Showing papers in "Journal of Plasma and Fusion Research in 2005"

Diagnostics of Recombining Plasmas in Divertor Simulator MAP-II

Abstract: Atomic and molecular processes in volumetric recombination phenomena relevant to divertor detachment are investigated in a divertor simulator MAP (Material and Plasma) -II. In the recombining plasmas, quantitative measurements of parameters, especially using a Langmuir probe, are difficult, so that the development of alternative diagnostics is important. Recombination can be induced in He plasma by puffing of He or H2. In the He puffing case, the Rydberg spectra show an electron temperature of lower than 0.1 eV, while in the H2 puffing case, the Rydberg spectra disappear even though the reduction of the ion flux is apparent, showing that another type of recombination occurs. Negative hydrogen ions are observed in the peripheral region of the plasma column.

Evaluation of Para-Perp Type Mach Probe by Using a Fast Flowing Plasma

Abstract: Mach probe with two collecting tips facing to the direction parallel and perpendicular to the plasma fl ow, named para-perp type one, is evaluated by using a directional Langmuir probe (DLP) in a fast fl owing plasma produced by an MPD (Magneto-Plasma-Dynamic) arcjet. We present simple formulas to determine an ion acoustic Mach number Mi from the ratio of collected ion saturation currents. The formulas and experimentally-obtained data are compared with Hutchinson’s simulation results which are calculated using a particle-in-cell(PIC) code in an unmagnetized plasma condition. [I.H. Hutchinson, Plasma Phys. Control. Fusion, 44 1953 (2002).] Good agreements are obtained among these data in both subsonic and supersonic plasma fl ows. Correction factor κ to determine Mi is presented under various conditions of Ti/Te and the specifi c heat ratio for ions γ i.

Thirty-Minute Plasma Sustainment by ICRF, EC and NBI Heating in the Large Helical Device

Abstract: Steady-state plasma heating was successfully performed and sustained for more than 30 min in the LHD. By using ICRF heating and additional EC and NBI heating, a total input energy of 1.3 GJ was achieved. The average input power was 680 kW and the plasma duration was 31 min 45 sec. The hardware of the ICRF and divertor plates was much improved and the position of the ICRF antenna was optimized. The heat load to the divertor plates was effectively dispersed by the magnetic axis swing technique, which caused large changes in the heat load distribution along the divertor leg traces.

A Shell Model for the Hall MHD System

Abstract: A shell model has been formulated for the Hall MHD system, where the Hall term (scaled by the ion skin depth li) brings about a new singular perturbation in addition to the resistivity and viscosity effects. The model equations are derived under the constraint to conserve the energy and two helicities in the inertial range. In the limit of li → 0, the system reduces to the conventional MHD shell model.

Probe Measurements: Fundamentals to Advanced Applications

Abstract: This intensive course gives fundamentals of probe measurement and its advanced applications. The first part reviews the theory of the electrostatic probe used in the plasma and methods for determining plasma parameters (space potential, electron temperature, plasma density, energy distribution, etc.). Several types of conventional probes are presented. The second part presents new methods and techniques that have been developed to measure complicated cases: electronegative plasma, flowing plasma, strongly magnetized (fusion) plasma, space plasma and (high pressure) microplasma by giving some examples.

High power electron heating experiments at the plug region of GAMMA 10

Abstract: In the GAMMA 10 tandem mirror, a high power ECRH experiment at the plug region is now in progress. As the first step of this experiment, high power operation of existing gyrotrons for plug ECRH has been carried out at a power Pplug exceeding their nominal powers of 200 kW. In this step, the highest recorded value of the axial ion confining potential φc for a hot ion mode plasma is 1.4 kV at Pplug=240 kW. Then, in the second step, a newly developed high power gyrotron has been installed at one side of the both end plugs. The value of φc increased with Pplug and attained 2.1 kV at Pplug=370 kW. The potential difference ΔΦ from the plug potential to the end plate potential exceeded 5 kV. The effective temperature Teff as a mean energy of the end loss electrons reached 3 keV, and the scaling between ΔΦ and Teff has been expanded. Functions of the plug ECRH on electrons are studied from the viewpoint of velocity space diffusion, and a picture of ECRH-induced axial electron motion is presented.

Electron Pressure Profiles in High-Density Neutral Beam Heated Plasmas in the Large Helical Device

Abstract: In the Large Helical Device (LHD), electron pressure profiles in gas-fueled high-density discharges tend to have a similar shape, as if these were frozen. This frozen profile is insensitive to variations in the magnetic field strength and moderate changes in the neutral beam heat deposition profile. At the same time, however, the absolute value of the electron pressure itself increases with the heating power, the electron density, and the magnetic field strength. In this study, a reference model for the electron pressure is proposed which consists of the frozen profile and parametric dependences derived from experimental observations. It is possible to define an operational regime where this typical profile appears by comparing the electron pressure profiles with this model. In the standard configuration, at which the maximum plasma stored energy in LHD has been obtained, the frozen profile appears in the plateau to the Pfirsh-Schluter regimes. As the collisionality decreases to the collisionless regime, the electron pressure becomes smaller than the prediction of the model and the deterioration is significant in the plasma core region. This tendency is enhanced in the configuration with the outward-shifted magnetic axis. The global energy confinement time, τE, in the high-collisionality regime has a weaker density dependence together with the mitigated power degradation, scaling as τE∝nebar0.28P-0.43 (nebar and P are the line-averaged density and the heating power, respectively), compared with the International Stellarator Scaling 95, where τE∝nebar0.51P-0.59.

Semi-Empirical Approach to Pulsed Wire Discharges in Water as a Method for Warm Dense Matter Studies

Abstract: Hydrodynamic behaviors accompanied by a pulsed thin wire discharge in water have been observed via a fast framing/streak camera, together with the basic electric characteristics. Results show that the discharge plasma is tamped and stabilized by the surrounding water and it evolves through a warm dense state with high degree of symmetry and reproducibility up to a 2 μs discharge time. Numerical simulations show that the shock wave trajectories driven in the water are strongly dependent on equation of state (EOS) models of the plasma. Those results indicate that a semiempirical fitting of the shock traces to the experimental observation is a useful method for studying the EOS models of matter in a warm dense state.

Further Roughing of Tungsten Surface due to Transient Heat Load by Laser Irradiation during Helium Plasma Exposure

Abstract: 745 J. Plasma Fusion Res. Vol.81, No.10 (2005) 745 746 Tungsten is one of the candidates for divertor materials because of its high thermal property and low sputtering yield. However, recent experimental observations have revealed that bubbles and holes are formed on the surface by helium plasma exposure even with an incident ion energy less than the threshold energy of sputtering [1]. The formation of holes and bubbles may lead to degradation of the superior properties of tungsten, especially, its thermal diffusivity. If the thermal diffusivity is degraded, transient heat loads owing to the ELMs (Edge Localized Modes) [2] and disruption may pose serious problems, such as the melting of high-Z materials. In this paper, the transient heat load to the tungsten target during exposure to helium plasmas was demonstrated using pulsed laser irradiation. Although the heat pulse duration was several ns, which is much shorter than the characteristic time of the ELMs (<~10 −4 s), the experimental observations indicate the effect of transient temperature increases on the surface modifi cation. Experiments were performed in the divertor simulator NAGDIS-II (NAGoya university DIvertor Simulator-II). A schematic view of the experimental setup is shown in Fig. 1. The helium plasma was irradiated to a W sample situated at about a 45-degree angle to the magnetic fi eld line. The specimen was powder metallurgy W 0.2 mm in thickness. Second-harmonic pulses of an Nd:YAG laser (Continuum: SLII-10), with a wavelength of 532 nm, were used for the photon source. The pulse duration was 5–7 ns, and the pulse interval was 0.1 s, suffi ciently longer than the characteristic thermal relaxation time. The laser beam was injected through a quartz viewing port at a 90-degree angle to the magnetic fi eld line. The laser-pulse energy was measured before the viewing port, and the energy per unit area at the target was deduced by taking into account the transmission rate of the viewing port and the angle of the laser beam to the tungsten sample. SEM images of the tungsten sample exposed to helium plasmas without laser irradiation are shown in Fig. 2 (a) (from top) and Fig. 3 (a) (cross section). The ion fl ux was Γi = 1.7 × 1023 m–2/s, and the incident ion energy was 27 eV. The exposure time to the helium plasma was 1800 s; the tungsten surface temperature measured with a radiation pyrometer was ~1700 K. Micron-bubbles and holes are observed on the surface, and the penetrating depth of the surface modifi cation was 1– 2 μm. Figure 2 (b) shows SEM images of the laser irradiated tungsten surface exposed to the helium plasma under the same

核融合と超伝導工学 1.核融合用超伝導コイル

Abstract: Magnetic fusion reactors need a superconducting coil system with the huge magnetic energy of approximately 100 GJ. A number of superconducting fusion experimental devices have been constructed since 1970. Accumulated technologies and experiences for the past 35 years now allow us to design the actual reactors. However, there still remain some engineering problems in order to design the commercial reactors that will need a low construction cost, a high field and high reliability. It is necessary to develop high-strength supporting materials, next-generation superconductors and high-current-capacity conductors.

Characteristics of Post-Disruption Runaway Electrons with Impurity Pellet Injection

Abstract: Characteristics of post-disruption runaway electrons with impurity pellet injection were investigated for the first time using the JT-60U tokamak device. A clear deposition of impurity neon ice pellets was observed in a post-disruption runaway plasma. The pellet ablation was attributed to the energy deposition of relativistic runaway electrons in the pellet. A high normalized electron density was stably obtained with nebar/nGW ˜ 2.2. Effects of prompt exhaust of runaway electrons and reduction of runaway plasma current without large amplitude MHD activities were found. One possible explanation for the basic behavior of runaway plasma current is that it follows the balance of avalanche generation of runaway electrons and slowing down predicted by the Andersson-Helander model, including the combined effect of collisional pitch angle scattering and synchrotron radiation. Our results suggested that the impurity pellet injection reduced the energy of runaway electrons in a stepwise manner.

Present Status of ITER Test Blanket Development

Abstract: As one of the engineering test programs in ITER, the test blanket program, which utilizes ITER as a test bed to develop breeding blanket systems for fusion DEMO plants, has been planned and advanced. The Test Blanket Working Group (TBWG) has been organized to review and coordinate test programs on Test Blanket Modules(TBM). Five breeding blanket concepts have been proposed and discussed in the TBWG, and Japan is participating in the development of all of these concepts. This paper describes the status of TBM design, blanket technology and materials development for ITER TBMs. The current design and R&D status of candidate TBM concepts, which include 1) a water-cooling system with a solid breeder, 2) a helium gas-cooling system with a solid breeder, 3) a lithium-lead (LiPb) breeding system, 4) a liquid lithium breeding system, and 5) a molten salt breeding system, are presented and discussed.

The Configuration Dependence of Ripple Transport in LHD

Abstract: The GIOTA code [1] has been applied to clarify the configuration dependence of ripple transport properties in LHD. The effective helical ripple (eeff), as a measure of the level of ripple transport, is calculated with a wide range scan of magnetic axis position, (Rax) and plasma beta value (β). Our results show that eeff takes a minimum value in a wide radial portion around the configuration with Rax = 3.53-3.55 m at vacuum. It is also revealed that eeff decreases as β is increased for configurations with Rax of less than the above mentioned range.

Efficient Radio Frequency Inductive Discharges in Near Atmospheric Pressure Using Immittance Conversion Topology

Abstract: A radio frequency (rf) inductive discharge in atmospheric pressure range requires high voltage in the initial startup phase and high power during the steady state sustainment phase. It is, therefore, necessary to inject high rf power into the plasma ensuring the maximum use of the power source, especially where the rf power is limited. In order to inject the maximum possible rf power into the plasma with a moderate rf power source of few kilowatts range, we employ the immittance conversion topology by converting a constant voltage source into a constant current source to generate efficient rf discharge by inductively coupled plasma (ICP) technique at a gas pressure with up to one atmosphere in argon. A novel T-LCL immittance circuit is designed for constant-current high-power operation, which is practically very important in the high-frequency range, to provide high effective rf power to the plasma. The immittance conversion system combines the static induction transistor (SIT)-based radio frequency (rf) high-power inverter circuit and the immittance conversion elements including the rf induction coil. The basic properties of the immittance circuit are studied by numerical analysis and verified the results by experimental measurements with the inductive plasma as a load at a relatively high rf power of about 4 kW. The performances of the immittance circuit are also evaluated and compared with that of the conventional series resonance circuit in high-pressure induction plasma generation. The experimental results reveal that the immittance conversion circuit confirms injecting higher effective rf power into the plasma as much as three times than that of the series resonance circuit under the same operating conditions and same dc supply voltage to the inverter, thereby enhancing the plasma heating efficiency to generate efficient rf inductive discharges.

Alignment Method of ECH Transmission Lines Based on the Moment and Phase Retrieval Method Using IR Images

Abstract: New alignment methods of millimeter-wave transmission lines for Electron Cyclotron resonance Heating are proposed and evaluated on a high power level. These methods are based on the measured data of infrared images on the target, which is irradiated by the high power millimeter-waves at several positions. The first and second moments and retrieved phase obtained from these data are used to determine the propagation direction of the millimeter-wave beam along the waveguide axis. It is demonstrated that these methods have sufficient resolution to discriminate 0.1 deg., which is required to restrict the transmission loss below 1% over the 100 GHz range.

Experimental Conditions for Improved Confinement Modes in Heliotron J

Abstract: ECH/NBI experiments in Heliotron J have revealed the existence of a spontaneous transition to improved confinement modes Based on the experimental database obtained up to now, the characteristics of the transition and the density and power thresholds are discussed paying attention to the configuration effects For ECH+NBI discharges with injection powers of PECH ˜ 029 MW and PNBI ˜ 057 MW, transition phenomena were observed in almost all ι(a)/2π configurations The global plasma confinements before and after the transition are affected by the value of the edge rotational transform ι(a)/2π During the improved mode, the ι(a)/2π-dependence of the plasma stored energy is mitigated compared to the pre-transition phase The database indicates the existence of a critical value in the line-averaged electron density under which the transition cannot be observed This threshold line-averaged density is in the range of 11-20×1019 m3 in most configurations, indicating that it is not a strong function of the injection power and/or the heating method The existence of an ι(a)/2π value where the transition was not observed for ECH-only discharges but observed for NBI-only or ECH+NBI discharges suggests the existence of a configuration effect and/or heating-method effect on the threshold power

Multi-Dimensional Structure of the Electric Field in Tokamak H-Mode

Abstract: In tokamak H-mode, a large poloidal flow exists in an edge transport barrier, and the electrostatic potential and density profiles can be steep both in the radial and poloidal direction. Two-dimensional structures of the potential, density and flow velocity near the edge of a tokamak plasma are investigated. The model includes the nonlinearity in bulk-ion viscosity and turbulence-driven shear viscosity. For the case with a strong radial electric field (H-mode), a two-dimensional structure in a transport barrier is obtained, giving a poloidal shock with a solitary radial electric field profile. The poloidal electric field induces convective transport in the radial direction, and poloidal asymmetry makes the flux-surface-averaged particle flux direct inward with a pinch velocity on the order of 1 [m/s]. A large poloidal flow with radial shear enhances the inward pinch velocity. The abrupt increase of this inward ion and electron flux at the onset of L-to-H-mode transition explains the rapid establishment of the density pedestal at the transition.

High-Z Dust Generation on Tungsten Surfaces due to Synergetic Erosion of Deuterium/Helium Plasma Exposures

Abstract: Low-energy and high-flux plasma exposures on tungsten surfaces result in blister/hole formation. Subsequent He plasma exposure at 2,000 K on the blister-rich surface forms a hole structure on the surface, including the blister surface, and some blisters collapse due to recrystallization and bubble formation on the blister skin. Grain ejections were observed after deuterium plasma exposure at 550 K on a W surface on which a hole structure had formed due to He pre-exposure at 1,600 K.

Sustained Detachment with the Self-Regulated Plasma Edge beneath the Last Closed Flux Surface in LHD

Abstract: Self-sustained detachment has been obtained in the Large Helical Devise (LHD). Strong hydrogen gas puffing of ~200 Pa·m3/s after a density feedback phase successfully detaches the plasma from the divertor plate with high reproducibility. High electron density of over 1 × 1020 m–3 is sustained without gas puffing until the heating beam stops and the high-density flat top for 2 seconds has been demonstrated. Throughout the detachment phase, the minor radius of the hot plasma column shrinks to ~90 % of the last closed flux surface, which corresponds to the ι /2π = 1/q = 1 rational surface.

First Results of Magnetized Plasma Flow Injection on the TPE-RX Reversed-Field Pinch

Abstract: 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.

Observation of a Rotating Radiation Belt in LHD

Abstract: A poloidally rotating radiation belt with helical structure was observed during the high density discharges with detachment by photodiode fan arrays and a fast camera in LHD. The peak of radiation rotates inside the last closed flux surface, and the direction and mode number of the poloidal rotation are electron diamagnetic and one, respectively. During the recombination phase after termination of the plasma heating, the rotation continues, and its rotating radius shrinks with shrinking of the plasma column. The poloidal rotating frequency depends on the heating power, and increases from the orders of several tens of Hz to several hundreds of Hz with shrinking of the rotation radius. The mechanism of the rotation remains uncertain.

Spontaneous Formation of Low-Aspect-Ratio Torus Equilibria by ECH under Steady Vertical Field

Abstract: Spontaneous current jump resulting in the formation of closed magnetic flux surfaces was observed in electroncyclotron-heated toroidal plasmas under a steady vertical magnetic field in the LATE device. This bridges the gap between the open field equilibrium maintained by a pressure-driven current in the external vertical field and the closed field equilibrium at a larger plasma current.

Development of a Gas Jet-Type Z-Pinch EUV Light Source for Next-Generation Lithography

Abstract: A new gas jet-type Z-pinch EUV light source having double gas jet electrodes has been developed. It has two nozzles and two diffusers. A xenon Z-pinch plasma that emits EUV light is produced between the inner nozzle and the corresponding diffuser. A cylindrical shell consisting of a He gas curtain produced by the outer nozzle is specially designed for shielding the debris and suppressing the inner gas expansion. We have succeeded in generating EUV energy of 1.22 mJ/sr/pulse (2% in-band at 13.5 nm) and EUV emitting plasma of 0.07 mm FWHM diameter and 0.34 mm FWHM length by using this He gas curtain.

Multi-Stage Ionization Dynamics of Carbon Film Irradiated by High Power Lasers

Abstract: By using a particle code including ionization and atomic processes, we investigated the ionization dynamics of carbon film irradiated by an intense laser pulse. We found two types of ionization dynamics, namely, a fast time scaleconvective propagation of the ionization front with C4+ triggered by induced plasma waves, and a slow front with C5+ and C6+ triggered by heated electrons due to non-local thermal conduction. Thus, ionization dynamics in solids werefound to evolve through multiple stages.

Evidence of Electron Bernstein Wave Heating on the TST-2 Spherical Tokamak

Abstract: Electron Bernstein wave heating experiments based on the X-mode to Bernstein mode mode-conversion scenario were performed on the Tokyo Spherical Tokamak -2 (TST-2). Up to 140 kW of microwave power at 8.2 GHz was injected perpendicularly from the low-field side. Evidence of electron heating was observed as increases of the stored energy (by about 15%) and soft X-ray (hν > 1 keV) emission.

Burning Plasma Simulation Initiative and Its Recent Progress

Abstract: The purpose and recent progress of the Burning Plasma Simulation Initiative (BPSI) are discussed. Simulation of burning plasmas requires integrated modeling of various physics phenomena with wide-ranging spatial and time scales. The activities of the BPSI are of three types: development of the framework for integrated simulation codes, development of integrated modeling of multi-scale physics, and implementation of distributed parallel processing. Similar activities have been reported in the United States and the European Union. Features of the integrated transport code, TASK, being developed as a reference code for BPSI, are also described. Finally, asummary is given and future issues are discussed.

Quasi-Optical Beam Analysis Based on Direct Phase Measurement at Low Power Level

Abstract: Alignments of the beam size and position, and the tilt of the propagation axis, have been pointed out to be important in transmission lines for Electron Cyclotron Heating in order to achieve high transmission efficiency. Beam size evolution along the propagation of a Gaussian-like beam, and its beam tilt are analyzed with a quasi-optical moment theory by using measured intensity and phase patterns at a low power level. The Gaussian-like beam is coupled into a corrugated waveguide in the tilted injection. The desired Gaussian content after the coupling is also evaluated in terms of the measured phase as well as the intensity. It is indicated that the direct phase measurements are essential to evaluate the beam size evolution along the propagation, the tilt angle, and the Gaussian content of the beam, which are key issues in the alignment problem.

Application of All Solid-State Pulsed Power Generators to Equipment Using Plasmas

Abstract: Recently, high-repetition-rate all-solid-state pulsed power generators with long life time and high reliability, have been developed for industrial applications with plasmas, such as high-repetition-rate pulsed gas lasers, high energy density plasma (EUV sources) and water discharges. Nowadays, power semiconductor device technology can improve the performance of fast and high-power switching devices. In practical systems, however, semiconductor switches are used with the assistance of magnetic switches because the semiconductor switches are not capable of driving the usual generators by themselves. These generators consist of semiconductor switches, step-up pulse transformers and magnetic switches. Progress of all-solid-state pulsed power generators is reviewed with particular emphasis on industrial applications with plasmas.