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An investigation of the edge region in a large reversed-field pinch (RFP) device has been undertaken for the first time using a high time resolution gas-puff imaging (GPI) system, applied to the TPE-RX experiment. The GPI diagnostic is aimed at the investigation of turbulence in the edge region under the severe heat flux conditions of thermonuclear fusion-grade plasmas, where conventional electrostatic probes cannot be used. To compare the data obtained by the GPI during low power experiments, a retractable array of electrostatic probes has been added. The whole diagnostic system provided high quality measurements of fluctuations with fine spatial and temporal resolution, as well as radial profiles of relevant plasma parameters. The measurements confirm several important properties of plasma edge already found in other experiments, like the presence of a strong ExB shear layer and the 'intermittent' behaviour of the measured quantities, which display non-Gaussian fluctuation spikes at different time scales. Along with the conventional power spectra and correlation analyses, the application of continuous wavelet transform techniques to GPI data allowed a detailed description of the spatial structure and of the propagation of these localized 'intermittent events'.

Low-aspect-ratio reversed field pinch (RFP) plasmas are expected to produce equilibria in which the resonant surfaces for tearing modes are well separated compared with high-aspect-ratio RFP plasmas. The profile of the safety factor, q, determines the mode separation of the resonant surfaces. The q profiles, the ratio of plasma pressure to magnetic energy, β, and the parallel current density to the magnetic field lines, μ, are studied with the varying aspect ratio, A. A new Mercier stable RFP plasma is proposed by introducing a finite surface toroidal current density at the plasma edge. The aim of this study is to investigate the effect of a finite surface toroidal current with a low-aspect-ratio (low A) on the profiles of q, μ, β and the ratio of plasma pressure to poloidal magnetic energy, β p . The investigation is performed using an equilibrium model with a finite pressure, in which unknown functions in the Grad-Shafranov (GS) equation are specified. Specified functions are composed of some numerical parameters that allow systematic variation of equilibria. The GS equation is solved in the toroidal coordinate for various A values, where the β limit of Mercier stable equilibria is determined. First we confirmed that q at the centre, q 0 , increases as A decreases. The result shows that the profiles of q, β and β p , strongly depend on A. The β and β p obtained decrease with decreasing A. In particular, the decrease in β differs from the case of a tokamak. The decrease in β is recovered, in spite of a low A, by setting the toroidal current to be finite at the plasma edge as a boundary condition.

A database of the confinement properties of the toroidal pinch experiment (TPE) series reversed-field pinch (RFP) devices was established, and an empirical scaling law for the energy confinement time, τE, was deduced for a consistently selected set of the database (Yagi Y. et al 2003 Nucl. Fusion 43 1787). The scaling for τE [TPE-scaling; ], in particular, is a power law similar to that predicted on the basis of the tearing modes, as a function of the plasma minor radius (a), plasma current (Ip), and Ip/N, where N is the column density. On the other hand, the TPE-scaling has a strong dependence on the pinch parameter, Θ, namely τE ~ Θ3. Recently, we have investigated the database of the improved confinement in the pulsed poloidal current drive (PPCD). We show that the TPE-scaling agrees well with the improved τE in the PPCD database, because of its strong Θ dependence. Namely, Θ3 is a factor of merit for RFP plasmas. We discuss why this agreement is obtained in spite of the general difference between the underlying transport mechanisms of the standard and PPCD discharges. We also show that this improvement, represented by Θ3, is related to the increase in magnetic shear with Θ.

According to a simplified costing algorithm for a steady-state fusion reactor power plant, the relative attractiveness of advanced physics modes mainly depends on the stability and noninductive current drive of the equilibrium. The high-stability beta and the good alignment of the equilibrium current profile with a self-induced plasma current profile are compatible with the low-aspect-ratio neoclassical Reversed-Field-Pinch equilibrium solved self-consistently considering the self-induced plasma current. The high-stability beta is due to the hollow current profile making the magnetic shear increase and the force-free field dominant. The good alignment of the current profile significantly reduces the power required for noninductive current drive to generate the steady-state magnetic field configuration. As a result, the fusion power plant based on the neoclassical RFP equilibrium with low aspect ratio enables electricity to be generated at relatively low cost.

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.

A confinement database of the reversed-field pinch (RFP) devices of the toroidal pinch experiment (TPE) series has been established. The TPE database contains global confinement properties under various experimental conditions for four different devices and seven different cases, which cover two decades of experiments. Characteristics of the TPE database have been extracted under consistent experimental conditions of conventional RFP discharges, and scaling laws of the poloidal beta, ?p and energy confinement time, ? E, are given against independent parameters that include the size of the plasma and the pinch parameter ?. Scaling laws of and are obtained for a set of scans, where a is the plasma minor radius, Ip the plasma current and N the column density. The scaling of ? E on a, Ip, and I/N is similar to that dominated by the transport under tearing instability. Another scaling shows that ? E is inversely proportional to the Joule input power. The scaling for the apparent effective charge shows a positive correlation both with Ip and I/N.

The locked mode (LM), which is locked both in phase and to the wall, is successfully controlled using a rotating toroidal field (RTF) in TPE-RX (large reversed-field pinch machine with R/a = 1.72 m/0.45 m and plasma current Ip up to 480 kA) at a low Ip (~200 kA) and high filling pressure (~0.8 mTorr). It is demonstrated that the onset of the LM can be delayed or even eliminated by applying the RTF. Only the Fourier component with m=0/n=1 of the magnetic fluctuation exhibits rotation with the RTF, but other modes do not exhibit systematic movement synchronizing with the m=0/n=1 mode rotation. The phase relation between the m=0/n=1 mode and the other modes becomes random, which probably prevents the phases of the other modes from locking and results in the elimination of the LM. It is found, however, after the LM has taken place, it cannot be controlled by the RTF and hence an early start and fast rotation of the RTF are required for control. It is not possible to control the LM at a high Ip (\gtrsim250 kA), since the LM takes place from the early stage of discharge. Compared with discharges without the LM, plasma performance is degraded by the LM. The performance can be recovered by eliminating the LM with the RTF. However, the RTF itself degrades the plasma performance when applied to LM-free discharges with a RTF value comparable to that necessary for controlling the LM.

A fully superconducting tokamak named JT-60SC is designed for the modification programme of JT-60 to enhance economical and environmental attractiveness in tokamak fusion reactors. JT-60SC aims at realizing high-β steady-state operation in the use of low radio-activation ferritic steel in a low ν* and ρ* regime relevant to the reactor plasmas. Objectives, research issues, plasma control schemes and a conceptual design for JT-60SC are presented.

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, τE, relative to standard discharges. Double-pulsed PPCD yields better performance than that of single-pulsed PPCD operation where twofold improvement in τE was obtained. It is shown that the enhancement factor of τE in the double-pulsed PPCD experiment in TPE-RX is consistent with the trends, observed previously, versus magneti...

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 Czerny–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.

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, Ip

Pulsed poloidal current drive (PPCD) (Sarff J S et al 1994 Phys. Rev. Lett. 72 3670) is conducted in a reversed-field pinch (RFP) machine, TPE-RX. The PPCD yields a twofold improvement of poloidal beta and energy confinement time. A quiescent phase is observed in the magnetic fluctuations, δb, during the PPCD. The result is discussed in terms of the change of the equilibrium configuration along the F-Θ trajectory (F and Θ are the reversal and pinch parameters, respectively). Representative mode amplitude is numerically simulated. The result indicates that a transient nature of the PPCD, where τPPCD (characteristic time of the PPCD operation) <

One of the hypotheses for the cause of the locked mode in reversed-field pinch (RFP) plasmas is examined using a large RFP machine, TPE-RX. A hypothetical model was proposed [Phys. Plasmas 6 (1999) 3824], which speculates that the halo current in the vacuum vessel might be the cause of the locked mode. We directly measured the toroidal distribution of the vessel current and correlated it with the position and the magnitude of the locked mode. The vessel current is found to flow into the contact point of the last closed flux surface where the locked mode exists. Magnitude of the vessel current is much smaller than the level expected to form the locked mode. From these observations, we conclude that this vessel current is a result of the local shift of the plasma column due to the locked mode, and may not be the cause of the locked mode.

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...

A design and related studies of equilibrium field control coil system of TPE-RX reversed field pinch (RFP) machine are described. Here the equilibrium field control coil system stands for the coils to produce poloidal magnetic field necessary for the plasma equilibrium condition. The configuration and the number of turns of the equilibrium field control coil system were designed so as to minimize the field error for reducing the plasma–wall interaction, especially at the shell gap of the thick metal shell. The design method of the equilibrium field control coil system taking account of an iron core with two yokes and the thick metal shell with the poloidal shell gap is described. As a result of the design of the equilibrium field control coil system, field error at the plasma surface is less than 1.0%. The field error can be canceled for wide range of the plasma equilibrium condition by adding the compensatory vertical field. Construction of TPE-RX was successfully completed at the end of 1997 and the coil system showed the expected performances.

The plasma rotation velocities in both toroidal and poloidal directions are measured by the Doppler shift of the impurity spectral line in a new large reversed-field pinch device, TPE-RX. It is found that the magnitude of the toroidal rotation velocities varies with the plasma current. The poloidal rotation velocities are almost zero in the plasma current region studied here. The radial electric field around the plasma edge region varies with the plasma current under some assumptions. This behavior is discussed in relation to both the finite Larmor radius effect and the magnetic field line stochasticity.

The effects of field errors at the poloidal gap of the thick shell on the plasma performance and the wall-locking are intensively studied. The coil system of TPE-RX (major/minor radii, R /a = 1.72/0.45 m) has several features for cancelling the field errors at the thick shell gap and for controlling the equilibrium position of the plasma column. It is confirmed that these field error cancellation schemes work well for the normal operating conditions in TPE-RX. It is shown quantitatively that the global performance of the revised-field pinch operation is affected if the shell gap field error exceeds about 5% of the poloidal magnetic field. As expected, the probability of the mode to lock at the shell gap increases as the field error increases.

An extensive magnetic measurement system (MMS) which contains 1201 sensors is developed for the new large reversed-field pinch machine, TPE-RX. The system facilitates a variety of magnetic signals for axisymmetric information to obtain equilibrium profiles as well as for nonaxisymmetric information of the magnetic perturbation mode spectra. The system is now routinely in operation for physics experiments on TPE-RX. The physical quantities obtained from the system, locations and specifications of coils and loops, calibration, the method of managing large amounts of data and examples of data are reported.

TPE-RX is a large-sized reversed-field pinch machine newly constructed at the Electrotechnical Laboratory. In this paper the design concepts, procedures to determine the machine size and the flux swing to drive the plasma current, and the prediction of the global confinement properties of the TPE-RX are reported. From these considerations, major and minor radii (R, a) are decided to be R/a=1.7175/0.45 m, respectively. It is estimated that the flux swing of the iron core of more than 3.9 Wb is necessary to drive 1 MA of plasma current, Ip, which is the given constraint of the machine. Energy confinement time in the range of 3–18 ms is predicted to be attained at Ip=1 MA depending on the models and assumptions.

Key design points of the front-end system of TPE-RX reversed-field pinch (RFP) machine are described. Here the front-end system is the components of the machine between the thick shell and the plasma surface and it consists of the vacuum vessel, shell system and pulsed vertical field coil (PVC). The effect of the multi-layered shell system is examined in terms of the relative radial magnetic perturbation. A summary of the port error field and the magnetic field produced by the PVC are also shown. The actual construction procedure is also described. Construction of the TPE-RX was completed at the end of December 1997 and it is now routinely in operation with RFP configuration.

Fast magnetic fluctuation (δB) levels are measured using a newly developed complex edge probe (CEP) in the edge region of TPE-RX reversed-field pinch plasma. The CEP is installed inside a vacuum vessel and is sensitive to fast δB. The δB levels measured using the CEP are compared with those measured using an extensive magnetic measurement system (MMS), which is located outside the vacuum vessel and has little sensitivity to fast δB. The δB levels before and after the appearance of the locked mode (LM) are compared in both the CEP and MMS signals. It was found that the rapid increase in the δB signal level obtained using the MMS just after the appearance of LM is mainly caused by the slowing of plasma rotation.

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The particle confinement time of boron, tau B, is studied in the TPE-1RM20 reversed field pinch plasma by means of a laser ablation technique. The experimental conditions were as follows: pinch parameter, Theta , and plasma current, Ip, are scanned, and tau B obtained from a zero dimensional approximation is compared with the energy confinement time, tau E. The results show that tau B increases almost linearly with tau E, and the implication of this is discussed in terms of energy loss channels. The increase of tau B with Theta supports the recently obtained improved confinement at high Theta . Laser ablation results are compared with the particle confinement time of deuterium, tau p, from the Dalpha line spectrum measurement. It is shown that tau b approximately= tau p, which indicates a weak mass dependence in particle transport. Numerical simulation is also conducted and compared on a one dimensional basis with the experimentally obtained spatial distribution of the line emission intensity of the boron impurity injected by the laser ablation. The result shows reasonable agreement with experiment.

Results of self-organization of the magnetic field and associating plasma loss are reviewed in the reversed field pinch (RFP) experiments on TPE machines. It is shown that the RFP plasma has a strong tendency to relax to a certain magnetic configuration similar to the energy minimum state predicted by Taylor. Thus the RFP configuration is self-organized and self-sustained with appropriate control of experimental conditions. Interestingly, however, it is observed in some cases that the relaxation can take place without the conservation of total magnetic helicity. The mechanisms of the self-organization and associated loss are discussed in some detail. In the low pinch parameter region (< 1.6), the self-organization is continuous and the main loss mechanism seems to be the electron motion along the stochastic magnetic field line caused by the overlapping of multiple modes of the magnetic fluctuations being excited simultaneously. In the high pinch parameter region, self-organization is characterized by the pulse-like relaxation (intermittent in many cases), which seems to destroy the magnetic surface in a certain position of the torus and causes rapid loss of plasma energy. Attempts to improve RFP confinement by controlling the relaxation are described. One example is the improved high-theta mode in TPE-1RM20 and another is the pulsed poloidal current drive in MST.

Discharges with improved plasma confinement in a high pinch parameter region ( Theta ~2) have been discovered in the TPE-1RMSO reversed field pinch experiment. These discharges become possible with careful control of the waveforms of the plasma current and the toroidal magnetic field after intensive discharge cleaning and are called the `Improved High Theta Mode' (IHTM). In these discharges the increase of electron density is distinct when compared with low Theta cases (~1.55). The electron temperature increases slightly with Theta but the ion temperature remains almost constant. Consequently, the poloidal beta increases, from 8% at Theta ~1.55 to 18% at Theta ~2.0, and the energy confinement time increases from 0.2 to 0.4 ms. The fluctuation level of the magnetic field corresponding to the dynamo activities is reduced by about a factor of 3 in IHTM, which may explain this improvement. Possible causes of IHTM are discussed. It is found that the m=1 mode rotates continuously during IHTM, while it always stops rotating just before a large soft X-ray emission crash occurs

A series of the detailed experiments has been conducted in three laboratory plasma devices to measure the dynamo electric field along the equilibrium field line (the {alpha} effect) arising from the correlation between the fluctuating flow velocity and magnetic field. The fluctuating flow velocity is obtained from probe measurement of the fluctuating E x B drift and electron diamagnetic drift. The three major findings are (1) the {alpha} effect accounts for the dynamo current generation, even in the time dependence through a ``sawtooth`` cycle; (2) at low collisionality the dynamo is explained primarily by the widely studied pressureless Magnetohydrodynamic (MHD) model, i.e., the fluctuating velocity is dominated by the E x B drift; (3) at high collisionality, a new ``electron diamagnetic dynamo`` is observed, in which the fluctuating velocity is dominated by the diamagnetic drift. In addition, direct measurements of the helicity flux indicate that the dynamo activity transports magnetic helicity from one part of the plasma to another, but the total helicity is roughly conserved, verifying J.B. Taylor`s conjecture.

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 H-alpha 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. (C) 2004 American Institute of Physics.

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 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

application/pdf, 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.

Zero-dimensional power balance is analyzed, and an operation boundary is deduced in a `beam-assisted reversed-field pinch`; the latter utilizes partial poloidal current drive by neutral beams so that transport losses arising from magnetohydrodynamics (MHD)-dynamo, i.e., tearing mode instability are reduced. Changes of power flow and heat conductivity due to a beam driven current are treated by considering an MHD-dynamo-based power balance model that assumes linear dependence of magnetic fluctuation level on the externally driven current. It is shown that a ratio of a beam driven current to a dynamo current must not exceed nearly 40% regarding a beta-limit in the next generation of plasma experiments (minor radius/major radius = 0.6 m/1.8 m, plasma current = 1 MA, poloidal beta = 0. 1). At that point, the energy confinement time is predicted to increase by a multiple or so of that estimated from the MHD dynamo model without a current drive. 41 refs., 12 figs., 2 tabs.

Fluctuation-induced dynamo electric fields are measured over a wide range of electron collisionality in the edge of TPE-1RM20 reversed-field pinch (RFP). In the collisionless region the magnetohydrodynamic dynamo alone can sustain the parallel current, while in the collisional region a new dynamo mechanism resulting from the fluctuations in the electron diamagnetic drift becomes dominant. A comprehensive picture of the RFP dynamo emerges by combining with earlier results from MST and REPUTE RFPs.

Confinement properties of a reversed field pinch (RFP), TPE-1RM20, are intensively presented. Plasma current, I{sub p}, dependencies of confinement properties are particularly shown in comparison with the forerunner machine, TPE-1RM15. The results without any active density controls are presented in this paper. It is shown that both machines have almost the same, relatively high, I/N values (

Edge plasma fluctuations are studied with inserted triple Langmuir probes and magnetic coils in the TPE‐1RM20 reversed‐field pinch [Y. Yagi et al., in Plasma Physics and Controlled Nuclear Fusion Research 1992 (International Atomic Energy Agency, Vienna, 1993), Vol. 2, p. 611]. Two‐point measurements show that density and potential fluctuations have relatively low mode numbers (m

In the high current density plasma on TPE-1RM15 reversed field pinch, large-amplitude oscillations with m=0, n=0 are observed in soft X-ray (SXR) emission for a wide range of theta values, even at low- theta ( theta

Experimental data on two quite different types of devices, a simple toroidal Z pinch without toroidal field coils and a reversed field pinch (RFP), are analyized by using the partially relaxed state model (PRSM). It is shown that the experimental data on both two devices fit fairly well theoretical curves by the PRSM with common parameter range included in the model, suggesting that some common physical process of relaxation is taking place together with some common loss process.

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, Wurzburg (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 fφ=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 sat...

In this paper we present a statistical study of edge fluctuations taken with the Gas Puffing Imaging (GPI) diagnostics. We carry out a comparison of GPI signal from an extensive database including four devices (two Tokamaks and two Reversed Field Pinches). The data are analyzed in terms of their statistical moments Skewness and Kurtosis, as done in [B. Labit, et al, Phys. Rev. Lett. \textbf{98}, 255002 (2007)]. The data align along parabolic curves, although different from machine to machine, with some spread around the best-fitting curve. A discussion about the meaning of the parabolic trend as well as the departure of real data from it is provided. A phenomenological model is finally provided, attempting to accomodate experimental evidence., To appear in Plasma Physics and Controlled Fusion

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 electron distribution function is statistically examined from measurements by Thomson scattering at +or-7 cm off-axis points (about half of the minor radius, 13.5 cm) from the center of the vacuum vessel of the TPE-1RM15 reversed field pinch (RFP) machine. Four different RFP discharge conditions are examined at both ports on a shot-by-shot basis. The wavelength shift of the distribution function and corresponding poloidal current density are deduced so that the same bulk electron temperatures at both the inside (-7 cm) and outside (+7 cm) ports are obtained. The deviation of the distribution function from the shifted Maxwellian is examined from the difference in fitted temperature with different weighting functions. The results indicate that in two of the experimental groups, the plasma current density carried by the tail electrons above 2.6 keV exceeds that carried by the shifted bulk Maxwellian distribution. For all of the data groups, the ratio of the current density carried by tail electrons to the total current density has a positive correlation with both E/EC (=0.14-0.22) and eta k/ eta S, where EC is the critical electric field for thermal electrons to run away, and eta k and eta S, are the plasma resistivity estimated from the helicity balance equation and Spitzer's formula, respectively. These tail electrons observed in the core plasma of an RFP can be attributed to the origin of the high-energy electrons recently observed at the edge region of RFPs.

The effect of the pressure on the mode transition point of the relaxed state from the cylindrical state to the mixed helical one is investigated, by using the associated eigenvalue problem derived from the reminimization of the energy integral for the nonideal MHD plasma. The eigenvalue which gives the mode transition point is solved to the 1st order approximation by the perturbation method. Numerical results by the eigenvalue show that the pressure effect leads to fairly large shifts of the mode transition points and makes them closer to the experimental operating region reported so far. Comparison between the theoretical and the experimental data shows that the experimental data tend to evolve along the line of constant β ax .

In order to understand the driving mechanisms that underlie the turbulence in the plasma edge of fusion devices, contemporary measurements of light emission fluctuations with the gas puff imaging diagnostic and of the electrostatic potential with Langmuir probes are carried out in the TPE-RX plasma device. The analysis made with the conditional average technique demonstrates a clear link between the two types of structures, with a phase shift of pi/2. This relation suggests that the curvature driven modes are a possible source of free energy for edge turbulence in this machine, as similarly found in other plasma devices.

The MHD instabilities that sustain the reversed-field pinch (RFP) configuration tend to phase-lock together and also to wall-lock, forming a bulging of the plasma column, called "locked mode". This phenomenon is of particular interest, since the locked mode causes a larger plasma resistivity, plasma cooling. and, in some cases, anomalous discharge termination. Up to now, studies of the locked mode have been focused on in = I modes (being in the poloidal mode number). In this Letter we show that in = 0 modes also play a role, based on the cross-check between magnetic spectra and toroidally resolved D, array measurements.

The quasi-single-helicity (QSH) state of a reversed-field pinch (RFP) plasma is a regime in which the RFP configuration can be sustained by a dynamo produced mainly by a single tearing mode and in which a helical structure with well-defined magnetic flux surfaces arises. In this Letter, we show that spontaneous transitions to the QSH regime enhance the particle confinement. This improvement is originated by the simultaneous and cooperative action of the increase of the magnetic island and the reduction of the magnetic stochasticity.

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 (∼−0.2mT) after the setting up phase and the following fast current rising phase. After a certain period at this shallow reversal (∼15–25ms), 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 ∼45ms) 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 susta...

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 5Bf(a)/^Bf&. Two main basins of high QSH probability have been found. The first one, at high current and low uFu, 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.

Summary form only given, as follows. Summary form only given. Every kind of plasma instability exists in a reversed field pinch (RFP) plasma, resulting in the behavior of plasma dynamics becoming very complex. In this process magnetic flux is self-generated by a turbulent electric field, which is deeply concerned with plasma instabilities. In a RFP plasma, this self-organization process plays an essential role in plasma formation. We describe here an experimental study of chaotic behavior in RFP plasmas. To investigate the property of the self-organization process in RFP plasma, the concept of chaos is applied to the analysis of its fluctuation signal. Although in past years, research regarding the chaotic phenomena in RFP plasma had concentrated on making sure that chaos exists in RFP plasma or not, low dimensional chaos was not still found. In our research, to investigate the chaotic behavior in RFP plasma in detail, we divide fluctuation data into several frequency bands and chaotic analysis is applied to these divided fluctuation signals.

This paper reports the study of the magnetic field error due to the port holes of the passive stabilizing shell of a Reversed-Field Pinch (RFP) machine, TPE-RX, now operating at the Electrotechnica...