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The TPE-RX Neutral Beam Injector, which provides a 25 keV positive ion beam energy with a maximum current of 50 A for a pulse duration of 30 ms, will be installed on RFX-mod thanks to the agreement with the AIST Institute of Tsukuba (Japan). The main scientific objective is the study of the behavior of the fast ions, which in the RFP helical equilibrium have exhibited very long confinement times. The integration of TPE-RX NBI on RFX-mod requires the development of several new components: a mechanical interface between the RFX-mod vacuum vessel and neutralizer; a Magnetic Residual Ion Dump; a new vacuum pumping system designed to maximize pumping and minimize beam stopping due to reionization. As regards the power supplies the compliance of the Japanese equipment to the Italian safety rules has been considered and layout studies have been carried out; the integration of the NBI control system in the RFX timing sequence has been studied as well.

In a low-temperature atmospheric pressure plasma jet using helium gas, emission-propagation phenomena, such as streamers and striations were measured using a high-speed intensified charge-coupled device camera. A particular focus was placed on the study of the dependence of the phenomena on the distance between the nozzle of the plasma device and a target plate. When the distance decreased, a transition from the positive streamer to a spatially continuous emission resulted. A further distance reduction resulted in a new propagation mode in which the positive and negative streamers appeared alternately with different current waveforms over two cycles of applied voltage. This phenomenon may be related to residual charges of the preceding cycle when streamer propagation begins. Striation structures were observed in the tail of the positive streamer head and in the successive spatially continuous-emission region. These structures can be measured only within a shorter period than one voltage cycle.

The confinement properties of a reversed-field pinch (RFP) device are limited by the high level of magnetic stochasticity produced by the interaction of tearing modes in the plasma core. A way to reduce the core magnetic stochasticity is the application of the pulsed poloidal current drive (PPCD) technique. By applying the PPCD, tearing modes are stabilized, magnetic stochasticity and turbulence are reduced and hence the confinement is enhanced. In the TPE-RX device, standard RFP plasmas are characterized by magnetic and electrostatic edge turbulence dominated by intermittent phenomena; on the contrary, the improved confinement regime generated by the PPCD is characterized by turbulence with a self-similar nature. We show that the increase in τp is well correlated with the change in the nature of both the magnetic and the electrostatic turbulence. While plasmas with low confinement are dominated by intermittent events and by magnetic and electrostatic activity with a non-self-similar nature, plasmas with high confinement show turbulence with a self-similar character.

An investigation of the edge region in a large reversed-field pinch (RFP) device has been undertaken for the first time using a high time resolution gas-puff imaging (GPI) system, applied to the TPE-RX experiment. The GPI diagnostic is aimed at the investigation of turbulence in the edge region under the severe heat flux conditions of thermonuclear fusion-grade plasmas, where conventional electrostatic probes cannot be used. To compare the data obtained by the GPI during low power experiments, a retractable array of electrostatic probes has been added. The whole diagnostic system provided high quality measurements of fluctuations with fine spatial and temporal resolution, as well as radial profiles of relevant plasma parameters. The measurements confirm several important properties of plasma edge already found in other experiments, like the presence of a strong ExB shear layer and the 'intermittent' behaviour of the measured quantities, which display non-Gaussian fluctuation spikes at different time scales. Along with the conventional power spectra and correlation analyses, the application of continuous wavelet transform techniques to GPI data allowed a detailed description of the spatial structure and of the propagation of these localized 'intermittent events'.

A reversed-field pinch (RFP) start-up assisted by a magnetized plasma flow injection was demonstrated for the first time on a TPE-RX machine. This sequence of experiments aimed to establish a new method of ionization, gas-fill and helicity injection in the start-up phase of an RFP. In this start-up method, magnetized and well-ionized plasma is formed by a magnetized coaxial plasma gun and injected into the torus chamber as an initial pre-ionized plasma for RFP formation. In the initial experiments, attenuated density pump-out and comparatively slow decay of the toroidal flux and plasma current were observed as evidence of its being an effective start-up method.

A spectroscopic system equipped with image magnifying optics has been developed for measuring a time-resolved plasma velocity at one discharge of a reversed-field-pinch plasma of the TPE-RX at National Institute of AIST. This spectroscopic system enables a simultaneous measurement from two different lines of sight, using two sets of quartz optical fiber bundles. This system provides a precise measurement of Doppler shift without the need for a calibration of the central location of a spectral line. Two fiber bundles are coupled to the entrance slit of a modified Czerny–Turner-type, 1-m-focal-length spectrometer with an 82×82 mm2 grating. In order to adjust the spectral image of the exit slit to each surface of two sets of one-dimensional detector arrays, an optical system equipped with a toroidal mirror, a cylindrical mirror, and splitting mirrors has been developed. The focal lengths of the toroidal mirror are selected for focusing vertically on the splitting mirror and horizontally (wavelength direction) on detectors. Plasma rotation and ion temperature for O V impurity ions (278.1 nm) are successfully measured at one plasma shot with a 25-μs time resolution in the TPE-RX.

This paper reports the main recent results of the RFX-mod fusion science activity. The RFX-mod device is characterized by a unique flexibility in terms of accessible magnetic configurations. Axisymmetric and helically shaped reversed-field pinch equilibria have been studied, along with tokamak plasmas in a wide range of q(a) regimes (spanning from 4 down to 1.2 values). The full range of magnetic configurations in between the two, the so-called ultra-low q ones, has been explored, with the aim of studying specific physical issues common to all equilibria, such as, for example, the density limit phenomenon. The powerful RFX-mod feedback control system has been exploited for MHD control, which allowed us to extend the range of experimental parameters, as well as to induce specific magnetic perturbations for the study of 3D effects. In particular, transport, edge and isotope effects in 3D equilibria have been investigated, along with runaway mitigations through induced magnetic perturbations. The first transitions to an improved confinement scenario in circular and D-shaped tokamak plasmas have been obtained thanks to an active modification of the edge electric field through a polarized electrode. The experiments are supported by intense modeling with 3D MHD, gyrokinetic, guiding center and transport codes. Proposed modifications to the RFX-mod device, which will enable further contributions to the solution of key issues in the roadmap to ITER and DEMO, are also briefly presented. © 2017 IAEA, Vienna.

In remodeling the RFP (Reversed Field Pinch) device TPE-2M [Y. Sato, K. Hayase, S. Kiyama, Realization of fast reversal time for toroidal field in TPE-2M, Proc. 20th SOFT (1998) pp. 707–710.], we adopted the thick stainless steel (SUS) vacuum chamber with minimized insulation gap/cut (one toroidal gap and no poloidal cut), which is expected to be highly resistant against a heavy heat flux, instead of a thin bellows chamber. Furthermore, the SUS chamber, combined with an aluminum shell, will increase the wall stabilization effect for equilibrium and fluctuation of RFP plasma. In this paper, the design and construction of new front-end system and the first results from testing and operation are described.

Poloidal and toroidal rotations of the reversed field pinch plasmas induced by the magnetohydrodynamic resistive tearing modes are numerically studied by using the ohm's law with the ion inertia term. The modes with the poloidal mode number m =1, which mainly grow around the center in the cylindrical plasma are taken into account. In the case of the non-resonant mode, the direction of the induced toroidal rotation velocity of the plasma around the center is same as that of the toroidal magnetic field. The toroidal velocity at the center is decelerated about the time when the magnetic energy of the unstable mode reaches the nonlinear saturation level. The direction of the toroidal velocity of the mode at the boundary is opposite to that of the plasma around the center before the time the instability reaches the nonlinear saturation level. In the case of the resonant modes the reversal of the rotations appears around the singular point. In the multi-mode case the behavior of the plasma rotations shows such ...

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.

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.

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.

The dynamical process of pulsed laser ablation is investigated by means of Monte Carlo simulations in order to develop a new cluster source. The interaction between the ablated Cu vapor plume and the He ambient gas induces strong shock compression of the gas by which the gas density is one order of magnitude increased and forms a stable gas layer mixed with the plume at the contact front. It is found that even with the target-wall distance as short as about 80 μm, a mixed layer more than 10 μm in thickness can be formed at about 15 μm away from the source wall. This layer can be locally-confined for a considerable long time of hundreds of ns. Frequent collisions occur among the atoms of vapor and gas within the confined layer which induce effective cluster aggregation. The thermodynamic state of this region is well defined, so that the mono-dispersed growth condition might form cluster beam with well-defined internal freedom.

Fast magnetic fluctuation (δB) levels are measured using a newly developed complex edge probe (CEP) in the edge region of TPE-RX reversed-field pinch plasma. The CEP is installed inside a vacuum vessel and is sensitive to fast δB. The δB levels measured using the CEP are compared with those measured using an extensive magnetic measurement system (MMS), which is located outside the vacuum vessel and has little sensitivity to fast δB. The δB levels before and after the appearance of the locked mode (LM) are compared in both the CEP and MMS signals. It was found that the rapid increase in the δB signal level obtained using the MMS just after the appearance of LM is mainly caused by the slowing of plasma rotation.

A method for blood coagulation using low-energy atmospheric-pressure plasma (LEAPP) is confirmed as an alternative procedure to reduce tissue damage caused by heat. Blood coagulation using LEAPP behaves differently depending on working gas species; helium is more effective than argon in promoting fast coagulation. To analyse the difference in reactive species produced by helium and argon plasma, spectroscopic measurements were conducted without and with a target material. To compare emissions, blood coagulation experiments using LEAPP for both plasmas were performed under almost identical conditions. Although many kinds of reactive species such as hydroxyl radicals and excited nitrogen molecules were observed with similar intensity in both plasmas, intensities of nitrogen ion molecules and nitric oxide molecules were extremely strong in the helium plasma. It is considered that nitrogen ion molecules were mainly produced by penning ionization by helium metastable. Near the target, a significant increase in the emissions of reactive species is observed. There is a possibility that electron acceleration was induced in a local electric field formed on the surface. However, in argon plasma, emissions from nitrogen ion were not measured even near the target surface. These differences between the two plasmas may be producing the difference in blood coagulation behaviour. To control the surrounding gas of the plasma, a gas-component-controllable chamber was assembled. Filling the chamber with O2/He or N2/He gas mixtures selectively produces either reactive oxygen species or reactive nitrogen species. Through selective treatments, this chamber would be useful in studying the effects of specific reactive species on blood coagulation.

A low-energy ion beam system operating at a dc voltage of less than 300 V was developed using an ion source with a multicusp magnetic field. A high-current-density ion beam of 6.9 mA∕cm(2) was successfully extracted at the electrode. The beam extraction characteristics for flat and concave electrodes were compared. In the case of a concave electrode with a designed focal length of 350 mm, it was observed that the beam profile was sharper than that obtained using a flat electrode.

The MW level TPE-RX Neutral Beam Injector is planned to be installed on the RFX-mod Reverse Field Pinch (RFP) experiment. The main purpose is to study the behavior of the fast ions, which in the helical equilibrium of the RFP have been seen to feature very long confinement times. Despite the many differences in several aspects, the use of a NBI on RFX has to face similar problems as the ITER NBI on ITER In both cases for instance, the stray poloidal magnetic field generated by the machine, has an impact on the transmission of the neutral beam heating power.

This paper summarizes the main achievements of the RFX fusion science program in the period between the 2008 and 2010 IAEA Fusion Energy Conferences. RFX-mod is the largest reversed field pinch in the world, equipped with a system of 192 coils for active control of MHD stability. The discovery and understanding of helical states with electron internal transport barriers and core electron temperature >1.5 keV significantly advances the perspectives of the configuration. Optimized experiments with plasma current up to 1.8 MA have been realized, confirming positive scaling. The first evidence of edge transport barriers is presented. Progress has been made also in the control of first-wall properties and of density profiles, with initial first-wall lithization experiments. Micro-turbulence mechanisms such as ion temperature gradient and micro-tearing are discussed in the framework of understanding gradient-driven transport in low magnetic chaos helical regimes. Both tearing mode and resistive wall mode active control have been optimized and experimental data have been used to benchmark numerical codes. The RFX programme also provides important results for the fusion community and in particular for tokamaks and stellarators on feedback control of MHD stability and on three-dimensional physics. On the latter topic, the result of the application of stellarator codes to describe three-dimensional reversed field pinch physics will be presented. © 2011 IAEA, Vienna.

A high poloidal beta, βp, was achieved using pulsed poloidal current drive (PPCD) in a toroidal pinch experiment-RX (TPE-RX). The plasma electron density and temperature increased, hence improving βp from 5 to 30% during PPCD. βp is almost equal to the total beta in the reversed-field pinch (RFP). D-alpha emission from recycling deuterium by the plasma-wall interaction decreased during the PPCD. An improved particle confinement time was indicated by a ten fold increase in the ratio of the total number of particles to the D-alpha emission. Single pellet injection into the good particle confinement plasma, achieved by the PPCD, produced high density plasma with high beta values. The plasma electron density rapidly increased to triple that of standard plasma, and high density was maintained till the end of the PPCD period. The electron temperature was lower than that of PPCD without ice pellet injection, but βp remained almost the same because the plasma electron density was higher with pellet injection.

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 analyzed in terms of their statistical moments Skewness and Kurtosis, as done in [B. Labit, et al, Phys. Rev. Lett. \textbf{98}, 255002 (2007)]. 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 accomodate experimental evidence., To appear in Plasma Physics and Controlled Fusion

A strongly focusing high-intensity He(+) ion source equipped with three concave electrodes has been designed and constructed as the beam source for a high-energy He(0) neutral beam probe system to diagnose fusion-produced alpha particles in thermonuclear fusion plasmas. The reduction of heat load onto the concave extraction electrodes is particularly important for a long pulse operation, as the heat load deforms the electrodes and thus the beam focal length. The effects on the arc efficiency (beam current/arc power) of the ion source due to the discharge filament structure (straight-type and L-shape-type filaments), size (filament diameters of 2 and 1.5 mm), number, and the locations have been studied. Choice of the appropriate filament structure improved the arc efficiency by 17%.

In order to understand the driving mechanisms that underlie the turbulence in the plasma edge of fusion devices, contemporary measurements of light emission fluctuations with the gas puff imaging diagnostic and of the electrostatic potential with Langmuir probes are carried out in the TPE-RX plasma device. The analysis made with the conditional average technique demonstrates a clear link between the two types of structures, with a phase shift of pi/2. This relation suggests that the curvature driven modes are a possible source of free energy for edge turbulence in this machine, as similarly found in other plasma devices.

To understand a strong focusing phenomenon that occurs in a low-energy hydrogen ion beam, the electron temperature, the electron density, and the space potential in an ion source with cusped magnetic fields are measured before and after the transition to the focusing state using an electrostatic probe. The experimental results show that no significant changes are observed before or after the transition. However, we found unique phenomena that are characterized by the position of the electrostatic probe in the ion source chamber. Specifically, the extracted ion beam current density and energy are obviously enhanced in the case where the electrostatic probe, which is covered by a dielectric material, is placed close to an acceleration electrode.

To understand the mechanism of turbulent enhancement phenomena of a neutral gas flow containing plasma ejected from the nozzle of plasma equipment, the schlieren optical method was performed to visualize the neutral gas behavior. It was confirmed that the turbulent starting point became closer to the nozzle exit, as the amplitude of discharge voltage (electric field) increased. To study the effect of electric field on turbulent enhancement, two sets of external electrodes were arranged in parallel, and the gas from the nozzle was allowed to flow between the upper and lower electrodes. It was found that the neutral gas flow was bent, and the bending angle increased as the amplitude of the external electric field increased. The results obtained using a simple model analysis roughly coincide with experimental data. These results indicate that momentum transport from drifted ions induced by the electric field to neutral particles is an important factor that enhances turbulence.

The formation of multiple stationary striations between a nozzle exit and a conductive target plate was clearly observed at regular intervals using a digital camera along an atmospheric pressure plasma jet of dielectric barrier discharge using a neon gas into ambient air. From the results of measuring using a high-speed camera during the positive current phase, the emission initially started in the middle between the nozzle and the target, and striations progressed in both upward and downward directions. During the negative current phase, the emission initially started in a region near the target, and the striations rapidly progressed to the nozzle.

A high current density (≈3 mA/cm(2)) hydrogen ion beam source operating in an extremely low-energy region (E(ib) ≈ 150-200 eV) has been realized by using a transition to a highly focused state, where the beam is extracted from the ion source chamber through three concave electrodes with nominal focal lengths of ≈350 mm. The transition occurs when the beam energy exceeds a threshold value between 145 and 170 eV. Low-level hysteresis is observed in the transition when E(ib) is being reduced. The radial profiles of the ion beam current density and the low temperature ion current density can be obtained separately using a Faraday cup with a grid in front. The measured profiles confirm that more than a half of the extracted beam ions reaches the target plate with a good focusing profile with a full width at half maximum of ≈3 cm. Estimation of the particle balances in beam ions, the slow ions, and the electrons indicates the possibility that the secondary electron emission from the target plate and electron impact ionization of hydrogen may play roles as particle sources in this extremely low-energy beam after the compensation of beam ion space charge.

Measurement of velocity distributions of fusion produced alphas in a next stage magnetic plasma confinement device is a challenging topic in thermonuclear fusion research. Active beam probe diagnostic method based on high energy neutral beam of3He is considered promising. The beam can be produced by auto electron detachment from an energetic negative helium ion (He-) during the transport in field free space. To find critical issues in forming a diagnostic beam, a double-charge-exchange He-source coupled to a simple beam transport systems is currently being constructed at Tohoku University. A He+beam of about 10 mA is extracted from the ion source and accelerated up to 10∼ 25 keV for efficient charge exchange. The beam qualities of both double-charge-exchanged He-and auto detached neutral He (He0) will be investigated by emittance meters, beam profile monitors, and a calorimeter.

The operating conditions for obtaining a quasi-single helicity (QSH) state with a good reproducibility are found in a reversed-field pinch (RFP) experiment on the large RFP machine, TPE-RX [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)]. In these conditions, the reversal of toroidal magnetic field (Bta) is maintained at a very shallow value (∼−0.2mT) after the setting up phase and the following fast current rising phase. After a certain period at this shallow reversal (∼15–25ms), the m∕n=1∕6 mode (m and n being the poloidal and toroidal Fourier mode numbers, respectively) rapidly grows and saturates before the termination of discharge. The growth of this mode dominates the other modes and the QSH state with m∕n=1∕6 is finally achieved. This QSH state can be sustained for a long period (up to ∼45ms) almost until the end of discharge by applying a delayed reversal of Bta with appropriate trigger timing and magnitude. The initial setup of the QSH states shows a reproducibility of almost 100%, but its susta...

The self-focusing phenomenon has been observed in a high current density and low energy ion beam. In order to study the mechanism of this phenomenon, a special designed double probe to measure the electron density and temperature is installed into the chamber where the high current density ion beam is injected. Electron density profile is successfully measured without the influence of the ion beam components. Estimated electron temperature and density are ∼0.9 eV and ∼8 × 10(8) cm(-3) at the center of ion beam cross section, respectively. It was found that a large amount of electrons are spontaneously accumulated in the ion beam line in the case of self-forcing state.

Spontaneous self-focusing of ion beam with high current density (Jc ∼ 2 mA/cm(2), Ib ∼ 65 mA) in low energy region (∼150 eV) is observed in a hydrogen ion beam extracted from an ordinary bucket type ion source with three electrodes having concave shape (acceleration, deceleration, and grounded electrodes). The focusing appears abruptly in the beam energy region over ∼135-150 eV, and the Jc jumps up from 0.7 to 2 mA/cm(2). Simultaneously a strong electron flow also appears in the beam region. The electron flow has almost the same current density. Probably these electrons compensate the ion space charge and suppress the beam divergence.

Carbon plasma is successfully sustained during 1000 s without any carrier gas in the bucket type ionization chamber with cusp magnetic field. Every several seconds, seed plasmas having ∼3 ms duration time are injected into the ionization chamber by a shunting arch plasma gun. The weakly ionized carbon plasma ejected from the shunting arch is also ionized by 2.45 GHz microwave at the electron cyclotron resonance surface and the plasma can be sustained even in the interval of gun discharges. Control of the gun discharge interval allows to keep high pressure and to sustain the plasma for long duration.

Magnetic helicity and plasma flow injection experiments using a magnetized coaxial plasma gun on a reversed-field pinch were performed for the first time. In the initial experiments, increases in the electron density and toroidal flux were observed. The dependence of flux change on the polarity of the injected helicity indicates the possibility of the magnetic helicity injection and the poloidal current drive.

This paper reports the highlights of the RFX-mod fusion science programme since the last 2010 IAEA Fusion Energy Conference. The RFX-mod fusion science programme focused on two main goals: exploring the fusion potential of the reversed field pinch (RFP) magnetic configuration and contributing to the solution of key science and technology problems in the roadmap to ITER. Active control of several plasma parameters has been a key tool in this endeavour. New upgrades on the system for active control of magnetohydrodynamic (MHD) stability are underway and will be presented in this paper. Unique among the existing fusion devices, RFX-mod has been operated both as an RFP and as a tokamak. The latter operation has allowed the exploration of edge safety factor q edge < 2 with active control of MHD stability and studies concerning basic energy and flow transport mechanisms. Strong interaction has continued with the stellarator community in particular on the physics of helical states and on three-dimensional codes. © 2013 IAEA, Vienna.

Transition to a highly focused state is observed in a hydrogen ion beam extracted from an ion source using concave-shaped electrodes. The beam has a low energy (∼95 eV) and a high current (∼60 mA) with a ∼60 mm diameter. The transition is induced by electron beam injection (1 keV and up to 50 mA) to a grounded electrode of an ion source from a direction obliquely opposite to the ion beam velocity. Probably, secondary electrons emitted from the electrode play an effective role in cancelling the beam ion space charge, since they are produced in the core region of ion beam immediately after the extraction. Comparison between the cases with and without an electron beam exhibits great improvements not only in a total ion beam current reaching a target, but also in the focusing of ion beam. The sudden transition to this highly focused state, which is triggered by a low electron beam current (∼9 mA), suggests that some kind of a nonlinear process plays an essential role in this focusing.

A new RFP device with a very small aspect ratio (A = 2.1) is designed and constructed. The aspect ratio of A = 2.1 is much smaller than those of existing devices (A > 3). At this small A, no rational magnetic surface appears from the axis to ψ = 0.4 in the normalized flux surface coordinate, which may reduce the chaotic properties of the plasma in the core region. The proposed examination of bootstrap current generation could also be done. For this new device, a compact and high voltage center coil and an aluminum vacuum vessel (R/rw = 0.53 m/0.25 m) are developed. The maximum plasma current is designed to attain 100 kA. The construction of the device has been completed successfully. In the initial series of discharges, a stable region with a very high Θ value up to ˜2.5 is newly found.

Turbulence in the reversed field pinch (RFP) plasma has been investigated by using the microwave imaging reflectometry in the toroidal pinch experiment RX (TPE-RX). In conventional RFP plasma, the fluctuations are dominated by the intermittent blob-like structures. These structures are accompanied with the generation of magnetic field, the strong turbulence, and high nonlinear coupling among the high and low k modes. The pulsed poloidal current drive operation, which improves the plasma confinement significantly, suppresses the dynamo, the turbulence, and the blob-like structures.

The magnetic fluctuations and electrostatic probe potential have been measured in the Toroidal Pinch Experiment - RX (TPE-RX) reversed-field pinch plasma [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)] (at the plasma surface r/a = 1.00). Fast electrons with energy comparable to or slightly higher than the core electron temperature are observed as many spikes in the electrostatic probe signal. These electrons are diffused by a fluctuating magnetic field from the core region. During the period of mild deepening of the reversal of the edge toroidal field, a significant reduction in the spike signal, increases in electron density and soft x-ray radiation, and a decrease in the Dα line radiation are observed, even though the reduction in magnetic fluctuations is not significant during the same period, which indicates that the mild deepening of the reversal of the toroidal field can improve the confinement of fast electrons.

The neutral beam injection facility in the National Institute of Advanced Industrial Science and Technology was used to irradiate a polycrystalline tungsten specimen with high energy and high flux helium and hydrogen particles. The incidence energy and flux of the beam shot were 25 keV and 8.8 × 1022 particles/m2 s, respectively. The duration of each shot was approximately 30 ms, with 6 min intervals between each shot. Surface temperatures over 1800 K were attained. In the two cases of helium irradiation, total fluence of either 1.5 × 1022 He/m2 or 4.0 × 1022 He/m2 was selected. In the former case, large sized blisters with diameter of 500 nm were densely observed. While, the latter case, the blisters were disappeared and fine nanobranch structures appeared instead. Cross-sectional observations using a transmission electron microscope (TEM) with the focused ion beam (FIB) technique were performed. According to the TEM image, after irradiation with a beam shot of total fluence 4.0 × 1022 He/m2 , there were very dense fine helium bubbles in the tungsten of sizes 1-50 nm. As the helium bubbles grew the density of the tungsten matrix drastically decreased as a result of void swelling. These effects were not seen in hydrogen irradiation case.

Two-dimensional (2D) turbulence measurement by using microwave imaging reflectometry (MIR) has been performed in the reversed field pinch (RFP) plasma in TPE-RX. In this work, the maximum entropy method (MEM) is used to estimate the 2D k-spectrum. The turbulences between the pulsed poloidal current drive (PPCD) and the high Θ plasmas are studied. The m = 1 modes are dominant in PPCD plasma, while in high Θ plasma the k spectrum expands to the high k range and the turbulence propagates in the electron drift direction. The preliminary result shows that PPCD plasma switches off the turbulence, and the high Θ plasma has strong turbulence.

A high-power-density ion beam system with high-repetition pulses was successfully developed. In the ITER (International Thermonuclear Experimental Reactor), it is anticipated that an intermittent thermal flux, due to the edge localized mode (ELM), to the plasma facing materials causes severe damage of the mechanical properties. Therefore, it is very important to study the effect of ELM phenomena. We already developed an ion beam system with a power density as high as ∼1 GW/m2 around the focal point of the beam. In order to imitate the intermittent high-power-density pulsed flux, we modified the beam operation method and part of the acceleration power supply. A pulsed helium ion beam with the beam width of 2 ms and 4 ms intervals between pulses was successfully extracted. In this case, beam energy, current and power were ∼22 keV, ∼40 A, and ∼0.88 MW, respectively. This high-repetition pulsed helium ion beam with high power density (∼300 MW/m2 ) was irradiated to a tungsten material. It was found that this repetitive short-pulse irradiation caused less surface damage compared with long-pulse irradiation, even when the total amount of irradiation fluence (1.5× 1022 particles/m2) was the same for each condition. This would provide important data for the design of ITER diverter.

We have measured fast magnetic fluctuations (> 10 kHz) using a complex edge probe in the edge region of TPE-RX reversed-field pinch plasma. We observed that the frequency of magnetic fluctuation at the peak power in the frequency spectrum varies with time, and this frequency variation is determined by the values of pinch and reversed parameters, although the amplitude of the fast magnetic fluctuations are almost independent of those parameter values.

A new method is proposed to measure the magnetic field direction in hot plasmas using helium (He) neutral beam injection. The injected He beam is ionized by collisions with field electrons, and the ionized He beam atoms emit line radiation during their gyromotion with their rotational axis in the direction of the magnetic field. The direction of that axis can be determined locally from the Doppler width of the line radiation in the region along the beam path. The current intensity and energy of the injected He beam necessary to obtain reasonable intensity of line radiation from He+ ions are estimated. This estimation includes the effect of ionization from the metastable state of the He beam atoms and takes the 468.6 nm line radiation from He+ ions as an example. It is shown that the radiation intensity and line broadening width obtained using a He neutral beam with energy of 1 keV and current of 10 mA - 100 mA are sufficient for spectral measurement in typical reversed field pinches. It is also shown that the beam with energy of 10 keV and current of 0.1 - 1 A is sufficient in medium sized tokamaks.

Microwave imaging reflectometry (MIR) was developed in TPE-RX, one of the world’s largest reversed field pinch (RFP) devices. The system optics are made of aluminum mirrors, Teflon lenses, and Plexiglas plates in order to reduce size. In this system, frequencies are stabilized so that noise can be reduced using narrow bandpass filters. A 4×4 2-D mixer array and phase detection system have also been developed. With this system, density fluctuations in the high-Θ RFP plasma, pulsed poloidal current drive (PPCD) plasma, and quasi-single helicity (QSH) plasma are observed in TPE-RX. This is the first MIR experiment in an RFP device.

By applying a small positive pulse (ΔBta) in toroidal magnetic field, the quasi-single helicity (QSH) state can be obtained with a controllable and reproducible manner in a reversed-field pinch (RFP) experiment on the large RFP machine, TPE-RX [Y. Yagi et al., Fusion Eng. Des. 45, 421 (1999)]. The QSH state in RFP is one of the states where the improved confinement can be observed, and is important for development toward the pure single helicity (SH) state. In the SH state, the dynamo-action for sustaining the RFP configuration will be driven by a single helical mode and its harmonics, and the anomalous plasma loss can be avoided which is caused by the multi-helicity dynamo action in ordinary RFPs. In the operating condition presented here, the reversal of toroidal magnetic field (Bta) is maintained at a shallow value (∼−1mT) for a certain period (∼20ms) after the setting up of the RFP configuration and then the positive ΔBta (≲5mT magnitude and ∼2ms width) is applied to the Bta, which is usually negative...