Yoshiki Maejima

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...

Self-organization and its effect on confinement in a reversed field pinch plasma

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.

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.

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.

Partially Relaxed Minimum Energy States of Reversed-Field-Pinch Plasma

An energy principle is formulated for construction of toroidal equilibria in partially relaxed minimum energy states. It is shown that additional global constraint yields an additional condition on the force-free current term of the MHD equilibrium equation. It is clarified that the partially relaxed state model (PRSM) for toroidal plasmas, such as the tokamak plasma and the reversed-field-pinch plasma (RFP plasma), represents the minimum-energy states with finite β-values under global invariants. Comparisons of numerical results by the PRSM for the RFP plasma with two typical experimental data indicate that RFP configurations close to the PRSM are realized in the two RFP experiments. The most advanced partially relaxed state with finite β-values is also discussed.

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.

Plasma current and energetic electrons in the core plasma of a reversed field pinch

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 Spitzers 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.