Haruhisa Koguchi

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.

Mode-locking phenomena in the TPE-RX reversed-field pinch plasma

The characteristics of the phase- and wall-locked mode found in a large-sized reversed-field pinch (RFP) machine TPE-RX [Y. Yagi et al. Plasma Phys. Controlled Fusion 41, 255 (1999)] are described in detail. The toroidally-localized radial magnetic field starts to grow after the setup of the RFP configuration in a current rising phase, reaching up to 2% of the poloidal magnetic field at the plasma surface, and stays at the same toroidal location throughout the discharge. The mode frequently locks to the thick shell gap position with a 20%–30% probability. The plasma–wall interaction is enhanced at the locked position where the thermal wall load is peaked by a factor of about 3 on average. The locked mode disappears in some experimental conditions. The probability for the locked mode to appear depends on the experimental conditions, especially on the filling pressure of the fueling gas and on the rise time of the plasma current. Possible causes of the locked mode are discussed from the braking effect of th...

Improved confinement in the TPE-RX RFP by means of the PPCD

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) <<τD(0) (resistive diffusion time of the core) holds, allows a trajectory with a deeper F which yields a less turbulent configuration than shot-by-shot F-Θ scans. It is shown that the improvement ratio of τE approximately scales as δb-2 for five cases of the PPCD experiments in three RFP machines, including the present work in TPE-RX.

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.

Extensive magnetic measurement system for TPE-RX

Abstract 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.

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...

Tumor-selective mitochondrial network collapse induced by atmospheric gas plasma-activated medium

Non-thermal atmospheric gas plasma (AGP) exhibits cytotoxicity against malignant cells with minimal cytotoxicity toward normal cells. However, the mechanisms of its tumor-selective cytotoxicity remain unclear. Here we report that AGP-activated medium increases caspase-independent cell death and mitochondrial network collapse in a panel of human cancer cells, but not in non-transformed cells. AGP irradiation stimulated reactive oxygen species (ROS) generation in AGP-activated medium, and in turn the resulting stable ROS, most likely hydrogen peroxide (H2O2), activated intracellular ROS generation and mitochondrial ROS (mROS) accumulation. Culture in AGP-activated medium resulted in cell death and excessive mitochondrial fragmentation and clustering, and these responses were inhibited by ROS scavengers. AGP-activated medium also increased dynamin-related protein 1-dependent mitochondrial fission in a tumor-specific manner, and H2O2 administration showed similar effects. Moreover, the vulnerability of tumor cells to mitochondrial network collapse appeared to result from their higher sensitivity to mROS accumulation induced by AGP-activated medium or H2O2. The present findings expand our previous observations on death receptor-mediated tumor-selective cell killing and reinforce the importance of mitochondrial network remodeling as a powerful target for tumor-selective cancer treatment.