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Turbulence in the reversed field pinch (RFP) plasma has been investigated by using the microwave imaging reflectometry in the toroidal pinch experiment RX (TPE-RX). In conventional RFP plasma, the fluctuations are dominated by the intermittent blob-like structures. These structures are accompanied with the generation of magnetic field, the strong turbulence, and high nonlinear coupling among the high and low k modes. The pulsed poloidal current drive operation, which improves the plasma confinement significantly, suppresses the dynamo, the turbulence, and the blob-like structures.

We have been studied the advanced Thomson scattering diagnostics from viewpoints of new concepts, laser technology and spectrum analysis. This paper summarizes results of development on technologies for advanced Thomson scattering diagnostics.

The confinement properties of a reversed-field pinch ( RFP) device are limited by the high level of magnetic stochasticity produced by the interaction of tearing modes in the plasma core. Away to reduce the core magnetic stochasticity is the application of the pulsed poloidal current drive ( PPCD) technique. By applying the PPCD, tearing modes are stabilized, magnetic stochasticity and turbulence are reduced and hence the confinement is enhanced. In the TPE-RX device, standard RFP plasmas are characterized by magnetic and electrostatic edge turbulence dominated by intermittent phenomena; on the contrary, the improved confinement regime generated by the PPCD is characterized by turbulence with a self-similar nature. We show that the increase in tau(p) is well correlated with the change in the nature of both the magnetic and the electrostatic turbulence. While plasmas with low confinement are dominated by intermittent events and by magnetic and electrostatic activity with a non-self-similar nature, plasmas with high confinement show turbulence with a self-similar character., QC 20120221

The cold pulse propagation is a powerful tool for the study of heat transport and the determination of perturbative electron heat diffusivity chi(CP)(e). In this paper the experimental study of cold pulses generated by pellet injection in a reversed-field pinch device is described. The most common phenomenology is an inward cold pulse propagation, from the edge to the core; however, we will describe that outward propagation can occur if the pellet ablates up to the plasma centre. A model to simulate the dynamics of the pulse (i.e. the relation between the arrival time of the pulse and its radial position) has been developed. With this model it is possible to explain the cold pulse phenomenologies as well as to estimate chi(CP)(e) in the plasma core. The heat diffusivity in standard and enhanced confinement plasmas has been determined and compared; in standard regimes chi(CP)(e) is three times larger than in enhanced confinement regimes. Finally, it has been possible to confirm a scaling between diffusivity and magnetic stochasticity., QC 20120221

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., QC 20120221

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 , 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., QC 20120221

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, tau(E), was deduced for a consistently selected set of the database (Yagi Y. et al 2003 Nucl. Fusion 43 1787). The scaling for tau(E) [TPE-scaling; tau(E) similar to a(1.7)I(p)(0.8) (I-p/N)(0.3)Theta(3)], in particular, is a power law similar to that predicted on the basis of the tearing modes, P as a function of the plasma minor radius (a), plasma current (I-p), and I-p/N, where N is the column density. On the other hand, the TPE-scaling has a strong dependence on the pinch parameter, Theta, namely tau(E) similar to Theta(3). 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 tau(E) in the PPCD database, because of its strong Theta dependence. Namely, Theta(3) 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 Theta(3), is related to the increase in magnetic shear with Theta., QC 20120131

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., QC 20120202

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., QC 20120221

Factors affecting the magnitude of sawtooth crashes in soft x-ray (SXR) signals (I-SXR) have been studied using data from high-Theta (pinch parameter) discharges in the toroidal pinch experiment TPE-1RM20 reversed-field pinch device [Y. Yagi , J. Plasma Fusion Research 69, 700 (1993)]. In TPE-1RM20, discharges with a high Theta that are free of large sawteeth (improved high-Theta mode discharges), spontaneously appeared [Y. Hirano , 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, [Delta(I-SXR)/I-SXR], is well correlated with the m (poloidal mode number)=0 mode amplitude in the postcrash phase. It is also shown that Delta(I-SXR)/I-SXR is smaller when the quasi-single helicity state precedes the crash rather than when the multiple-helicity state precedes the crash, and Delta(I-SXR)/I-SXR is smaller when E-parallel to (the parallel electric field at the plasma surface) is approximately zero in the precrash phase rather than E-parallel to being negative., QC 20120131

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 , 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-parallel to. The positive trend between E-parallel to 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., QC 20120221