Y. Hirano

Overview of quasi-single helicity experiments in reversed field pinches

We report the results of an experimental and theoretical international project dedicated to the study of quasi-single helicity (QSH) reversed field pinch (RFP) plasmas. The project has involved several RFP devices and numerical codes. It appears that QSH spectra are a robust feature common to all the experiments. Our results expand and reinforce the evidence that the formation of self-organized states with one dominant helical mode (Ohmic SH state) is an approach complementary to that of active control of magnetic turbulence to improve confinement in a steady state RFP.

Design concept and confinement prediction of TPE-RX reversed-field pinch device

Abstract 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.

Front-end system of the TPE-RX reversed-field pinch machine

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.

On the statistics of edge fluctuations: comparative study between various fusion devices

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 analysed in terms of their statistical moments Skewness and Kurtosis, as done in B Labit et al (2007 Phys. Rev. Lett. 98 255002). 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 accommodate experimental evidence.

Edge plasma fluctuations and transport in a reversed‐field pinch

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<3, n<40). High coherence (γ=0.5) with magnetic field fluctuations and similar mode spectra suggest that density and potential fluctuations are mainly caused by electromagnetic turbulence. Broadband magnetic fluctuations are dominated by m=0, low‐n modes and internally resonant m=1 and m=2 modes. A coherent (f=20–30 kHz) m=0, low‐n mode is also observed. Particle flux driven by electrostatic electric field fluctuations is 50%–100% of total flux obtained from Dα line intensity measurement. Low‐frequency fluctuations (f<100 kHz) give the main contribution to the total flux. Electrostatic fluctuation driven electron energy flux is on...

The first results of TPE-RX, a large reversed-field pinch machine

The first experimental results of a large reversed-field pinch machine, TPE-RX, are reported. A reversed-field pinch configuration in TPE-RX was successfully obtained in March 1998. The highest plasma current, Ip, of 480 kA and the longest pulse duration time of 70 ms have so far been obtained separately. A minimum loop voltage of about 15 V is obtained at Ip=150-250 kA. A locked mode has been found to exist in TPE-RX from the magnetic and vessel-temperature measurements, while the C++ Doppler spectrum shows a finite toroidal rotation.

Increased confinement improvement in a reversed-field pinch using double-pulsed poloidal current drive

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...

Optical and electrical diagnostics for the investigation of edge turbulence in fusion plasmas

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.

Improved stability period in high-current-density operation of reversed-field pinch on ETL-TPE-1R(M)

The improved stability period of a reversed-field-pinch (RFP) configuration with high current density (500 Acm−2, on average) is well understood on TPE-1R(M) with a metal liner whose size is the smallest among the RFP experimental devices being operated at present (r/R = 9/50 cm unit). Plasma parameters of Ip = 130 kA (duration time about 1 ms), ne0 = 6 × 1013 cm−3, Te0 ≈ 600 eV, and βp ≈ 10% are obtained. The improved stability period is characterized by a remarkably reduced fluctuation level (δBp/Bp ≈ 1%) and a steady improvement of plasma confinement. Possible explanations for the interesting phenomena in the setting-up phase and the improved stability period of TPE-1R(M) are given, i.e. relaxation phenomena, toroidal-flux enhancement within the plasma, and current disruption. The scaling of electron temperature and density, depending on the plasma current in the improved stability period, are described with a discussion of the energy confinement time. The entire performance of TPE-1R(M) is discussed in relation to that of other RFP machines.

Self-organization towards helical states in the Toroidal Pinch Experiment reversed-field pinch

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.