Shigeyuki Sekine

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.

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...

Evolution process of the mode wall-locking and phase-locking in a reversed-field pinch plasma

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...

Excimer laser ablation of cryogenic NO2 films

Cryogenic NO2 films were ablated by a XeCl excimer laser. Only NO2 molecules were detected by a time-of-flight method. Velocity distribution of ablated NO2 molecules was found to be almost thermal. The derived temperature of the fitted Maxwell-Boltzmann distributions depended on the thickness of cryogenic NO2 film in the range from 300 K to 1100 K. The fastest NO2 observed in the present experiment had a translational energy of 0.5 eV.

Phase- and Wall-Locked Modes Found in a Large Reversed-Field Pinch Machine, TPE-RX

Phase- and wall-locked modes are found to exist in the toroidal pinch experiment called TPE-RX, a large-sized reversed-field pinch machine at the Electrotechnical Laboratory. General characteristics of the locked mode in TPE-RX are similar to those in the reversed-field pinch experiment called RFX, in spite of the difference in the vacuum vessel and the shell system. It is also found that there are certain experimental conditions under which phase-locking disappears. These experimental observations of the locked mode in TPE-RX can provide us with a general understanding of the cause of the locked mode in RFP plasmas.