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We demonstrate a drastic improvement in the efficiency of rare-element-free graphene nanomesh (GNM) magnets with saturation magnetization values as large as ~10-4 emu/mm², which are 10-100 times greater than those in previous GNM magnets hydrogenated by only annealing under a hydrogen molecule (H2) atmosphere, even at room temperature. This improvement is realized by a significant increase in the area of the mono-H-terminated pore edges by using hydrogen silsesquioxane resist treatment with electron beam irradiation, which can produce mono-H by detaching H-silicon (Si) bonds. This result must open the door for industrial applications of graphene magnets to rare-element-free magnetic and spintronic systems. [ABSTRACT FROM AUTHOR]

The so-called zigzag edge of graphenes has localized and strongly spin-polarized electrons. However, magnetoresistance (MR) behavior associated with the edge electrons has not been reported in graphenes. Here, we measure MR of graphene antidot-lattices, honeycomb-like arrays of hexagonal antidots with a large ensemble of hydrogen-terminated and low-defect antidot edges, prepared by a nonlithographic method using nanoporous alumina templates. We find anomalous MR oscillations arising from localized electron spins existing at the antidot edges. These are promising for realization of spintronic devices. [ABSTRACT FROM AUTHOR]

The pulsed poloidal current drive technique reduces the magnetic chaos that characterizes reversed-field pinch configurations and produces a regime with an improved confinement. In this paper, we describe that, in TPE-RX [Y. Yagi et al., Fusion Eng. Des. 45, 409 (1999)], the termination phase of this regime is due to the increase of the slinky structure that creates a stochastic region and produces the expulsion of energy in a localized toroidal position. Before the plasma reaches the improved confinement regime, the slinky distorts the chain of m=0 islands on the reversal surface. During this regime, the magnetic activity and the phase locking decrease, the distortion in the island chain disappears, and the confinement increases. At the termination of this regime the magnetic activity markedly increases, as well as the phase locking, recreating the distortion in the m=0 magnetic island chain. As a consequence, at the position of the distortion the plasma region inside the reversal surface is characterized by a rapid energy loss, and outside the reversal surface a toroidally localized energy expulsion is induced. [ABSTRACT FROM AUTHOR]

The operating conditions for obtaining a quasi-single helicity (QSH) state with a good reproducibility are found in a reversed-field pinch (RFP) experiment on the large RFP machine, TPE-RX [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)]. In these conditions, the reversal of toroidal magnetic field (Bta) is maintained at a very shallow value (∼-0.2 mT) after the setting up phase and the following fast current rising phase. After a certain period at this shallow reversal (∼15–25 ms), the m/n=1/6 mode (m and n being the poloidal and toroidal Fourier mode numbers, respectively) rapidly grows and saturates before the termination of discharge. The growth of this mode dominates the other modes and the QSH state with m/n=1/6 is finally achieved. This QSH state can be sustained for a long period (up to ∼45 ms) almost until the end of discharge by applying a delayed reversal of Bta with appropriate trigger timing and magnitude. The initial setup of the QSH states shows a reproducibility of almost 100%, but its sustainment for a long period shows a slightly reduced reproducibility (∼85%). The initial rapid growth of the single dominant mode is compared with the numerical results of linear stability and nonlinear three-dimensional (3D) calculations by assuming the experimental magnetic field profile estimated with a standard model. Linear calculations show that the m/n=1/6 mode has the maximum growth rate to the ideal magnetohydrodynamic instability and can explain the dominant growth of this mode. The 3D calculations also show a qualitative agreement with the experiment, where under some conditions the m/n=1/6 mode becomes dominant after an initial relaxation and continues to the end of the simulation. These results indicate that the present QSH state is the combined result of the linear growth and nonlinear saturation of a particular mode. [ABSTRACT FROM AUTHOR]

The close relationship between the locked mode (LM) and pulsed poloidal current drive (PPCD) regime in a reversed-field pinch device is described. If the modes lock close to the shell gap then the LM is enhanced and there is a high probability that the PPCD will not improve plasma performance. The physical reason will be discussed. If the position of the locked mode is far from the shell gap then the PPCD is effective with a high probability. LM energy is reduced and the more the LM energy decreases, the better the plasma performance. [ABSTRACT FROM AUTHOR]

Factors affecting the magnitude of sawtooth crashes in soft x-ray (SXR) signals (ISXR) have been studied using data from high-Θ (pinch parameter) discharges in the toroidal pinch experiment TPE-1RM20 reversed-field pinch device [Y. Yagi et al., J. Plasma Fusion Research 69, 700 (1993)]. In TPE-1RM20, discharges with a high Θ that are free of large sawteeth (improved high-Θ mode discharges), spontaneously appeared [Y. Hirano et al., Nucl. Fusion 36, 721 (1996)] among typical conventional discharges exhibiting large sawtooth crashes. The reason for the spontaneous appearance of sawtooth-free discharges has been unclear. It is shown that the normalized magnitude of the SXR crashes, [Δ(ISXR)/ISXR], is well correlated with the m (poloidal mode number)=0 mode amplitude in the postcrash phase. It is also shown that Δ(ISXR)/ISXR is smaller when the quasi-single helicity state precedes the crash rather than when the multiple-helicity state precedes the crash, and Δ(ISXR)/ISXR is smaller when E∥ (the parallel electric field at the plasma surface) is approximately zero in the precrash phase rather than E∥ being negative. [ABSTRACT FROM AUTHOR]

The neoclassical magnetohydrodynamic equilibrium of reversed-field pinch (RFP) is self-consistently solved considering the self-induced plasma current I[lowercase_phi_synonym]SI in a reactor relevant parameter regime. The current I[lowercase_phi_synonym]SI consists of bootstrap currents due to α particles as well as the bulk plasma, where the conventional neoclassical transport theory is applied without considering the finite banana-width effect, Pfirsch–Schluter current, and diamagnetic current. The neoclassical effect enhances I[lowercase_phi_synonym]SI in a low-aspect-ratio RFP (aspect ratio A=2), when β is high. An I[lowercase_phi_synonym]SI as high as 94.4% of the plasma current in the equilibrium with ellipticity κ=1.4 and triangularity δ=0.4 is obtained with a flat pressure profile, which provides a toroidal β of βt=63% (volume-averaged β, 〈β〉=66%) stable against both the ideal kink mode and Mercier mode. This dominant self-induced current makes the steady-state approach feasible. Its hollow current profile gives a strong magnetic shear and a high-stability β. [ABSTRACT FROM AUTHOR]

The application of the pulsed poloidal current drive (PPCD) technique in reversed-field pinch (RFP) devices leads to the improvement of the confinement properties of the plasma. In this article the conditions necessary to achieve high PPCD performance in the TPE-RX [Y. Yagi, S. Sekine, H. Sakakita et al., Fus. Eng. Des. 45, 409 (1999)] RFP device will be shown. One of the key parameters is the component of the electric field parallel to the magnetic field E∥. The positive trend between E∥ and the PPCD performance can be explained in terms of reduction of magnetic fluctuations. It will also be shown that important roles are played by the triggering time of the PPCD pulses, the filling pressure of the deuterium gas, and the wall condition. [ABSTRACT FROM AUTHOR]

In this paper results of an experimental study of magnetic fluctuations in the Toroidal Pinch Experiment (TPE–RX) [Y. Yagi, S. Sekine, H. Sakakita et al., Fus. Eng. Design 45, 409 (1999)] are reported. This study has permitted the identification of the experimental conditions under which the usually turbulent spectrum of magnetic fluctuations in TPE–RX turns into a single tearing mode, in the so-called quasi-single helicity (QSH) state. The large data set has allowed us to provide a complete two dimensional map of QSH probability in the current F plane, being F=Bϕ(a)/〈Bϕ〉. Two main basins of high QSH probability have been found. The first one, at high current and low |F|, confirms previous results on QSH in other machines; the second one is completely new, and is found at low current and high |F|. These studies show that QSH is a robust, device-independent phenomenon, and a significant step towards a laminar type of dynamo in the reversed-field pinch configuration. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The pulsed poloidal current drive (PPCD) [J. S. Sarff et al., Phys. Rev. Lett. 72, 3670 (1994)] experiment is conducted in a reversed-field pinch device, the toroidal pinch experiment RX (TPE-RX) after providing an auxiliary power supply system with increased energy in the main power supply system for the PPCD. The PPCD system thus provides double-pulsed operation with higher current in the toroidal coil than that in single-pulsed PPCD operation in TPE-RX [Y. Yagi et al., Plasma Phys. Controlled Fusion 44, 335 (2002)]. The central electron temperature, ion temperature, and electron density increase during PPCD, and there is, on average, a fivefold improvement in energy confinement, τ[SUBE], relative to standard discharges. Double-pulsed PPCD) yields better performance than that of single-pulsed PPCD operation where twofold improvement in τ[SUBE] was obtained. It is shown that the enhancement factor of τ[SUBE] in the double-pulsed PPCD experiment in TPE-RX is consistent with the trends, observed previously, versus magnetic fluctuation amplitude and versus Δγ, where Δγ is the difference in γ[=1-F)/&THgr;] between the start and the end of the PPCD period. [ABSTRACT FROM AUTHOR]

An overview of the present diagnostics system for a reversed-field pinch machine, the toroidal pinch experiment RX [TPE-RX, R/a = 1.72/0.45 m, I[sub p] < 1 MA (designed)], is presented. The diagnostic system of TPE-RX consists of measurement systems for (1) equilibrium configuration, (2) perturbing modes, and (3) global confinement properties. A concise explanation is given for these diagnostics systems. The mode-measurement system, in particular, is equipped with high toroidal resolution (up to the mode number of 32), which is necessary for the reversed-field pinch configuration. We have magnetic, thermal, and spectroscopic measurement systems for the toroidal distribution of the perturbing modes, which are useful for revealing the mode-locking structure. [ABSTRACT FROM AUTHOR]

Data acquisition and analysis systems for a reversed-field pinch (RFP) machine, the toroidal pinch experiment (RX) [TPE-RX, R/a = 1.72/0.45 m, I[sub p] < 1 MA (designed)], are presented. Many kinds of diagnostic systems are installed on TPE-RX, and 392 signal channels at maximum from the diagnostic systems are routinely acquired by using the Computer Automated Measurement and Control system. The data are acquired through a serial highway into a front-end computer. A custom made, special computer program system, called DAQR, has been developed for the data acquisition routine. Software libraries are also developed for the data analyses of the plasma parameters, perturbed mode structures of the RFP plasma, and radiation from plasma. [ABSTRACT FROM AUTHOR]

A spectroscopic system equipped with image magnifying optics has been developed for measuring a time-resolved plasma velocity at one discharge of a reversed-field-pinch plasma of the TPE-RX at National Institute of AIST. This spectroscopic system enables a simultaneous measurement from two different lines of sight, using two sets of quartz optical fiber bundles. This system provides a precise measurement of Doppler shift without the need for a calibration of the central location of a spectral line. Two fiber bundles are coupled to the entrance slit of a modified Czemy-Turner-type, 1-m-focal-length spectrometer with an 82 × 82 mm² grating. In order to adjust the spectral image of the exit slit to each surface of two sets of one-dimensional detector arrays, an optical system equipped with a toroidal mirror, a cylindrical mirror, and splitting mirrors has been developed. The focal lengths of the toroidal mirror are selected for focusing vertically on the splitting mirror and horizontally (wavelength direction) on detectors. Plasma rotation and ion temperature for O v impurity ions (278.1 nm) are successfully measured at one plasma shot with a 25-µs time resolution in the TPE-RX. [ABSTRACT FROM AUTHOR]

Wall-locking and phase-locking modes are studied in detail in a reversed-field pinch device, TPE-RX [Y. Yagi et al., Fusion Eng. Design 45, 409 (1999)]. These mode-locking phenomena arise from tearing instabilities. Wall locking means the stopping of mode rotations, and phase locking means the locking of the phases of multiple modes. Phase locking induces a toroidally localized enhanced magnetic amplitude. There are two types of mode-locking states in TPE-RX. One of them exhibits a clear phase-locked structure, while the other exhibits a weak toroidal localization. Both types show finite toroidal rotation during the current-rising phase of the discharge, and are eventually wall locked during the current flat-top phase. However, the rotation speeds are clearly different between the two types. Confinement properties are compared between the two types of mode-locking states. It is shown that the threshold for the mode amplitude necessary to wall lock the toroidal rotation, as well as the bifurcation of phase-locked structures, agrees with theoretical predictions IR. Fitzpatrick et al., Phys. Plasmas 6, 1168, 3878 (1999)]. [ABSTRACT FROM AUTHOR]

Magnetic field fluctuations have been measured in the TPE-1RM20 reversed-field pinch [in Plasma Physics and Controlled Nuclear Fusion Research, Proceedings of the 14th International Conference, 1992, Würzburg (International Atomic Energy Agency, Vienna, 1992) Paper IAEA-CN-56/C-4-3]. The dominant modes are m=1/n=7–9, resonant near the axis. The fluctuations are coherent, corresponding to nearly continuous rigid body rotation with poloidal and toroidal frequencies of fθ=130–170 kHz and f[lowercase_phi_synonym]=11–14 kHz, respectively. Discrete dynamo events (in which toroidal magnetic flux is generated) of two different types are observed at high values of pinch parameter (Θ>=1.6). The first type is triggered by a single, wall-locked m=1/n=7 mode. The slowing down of the mode rotation is in rough agreement with a simulation which assumes the effect of induced eddy current in the vacuum liner. In the subsequent relaxation event, the locked mode decays and the m=1 spectrum cascades to higher-n modes. However, in some cases the saturated m=1/n=7 mode exists for periods up to t=0.4 msec, resembling single helicity Ohmic states. The second type of event is characterized by phase locking of several m=1/n=7–9 modes and the appearance of large m=0/n=1–5 modes. Mode rotation continues through the event, similar to the normal machine operating condition. [ABSTRACT FROM AUTHOR]

Studies the dynamo electric field along the equilibrium line in laboratory plasma devices arising from the correlation between the fluctuating flow velocity and magnetic field. Alpha effect that accounts for the dynamo current generation; Dynamo at low collisionality; Fluctuating velocity at high collisionality; Transport of magnetic helicity by dynamo activity.

An ultrafine-particle plasma consisting of electrons, positive K+ ions, and large negative C−60 ions is produced by introducing "Buckminsterfullerene, C60" particles into a low-temperature (≃0.2 eV) potassium plasma column confined by a strong axial magnetic field. With an increase in the C60 fraction, the electron shielding decreases, yielding clear effects on plasma collective phenomena, which are demonstrated for low-frequency electrostatic plasma-wave propagations and instabilities. This plasma might be useful for producing new C60-based materials. [ABSTRACT FROM AUTHOR]

Focuses on edge plasma fluctuations with inserted triple Langmuir probes and magnetic coils in the TPE-1RM20 reversed-field pinch. Low mode numbers of the density and potential fluctuations; Low frequencies of the electrostatic field fluctuations in the same frequency range as magnetohydrodynamics fluctuations; Poloidal and toroidal mode number spectra.

Fluctuations of magnetic fields and related magnetohydrodynamic (MHD) phenomena are investigated in the TPE-1RM15 reversed-field pinch experiment [Plasma Physics and Controlled Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. 2, p. 453]. Mode analysis of fluctuations measured by multichannel coils reveals that nonlinear interactions between m=1 and m=0 modes, such as nonlinear coupling and phase locking, play significant roles during a dynamo event (i.e., the flux genertion process in the sustainment phase), resulting in transition from an unstable state to a stable state. Behaviors of these fluctuations are found to be toroidally asymmetrical due to strong nonlinearity. Study of the current ramping experiment shows that the inverse of global energy confinement time depends on the squared fluctuation level offset linearly, which is consistent with the prediction of the transport model based on the diffusion of stochastic field lines. By examining the dependence of the resistive part of the loop voltage on the fluctuation level, the input power to the electrons and ions are estimated to be about 70% and 30% of the total input power, respectively. [ABSTRACT FROM AUTHOR]

In this article, the measurement system for the total radiation and electron temperature profiles to be installed in a reversed-field pinch machine, toroidal pinch experiment, RX [TPE-RX, R/a=1.72/0.45 m, Ip

A space- and time-resolving vacuum ultraviolet (VUV) spectrograph system has been applied to diagnose impurity ions behavior in plasmas produced in the tandem mirror GAMMA 10 and the reversed field pinch TPE-RX. We have carried out ray tracing calculations for obtaining the characteristics of the VUV spectrograph and calibration experiments to measure the absolute sensitivities of the VUV spectrograph system for the wavelength range from 100 to 1100 Å. By changing the incident angle, 50.6°–51.4°, to the spectrograph whose nominal incident angle is 51°, we can change the observing spectral range of the VUV spectrograph. In this article, we show the ray tracing calculation results and absolute sensitivities when the angle of incidence into the VUV spectrograph is changed, and the results of VUV spectroscopic measurement in both GAMMA 10 and TPE-RX plasmas. [ABSTRACT FROM AUTHOR]

A soft x-ray (SXR) measurement system for tomography analysis on a reversed-field pinch machine. torodial pinch experiment, RX [TPE-RX, R/a=1.72/0.45 m, Ip<1 MA (designed)], is presented. The soft x-ray imaging system consists of two surface barrier detector (SBD) arrays that are vertical and horizontal. Thirteen SBDs are installed on the vertical ports and used for the measurement along vertical lines of sight. Eleven SBDs are installed on the horizontal port and used for the measurement along a fan-shaped line of sight. These detectors have 15-μm-thick Be foil with sensitivity in the soft x-ray range. This system is installed in order to study the structure of the SXR emission from the plasma core and to know the relation between global performance and magnetohydrodynamics dynamics. This system has been used under several operating conditions in addition to those of standard operation. The first results of these experiments are reported. [ABSTRACT FROM AUTHOR]

A new, two dimensional and fast diagnostic system has been developed for studying the dynamic structure of plasma turbulence; it will be used in the edge of the reversed-field pinch devices TPE-RX and RFX. The system consists of a gas-puffing nozzle, 32 optical channels measuring Hα emitted from the puffed gas (to study the optical emissivity of turbulent patterns and to analyze structures in two dimensions), and an array of Langmuir probes (to compare the turbulent pattern with the optical method and to measure the local plasma parameters). The signals can be acquired at 10 Msamples/s with 2 MHz band width. The design of the system, calibrations, and tests of the electronic circuitry and the optical sensors are presented. [ABSTRACT FROM AUTHOR]

The behavior of locked modes in two reversed-field pinch devices, the Toroidal Pinch Experiment (TPE-RX) [Y. Yagi et al., Plasma Phys. Control. Fusion 41, 2552 (1999)] and Extrap T2 [J. R. Drake et al., in Plasma Physics and Controlled Nuclear Fusion Research 1996, Montreal (International Atomic Energy Agency, Vienna, 1996), Vol. 2, p. 193] is analyzed and compared. The main characteristics of the locked mode are qualitatively similar. The toroidal distribution of the mode locking shows that field errors play a role in both devices. The probability of phase locking is found to increase with increasing magnetic fluctuation levels in both machines. Furthermore, the probability of phase locking increases with plasma current in TPE-RX despite the fact that the magnetic fluctuation levels decrease. A comparison with computations using a theoretical model estimating the critical mode amplitude for locking [R. Fitzpatrick et al., Phys. Plasmas 6, 3878 (1999)] shows a good correlation with experimental results in TPE-RX. In Extrap T2, the magnetic fluctuations scale weakly with both plasma current and electron densities. This is also reflected in the weak scaling of the magnetic fluctuation levels with the Lundquist number (∼S[sup -0.06]). In TPE-RX, the corresponding scaling is ∼S[sup -0.18]. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A dynamical mean-field theory is developed to analyze stochastic single-cell dynamics of gene expression. By explicitly taking account of nonequilibrium and nonadiabatic features of the DNA state fluctuation, two-time correlation functions and response functions of single-cell dynamics are derived. The method is applied to a self-regulating gene to predict a rich variety of dynamical phenomena such as an anomalous increase of relaxation time and oscillatory decay of correlations. The effective "temperature" defined as the ratio of the correlation to the response in the protein number is small when the DNA state change is frequent, while it grows large when the DNA state change is infrequent, indicating the strong enhancement of noise in the latter case.