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Describes a change in the detailed edge electron density profile during a positively biased limiter experiment with high temporal resolution of approximately 20 microns using a laser blowoff lithium beam probe technique in HYBTOK-II tokamak. Transient change in the electron density profile; Suppression of electrostatic fluctuations near the edge of the core region.

We study the influence of grain boundaries (GBs) on radiation-induced vacancies, as well as on the hydrogen (H) behavior in tungsten (W) samples with different grain sizes in the temperature range from 300 K to 573 K, both experimentally and by computer simulations. For this purpose, coarse-grained and nanostructured W samples were sequentially irradiated with carbon (C) and H ions at energies of 665 keV and 170 keV, respectively. A first set of the implanted samples was annealed at 473 K and a second set at 573 K. Object kinetic Monte Carlo simulations were performed to account for experimental outcomes. Results show that the number of vacancies for nanostructured W is always larger than for monocrystalline W samples in the whole studied temperature range and that the number of vacancies is only reduced in samples with a large density of grain boundaries and at temperatures high enough to activate the vacancy motion (around 573 K). Results also indicate that the migration of H along vacancy free grain boundaries is more effective than along the bulk, and that the retained H is trapped in vacancies located within the grains. These results are used to explain the experimental outcomes. [ABSTRACT FROM AUTHOR]

Three sets of triple probe arrays (four-tip) are applied to study the transport properties at the edge of a HL-2A tokamak. The Geodesic Acoustic Mode (GAM) exhibits intermittent characteristics during the discharge. The radial particle flux has been studied in different phases corresponding to the variable GAM intensity. The experimental results reveal that the radial particle flux contributed by the ambient turbulence ( 20 kHz – 100 kHz ) has been suppressed by 13% during the GAM bursts, contrasting weak GAM cases. Power of density fluctuations and coherence between density and potential fluctuations contribute to most reduction of the particle flux, while changing the cross phase between the density and potential fluctuations and the suppression of power of potential fluctuations hardly contribute to it. These results suggest that the GAM can regulate the turbulent transport mainly by changing the amplitude of ambient turbulence, rather than the cross phase between density and potential fluctuations. [ABSTRACT FROM AUTHOR]

The thickness x, of tungsten fuzz layers are measured for non-varying helium (He) plasma exposure conditions spanning four orders of ion fluence (1024–1028m−2) and flux (1019–1023 m−2 s−1), at 1000–1140 K under low energy He ion impact (50–80) eV. The data obtained are complemented by previously published data of similar growth conditions, and collectively analysed. The new analysis allows for the reconciliation of fast high flux growth with commonly observed slower growth at lower flux. It is demonstrated that the standing t1/2 time dependence is a special case of a more general expression for determining the layer thickness, , that depends on , an incubation fluence , and the growth constant m4, which is temperature dependent. The incubation fluence, which must be exceeded before the observation on the onset of fuzz surface morphology is determined to be m−2. In fuzz growth-erosion regimes, characterized by an erosion constant , that is proportional to the sputter yield, an analytic solution for has been found, by solving the growth-erosion equilibria problem of prior work with the Lambert W function. Simple limit expressions follow from the solution for determining the equilibrium fluence and fuzz thickness; the predictions of such being in good agreement with previous fuzz growth-erosion equilibria results in the literature. [ABSTRACT FROM AUTHOR]

Spatially resolved spectroscopic measurements of He-I line emission are used to study the causes of emission profile broadening radially across the cylinder of a weakly ionized helium plasma. The plasma consists of an ionizing core (r < 2 cm) surrounded by a recombining edge (r > 2 cm) plasma. The brightness profiles of low-n EUV He-I resonance lines are shown to be strongly radially broadened due to opacity. The brightness profiles of high-n visible lines are also found to be strongly radially broadened, but dominantly due to edge recombination. Visible low-n lines are less strongly radially broadened, apparently by a combination of both recombination and EUV opacity. The low-n visible He-I line ratio method with central opacity correction is found to calculate central electron density and temperature well, with poor agreement at the edge, as expected for recombining plasma. In the recombining edge, high-n Boltzmann analysis is found to accurately measure the cold (Te < 0.2 eV) edge temperature. Near the core, however, high-n Boltzmann analysis can be complicated by electron-impact excitation, giving incorrect (Te≈ 0.1 eV) apparent temperatures. Probe measurements were not able to capture the cold edge temperature accurately, probably due to large potential fluctuations, even when using fast triple probe measurements. Fast spectroscopic measurements show that this discrepancy is not explained by recombining plasma alternating with ionizing plasma in the edge region. [ABSTRACT FROM AUTHOR]

The evolution of diffusive and non-diffusive transport during pedestal buildup following a low-high (L-H) transition has been interpreted from a particle-momentum-energy balance analysis of the measured density, temperature, and rotation velocity profiles in the plasma edge (0.82<ρ<1.0) of a DIII-D [Luxon, Nucl. Fusion 42, 614 (2002)] discharge. In the discharge examined, there was an edge-localized-mode-free period of more than 600 ms following the L-H transition, and the majority of edge pedestal development occurred within the first 100 ms following the L-H transition. There appears to be a spatio-temporal correlation among the measured toroidal and poloidal rotation, the formation of a negative well in the measured radial electric field, the creation of a large inward particle pinch, the calculated intrinsic rotation due to ion orbit loss, and the measured formation of steep gradients in density and temperature in the outer region (ρ>0.95) of the edge pedestal. [ABSTRACT FROM AUTHOR]

A novel two-channel, high throughput, high efficiency spectrometer system has been developed to measure impurity ion temperature and toroidal velocity fluctuations associated with long-wavelength turbulence and other plasma instabilities. The spectrometer observes the emission of the n = 8-7 hydrogenic transition of C+5 ions (λair = 529.06 nm) resulting from charge exchange reactions between deuterium heating beams and intrinsic carbon. Novel features include a large, prism-coupled high-dispersion, volume-phase-holographic transmission grating and high-quantum efficiency, high-gain, low-noise avalanche photodiode detectors that sample emission at 1 MHz. This new diagnostic offers an order-of-magnitude increase in sensitivity compared to earlier ion thermal turbulence measurements. Increased sensitivity is crucial for obtaining enough photon statistics from plasmas with much less impurity content. The irreducible noise floor set by photon statistics sets the ultimate sensitivity to plasma fluctuations. Based on the measured photon flux levels for the entire spectral line, photon noise levels for Ti/Ti and Vi/Vi of ∼1% are expected, while statistical averaging over long data records enables reduction in the detectable plasma fluctuation levels to values less than that. Broadband ion temperature fluctuations are observed to near 200 kHz in an L-mode discharge. Cross-correlation with the local beam emission spectroscopy measurements demonstrates a strong coupling of the density and temperature fields, and enables the cross-phase measurements between density and ion temperature fluctuations. [ABSTRACT FROM AUTHOR]

The kinetic energy transfer between shear flows and the ambient turbulence is investigated in the Experimental Advanced Superconducting Tokamak during the L-H transition. As the rate of energy transfer from the turbulence into the shear flow becomes comparable to the energy input rate into the turbulence, the transition into the H-mode occurs. As the observed behavior exhibits several predicted features of zonal flows, the results show the key role that zonal flows play in mediating the transition into H-mode. [ABSTRACT FROM AUTHOR]

In neutral-beam-injection (NBI) heated plasmas of the Compact Helical System (CHS) heliotron/torsatron [K. Matsuoka et al., Plasma Physics and Controlled Nuclear Fusion Research, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 441], inward turbulent particle flux is observed at a particular radial location near the plasma edge. The observed reversal of the particle flux is connected to the change in the cross-phase between electron density and poloidal electric field fluctuations. This phenomenon is observed in the parameter regime of low collisionality at the electron temperature T[sub e]>15 eV, electron density n[sub e]<3×10[sup 18] m[sup -3], which approximately corresponds to the effective collision frequency normalized by the transit frequency of a circulating particle ν*≤1. The large inward particle flux (>=1×10[sup 20] m[sup -2] s[sup -1]) correlates with the positive shear of the radial electric field E[sub r][sup ′] (>=1×10[sup 6] V m[sup -2]), but does not have clear correlation with the radial electric field E[sub r] and the curvature E[sub r][sup ″]. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The bifurcation nature of the electrostatic structure is studied in the toroidal helical plasma of the Compact Helical System (CHS) [K. Matsuoka et al., Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411]. Observation of bifurcation-related phenomena is introduced, such as characteristic patterns of discrete potential profiles, and various patterns of self-sustained oscillations termed electric pulsation. Some patterns of the electrostatic structure are found to be quite important for fusion application owing to their association with transport barrier formation. It is confirmed, as is shown in several tokamak experiments, that the thermal transport barrier is linked with electrostatic structure through the radial electric field shear that can reduce the fluctuation resulting in anomalous transport. This article describes in detail spatio-temporal evolution during self-sustained oscillation, together with correlation between the radial electric field and other plasma parameters. An experimental survey to find dependence of the temporal and spatial patterns on plasma parameters is performed in order to understand systematically the bifurcation property of the toroidal helical plasma. The experimental results are compared with the neoclassical bifurcation property that is believed to explain the observed bifurcation property of the CHS plasmas. The present results show that the electrostatic property plays an essential role in the structural formation of toroidal helical plasmas, and demonstrate that toroidal plasma is an open system with a strong nonlinearity to provide a new attractive problem to be studied. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A two-dimensional vortex subjected to shear flow is considered in Rayleigh-Taylor mode of magnetized plasmas. It is shown that, when vortex-flow is almost absent, linear instability is suppressed due to shear-flow. In the presence of a vortex, however, there is a possibility of secondary instability excitation. It is demonstrated that the efficiency of this secondary instability depends on the sense of rotation and strength of vortex-flow and shear-flow. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The equation describing the radial transport of toroidal momentum in a collisional subsonic plasma with steep gradients has been obtained via a systematic expansion of the two-fluid equations. The diffusion rate is classical; the poloidal rotation, driven by the temperature gradient, generates, in turn, a toroidal flow gradient, also in Ohmic discharges. Moreover, important modifications of the parallel momentum equation are found to arise if Λ[sub 1]≡(ν[sub i]/Ω[sub i])(q[sup 2]R[sup 2]/rL[sub T]) is >=O(1/3); the poloidal rotation velocity is then no longer unique but obeys a cubic equation which may allow for bifurcated equilibria under certain conditions. The toroidal velocities predicted for Ohmic discharges compare well with those measured in PLT [Princeton Large Torus; S. Suckewer et al., Nucl. Fusion 21, 1301 (1981)]; the relevance of the extended equation providing the poloidal rotation velocity to selected experimental edge plasmas is discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Hollow neutral pressure profiles, with significant on-axis reductions in neutral pressure (up to 40%), are observed across the face of an inert wafer in discharges with uniform plasma density. These results show that significant neutral depletion, which may cause the nonuniform plasma process results, can occur in large-area high-density plasma sources with a wafer present. The neutral depletion is explained by the ion pumping effect, wherein electron impact ionization of neutral particles is followed by their rapid movement from the plasma to the chamber wall by the presheath electric field. Cooling of plasma electrons via inelastic neutral collisions is also observed at elevated fill pressure, and results in a reduction of the magnitude of neutral depletion, thus demonstrating the linkage between plasma equilibrium and neutral equilibrium conditions. Initial experiments have also been performed in O[sub 2] discharges. Similar hollow neutral pressure profiles are observed, suggesting that similar effects occur in chemically reactive discharges. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Absolute measurements of poloidal rotation velocity with the accuracy up to 1 km/s (2 pm in wavelength) were done using charge exchange spectroscopy in a large helical device. Radial profiles of the absolute Doppler shift of charge exchange emission with a beam are obtained from spectra measured with four sets of optical fiber arrays that view downward and upward at the poloidal cross section with and without neutral beam injection. By arranging the optical fiber from four arrays close to each other at the entrance slit, the apparent Doppler shift due to aberrations of the spectrometer and due to interference of the cold component (the charge exchange between He-like oxygen and thermal neutrals 8 pm from the charge exchange emission with a beam) can be eliminated from the measurements. The measured poloidal rotation velocity is 1-3 km/s in the electron diamagnetic direction at half of the plasma minor radius. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A detailed experimental investigation of plasma fluctuation and its modification due to radial electric field effects, both its shear and curvature, has been carried out on the Current Sustained Torus IV at Nagoya University [Takamura et al., J. Plasma Fusion Res. 74, 38 (1998)] through measurement with an array of Langmuir probes. The observed statistical dispersion relation indicates a drift-wave-type turbulence. An examination of the radial electric field shear and curvature effects on the fluctuations as well as comparisons to theoretical predictions are presented. The decorrelation of turbulence is found to be influenced by the radial electric field shear in our experiment. A modification of the poloidal correlation length and the two-poloidal-point coherency due to the radial electric field has also been identified. Nonlinear analysis of quadratic mode coupling indicates that the radial electric field can affect the fluctuation amplitudes by modifying the coupling between different spectral components of the fluctuations. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A theoretical model is presented which shows generation of a co-current directional central toroidal rotation by a nondirectional on-axis ion cyclotron heating in the Low-mode plasmas of Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. It is shown that a net-inward nonambipolar transport of resonant ions can induce such a rotation. The net-inward transport is a result of the passing ion dominant transport process in Alcator C-Mod. The opposite horizontal orbit shift between co- and counter-passing ions makes the wave absorption asymmetric in k[sub ||]. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The poloidal flow velocity and the density of fully ionized carbon were measured in the Compact Helical System [K. Ida et al., Phys. Rev. Lett. 67, 58 (1991)] using the charge exchange spectroscopy with the bidirectional viewing. The poloidal asymmetry of the poloidal flow velocity observed in the outward shifted plasma is due to the electrostatic potential being constant on the magnetic flux surfaces. The poloidal asymmetry of the ion density can be explained by the conservation of the poloidal ion flux under the strong toroidal viscosity. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Stationary solutions of the nonlinear equations, which govern the dynamics of ion-temperature-gradient (ITG) electrostatic modes, are re-examined without and with magnetic field inhomogeneities. It is shown that for both cases the nonlinear equations admit vortex street solutions in those parameter regimes where spatially bounded double vortex solutions are forbidden. The ITG-driven vortex streets can be associated with coherent nonlinear structures that are observed in laboratory and computer simulation experiments. Furthermore, vortex streets might play a detrimental role in the study of anomalous ion thermal transport in magnetically confined plasmas. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]