Reversed-field Pinch Machine Research Articles

First measurement of confinement properties in TPE-RX reversed field pinch

Electron and ion temperatures Te0 and Ti, respectively, are measured and global confinement properties are estimated at Ip ⩽ 400 kA for the first time in the TPE-RX reversed field pinch (RFP) using a single point Thomson scattering system and a single chord neutral particle energy analyser, respectively. The values of Te0 and Ti range from 0.2 to 1 keV and from 0.2 to 0.4 keV, respectively, at Ip ∼350 kA. The I/N value (N is the line density) is relatively high ( = 12 × 10-14A m) in comparison with those of other RFP machines. The poloidal beta βp and energy confinement time τE are comparable to those in similarly sized RFP plasmas such as the MST and RFX plasmas, namely, βp = 5-10% and τE = 0.5-1.0 ms. The behaviour of the global confinement properties and the implication of thisresult are discussed.

Effects of shell configuration on the performance of reversed field pinch plasmas

The experimental results of three different reversed field pinch machines at the Electrotechnical Laboratory in Tsukuba are compared. In particular, the results are summarized in terms of the difference in the configuration of the conductive shell system. The three configurations correspond to TPE-1RM15 (b/a = 1.18), TPE-1RM20 (b/a = 1.08) and TPE-1RM20mod (b/a = 1.12), where b/a is a measure of the shell proximity, and a and b are the minor radii of the plasma and the inner surface of the shell closest to the plasma, respectively. The magnetic fluctuations, global confinement properties and mode locking events induced by the external field error at the thick shell gap are studied. It is shown that the magnetic fluctuation amplitude increases with b/a, as expected from the linear growth rate of the dominant MHD modes, and that a multilayered, close fitting shell system improves the performance under extreme operating conditions in the high pinch parameter region and when external field errors are considered, while the performance under normal operating conditions is comparable within the shot to shot deviations of the data.

Multipoint vessel-temperature monitoring system for TPE-RX

A multipoint, vessel-temperature monitoring system is developed for a large reversed-field pinch machine, the TPE-RX at the Electrotechnical Laboratory. It consists of a total of 268 thermocouples attached mainly on the outer surface of the vacuum vessel and a multipoint, stand-alone data acquisition system. Simultaneous data sampling for all the signals every 0.5 s is possible with the data acquisition system dedicated to this purpose. This system is used to clarify the spatial distribution of the heat load on the first wall around a torus from a magnetically confined plasma. The entire system is currently used in routine operations and has been proven to be very effective for the detection of a localized plasma–wall interaction due to plasma distortion. Details of the specifications, characteristics and typical examples are reported.

Extensive magnetic measurement system for TPE-RX

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.

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

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, I p, which is the given constraint of the machine. Energy confinement time in the range of 3–18 ms is predicted to be attained at I p=1 MA depending on the models and assumptions.

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

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.

Search for low-dimensional chaos in a reversed field pinch plasma

An analysis of experimental data from the reversed field experiment (RFX) reversed field pinch machine [L. Fellin, P. Kusstatscher, and G. Rostagni, Fusion Eng. Des. 25, 315 (1995)] is carried out to investigate the possible existence of deterministic chaos in the edge plasma region. The mathematical tools used include Lyapunov exponents, Kaplan–Yorke dimension, minimum embedding dimension estimates and nonlinear forecasting. The whole analysis agrees in ruling out the possibility of low-dimensional chaos: The dimension of the underlying dynamical system is estimated to be >10. From a critical rereading of the literature it emerges that the findings of this work are likely to be common to all reversed field pinches.

Energy distribution of deuterium atoms recycling on molybdenum limiter surface

The emission spectrum of deuterium atoms (D α) was measured at the surface of a movable molybdenum limiter in the reversed field pinch machine (RFP: TPE-1RM20). The intensity of the D α line increased with the insertion depth of the limiter. The observed line profile was fitted by a combination of two Gaussian functions with different temperatures. The temperatures derived from the fitted functions were approximately 5000 K and 50 000 K. The temperatures were independent of the insertion depth. No Doppler shifted shoulder, which would correspond to neutralized reflected ions at the limiter surface, was observed in the present experiment.

Methods of magnetic measurements in the RFX fusion experiment

The Magnetic Measurement System of the Reversed Field Pinch machine RFX for research on Thermonuclear Fusion is briefly presented and the main magnetic analyses performed, on the basis of experimental data, are discussed. These analyses lead to significant results for plasma physics comprehension.

High heat load test of molybdenum

Three different types of molybdenum, powder metallurgical polycrystalline (PM-Mo), and as-forged polycrystalline and single crystalline of highly purified electron-beam-melted Mo (AFEB-Mo and SCEB-Mo), have been subjected to high heat load test with neutral beam injection (NBI) stands at Japan Atomic Energy Research Institute (JAERI) and National Institute for Fusion Science (NIFS). These materials have also been tested as a movable limiter in a reversed field pinch machine (RFP: TPE-1RM15) in Electrotechnical Laboratory (ETL). The results are summarized as follows. The SCEB-Mo shows the least damage with slight local melting after a very high heat load of 260 MW/m 2 for 250 ms with NBI, while for the PM-Mo the whole irradiated area melt with many crater due to impurity gas evaporation under less heat load (200 ms). All movable limiter heads of the RFP are severely damaged with partial melting. The appearance of the SCEB-Mo limiter after melting is not good and shows the crystalline cleavage. However, SEM observation of the microstructure opposes the surface appearance. In the SCEB-Mo, appreciable recrystallization is not observed and hence no crack is seen to go into the bulk except the crystalline cleavage. In the PM-Mo, on the other hand, the resolidification to columnar grains as well as the recrystallization is apparent, and the cracks not only go along the columnar grains but also separate the recrystallized region from the matrix. In the AFEB-Mo, a slight grain growth occurs and several cracks enter deep along the grain boundaries. Thus the SCEB-Mo is a very nice plasma-facing material if used under the critical heat load for melting.

First Results of Boronization in REPUTE-1 RFP

The first boronization in a reversed field pinch machine has been carried out and its effects on plasma performance have been studied. From a residual gas analysis, it was found that total and partial pressures were reduced. During the first several tens of discharges, the mean plasma density normalized by a filling pressure became much higher and was nearly constant during a discharge, but these phenomena became weaker shot by shot. Radiated power, plasma resistance, CrI line intensity and high-frequency magnetic oscillations decreased. The ion temperature from Doppler broadening of the CV line changed only slightly, whereas lower temperature from the OV line was found.

Application of microwave reflectometry for plasma density diagnostics in a reversed-field pinch machine

The implementation of reflectometry as a means of measuring plasma density is examined in relation to the reversed-field pinch machine. The effects of magnetic shear and density fluctuations are studied. Ray-tracing and simulation results are presented and the proposed reflectometer system for the RFX reversed-field pinch machine is described.

Automatic laser beam alignment for a Thomson scattering system

A system for the automatic alignment of a pulsed ruby laser beam is under development for the Thomson scattering diagnostics of RFX, a large reversed-field pinch machine now under construction. In this experiment the laser will be 11 m from the machine and the beam alignment at the 750×4-mm scattering volume will be actively maintained to within 0.5 mm. The beam direction in space is measured in two reference planes fixed to the collection optics frame by means of two 80-mm2 fast quadrant photodiodes. A double-channel preamplifier is used for each quadrant in order to measure both the 30-ns FWHM ruby pulse and a low-power cw He–Ne beam propagating through the same optical path. Every time the main laser is fired, the relative direction of the two beams is determined so that the position of the He–Ne beam can be used for feedback control of the steering optics between plasma shots.

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 A·cm−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.