understand the characteristics of electrons generated from cone targets. It is explained that there are two dominant acceleration processes which are distinctive in the laser-cone interaction. One is the acceleration and transport along the side wall of the cone target, where electrons are guided along the side wall surface towards the cone tip by surface magnetic and electric fields. The second is the ponderomotive acceleration at the cone tip by the laser field which is intensified by cone focusing. The understanding of these acceleration processes helps to design cone targets to control the electron energy characteristics.

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This paper describes diagnostics for transport analysis of energetic ions in JT-60U. Line integrated neutron emission profiles are measured using Stilbene neutron detectors, which are installed as a neutron emission profile monitor. The flux and energy distribution of charge exchange neutral particles are measured using a natural diamond detector (NDD). These measurements in the presence of bursting instabilities in the frequency range of Alfvén Eigenmode induced by negative-ion-based neutral beam injection, which are called Abrupt Large-amplitude Events, indicate that energetic ions are transported from the core region in the plasma due to a resonance interaction between energetic ions and the instabilities. Development of a digital signal processing (DSP) system for a neutron detector and a NDD enabling a sampling rate in the MHz class is proceeding. The DSP system for the neutron detector successfully discriminates between neutrons and gamma-rays in tests using neutron/gamma-ray sources.

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A new gyrokinetic plasma simulation model for electromagnetic phenomena is presented. In this model, the total characteristic method, where the δf particle-in-cell simulation model is complemented with the fluid model to satisfy the conservation properties, is applied to electrons. The electric field component parallel to the magnetic field is calculated from the time derivative of Ampère's law. It is demonstrated that both the real frequency and damping rate of kinetic Alfvén wave are computed correctly for various electron beta values. It is also shown that with respect to the number of marker particles, the numerical convergence of real frequency and damping rate is faster with the total characteristic method than with the conventional δf method. Specifically, it is demonstrated that the total characteristic method enables a simulation of a kinetic Alfvén wave with a grid size ten times larger than the electron skin depth, while the wave damps spuriously for the same physical condition in a conventional δf simulation.

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The dynamics of the tearing mode and microscopic resistive drift wave turbulence are studied by performing a nonlinear simulation based on a 4-field Reduced MHD model, placing an emphasis on the interaction between microscopic and transport processes. The simulation results show the importance of turbulent fluctuations for the onset of the tearing mode. The faster growth of microscopic fluctuations induces accelerated growth of the tearing mode, which is much faster than the linear growth rate. A turbulence-driven magnetic island is formed. This is based on the incoherent emission of the long wavelength mode by microscopic turbulence.

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The interactions between ultrahigh intense laser and overdense plasmas were investigated by the use of a 1-1/2 dimensional electromagnetic relativistic particle-in-cell code, EMPAC. When the effective electron plasma frequency is reduced below the laser frequency by increasing the inertial electron mass due to the relativistic effect, the ultrahigh intense short pulse laser can penetrate the overdense plasma, but is completely reflected after propagating to a certain extent, except for a portion of the absorbed laser. The pulse length of the reflected laser is expanded more than that of the incident laser by a modulation due to the anomalous penetration, and the pulse expansion factor can be predicted by the schematic model. The frequency of the reflected laser can be calculated by the Doppler shift formula coupled with a relativistic dispersion relation, and is good agreement with the simulation result. The anomalously penetrating pulse shows soliton-like behaviors in the plasma after the incident laser has vanished.

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In charge exchange spectroscopy (CXS), a simultaneous observation in different plasma toroidal cross sections and/or viewing ports is required to investigate radial distributions of ion temperatures T_i(r), and poloidal rotation velocities V_p(r) in magnetically confined toroidal plasmas. In recent studies of the edge transport barrier (ETB) in the Compact Helical System (CHS), a simultaneous viewing of the vertically elongated and the horizontally elongated plasma cross sections is used to improve the spatial resolution at the edge region. The 90 fibers used for this purpose are connected to one spectrometer, and a 256 × 243 pixel sampling CCD is used to detect the diffraction image. It is found that there is a localized edge ion temperature pedestal region with ΔT_i ≈ 100 eV and Δr/a ≈ 0.1, where r and a are flux surface averaged minor radii of measured surfaces and the outermost flux surface, respectively. The negative radial electric field at the edge is increased in the high confinement phase because of the increased ion pressure.

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Selectively launched electromagnetic waves with left- and right-handed polarizations for m = 0, +1, and -1 modes are investigated in terms of polarization reversal around an electron cyclotron resonance (ECR) region in inhomogeneously magnetized plasmas, where m is an azimuthal mode number. It is observed for the first time that a left-handed polarized wave for m = 0 mode is absorbed near the ECR point as a result of the polarization reversal in the axial direction. Dispersion analysis in bounded plasmas can explain this quantitatively. For m = +1 and -1 modes, on the other hand, polarization reversal occurs along the radial axis, i.e., wave polarization for m = +1 mode is right-handed (left-handed for m = -1 mode) around the central area and left-handed (right-handed for m = -1 mode) around the peripheral area of the cross section of the plasma column. Furthermore, we investigate plasma-potential structures formed by ECR of high-power m = +1 and -1 waves and, for the first time, demonstrate control of the plasma-potential structure by changing the azimuthal mode, i.e., a positive potential hill is formed near the ECR point around the central and peripheral areas for m = +1 and -1 modes, respectively.

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A frequency-modulated reflectometer with a frequency band of 26.5 to 40 GHz has been constructed and installed in the TST-2 spherical tokamak to measure fast density profile evolution. In order to calibrate the instantaneous frequency for various frequency sweep rates, a waveform matching method has been proposed and applied to the reflectometer. This method compares the interference patterns (i.e., waveforms) of a fixed target reflection, which do not depend on the sweep rate, and obtains the instantaneous frequency by fitting the target waveform time to the time for a reference sweep waveform. This allows evaluation of output frequency stability. The overall frequency error, including reproducibility, is around 0.1 GHz. Sweep rates up to 20 μs were used to measure the density profile evolution during rf heating.

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The ignition and burn properties of fast ignition DT targets are evaluated for various-sized core (ignition experiment ∼ high gain) on the basis of two-dimensional (2D) burn simulations. A core size of ρR 2.0 g/cm² is required to achieve explosive burning and then high gain. When the core size is smaller, the target gain drops sharply as core size decreases. Assuming the energy coupling efficiencies from laser to core of 5 % for implosion and 30 % for heating, a target gain of ∼170 is obtained with a 1 MJ implosion laser and a 70 kJ heating laser, under optimum heating conditions (10 ps duration, 15 μm spot radius, and 1.0 g/cm² heating depth). This requires a very high intensity heating laser (∼ 1 × 10²¹ W/cm²). In accordance with a scaling for temperature of fast electrons generated by long-duration intense lasers, such a intense laser will generate fast electrons having suitable stopping range for efficient core heating. The sensitivities of ignition condition and gain performance to heating conditions, and the influence of high-Z ions contained in a foam layer on ignition and gain performance are also discussed.

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A bursting mode, whose time scale is a few milliseconds, is excited in low density neutral beam injected plasmas in the compact helical system. A scintillator-based probe equipped with a high speed video camera has been used to investigate the energetic ion losses induced by this fast evolving mode. This instrument reveals loss in a region of the gyroradius and the pitch angle space that occurs only during a burst. Namely, it was found that the bursting mode induces the transport of energetic ions to the region where the energetic ions cannot exist without such an enhanced transport. The dependence of the newly observed loss on the electron density and the neutral beam injected power was also investigated.

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Investigations of heat- and particle-deposition distributions on helical divertor plates were performed both experimentally and numerically with the Large Helical Device (LHD). The distributions were measured by thermocouples and Langmuir probe arrays embedded in the divertor plates. They are similar to the distribution of field lines with long connection lengths, which are estimated numerically by field-line tracing calculations. Localized heat- and particle-deposition distributions were observed, and were found to be determined primarily by the magnetic-field line structure in the divertor region. They vary depending on the configuration − the intensively loaded areas move with a change of the magnetic configuration. Using this property, a swing of the magnetic axis successfully dispersed the heat and particle loads on the divertor plate during long-pulse discharges. The magnetic-axis swing around a certain magnetic-axis position is found to be very effective in changing the field-line distribution pattern drastically, and thus dispersing the heat load.

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