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Photoluminescence (PL) spectra of (000 1 ¯ ) N-polar p-type GaN fabricated by using the sequential ion-implantation of Mg and H with subsequent high temperature annealing exhibited the near-band-edge (NBE) emission at 300 K. The longest PL lifetime ( τ PL ) for the NBE emission of the sample with Mg and H concentrations of 1 × 1019 and 2 × 1020 cm−3, respectively, annealed at 1230 °C was 18 ps at 300 K. This value is almost comparable to that of the (0001) Ga-polar p-type Mg-doped GaN (p-GaN:Mg) homoepitaxial film of the same Mg concentration. By correlating τ PL and the concentration of major vacancy-type defects quantified using positron annihilation spectroscopy, the electron capture-cross-section (σn) of the major nonradiative recombination centers (NRCs), namely, clusters of Ga vacancies (VGas) and N vacancies (VNs) such as (VGa)3(VN)3, is estimated at a few times 10−13 cm2. This σn value is also comparable to that of the major NRCs in p-GaN:Mg epilayers, namely, VGa(VN)2 or VGa(VN)3, although the clustering sizes of the defects are different. These σn values are commonly larger than the hole capture-cross-section (σp = 7 × 10−14 cm2) of the major NRCs, VGaVN divacancies, in n-type GaN. [ABSTRACT FROM AUTHOR]

Laser phase contrast is a powerful diagnostic method to determine the spatial distribution of electron density fluctuations in magnetically confined plasmas, although its applicability depends on magnetic field configurations. The spatial resolution of fluctuations is linked with the resolution of the propagation direction that is derived from the two-dimensional spectral analysis of the wavenumber for the fluctuations. The method was applied to fluctuation measurements in a compact helical system. In order to improve the resolution of the propagation direction with a relatively small number of data points, the maximum entropy method with polar coordinates was employed. A spatial resolution of the order of 1 cm was obtained, which is satisfactory in a plasma with a 20 cm minor radius. [ABSTRACT FROM AUTHOR]

A near-infrared laser phase contrast optical system incorporating a folded beam was developed in order to measure the distribution of density fluctuations in a high-temperature plasma. The coherent light source used was an yttrium aluminum garnet laser stabilized by a ring oscillator. The probe beam system separates and reflects the incident and exiting beams with a polarizer and a fully reflective mirror with a waveplate. This system was employed with a compact helical system to detect fluctuations at the plasma edge. [ABSTRACT FROM AUTHOR]

The experimental studies on zonal flows and turbulence have been carried out in Compact Helical System [K. Matsuoka, S. Kubo, M. Hosokawa et al., in Plasma Physics and Controlled Nuclear Fusion Research, Proc. 12th Int. Conf., Nice, 1988 (International Atomic Energy Agency, Vienna, 1989, Vol. 2, p. 411] using twin heavy ion beam probes. The paper presents the experimental observations of stationary zonal flow, nonlinear couplings between zonal flow and turbulence, and the role of zonal flow in the improved confinement, together with the recent discovery of zonal magnetic field. The presented experimental results strongly support the new paradigm that the plasma transport should be considered as a system of drift wave and zonal flows, and provides the first direct evidence for turbulence dynamo that the structured magnetic field can be really generated by turbulence. [ABSTRACT FROM AUTHOR]

Heavy ion beam probe (HIBP) is an excellent diagnostic to measure the density and potential fluctuations simultaneously in magnetically confined plasmas. However, it has been well known that the density fluctuation measured with the HIBP is not local but contains the fluctuations along the beam orbits. In this article, a method is proposed to evaluate local density fluctuation in the HIBP measurements by removing the well-known path integral effects. The reconstructed density fluctuation amplitude and power spectrum are shown, for example, by applying the proposed method on the density fluctuation measurement data obtained in a toroidal helical plasma, Compact Helical System. [ABSTRACT FROM AUTHOR]

A neutral lithium beam probe for two-dimensional diagnosis of edge plasmas has been designed and installed on the compact helical system. A lithium beam with an energy of 15 keV and a current of 0.1 mA is used. The spatial resolution is about 10 mm, and the time response is about 10 ms. The beam penetration depth is expressed in terms of the line integral density , which is about 2×1018 m-2. The beam injection angle can be varied and the observation point covers the edge and separatrix region of the helical diverter configuration. Two-dimensional electron density profiles for electron cyclotron heating and neutral beam injection (NBI) heated plasmas are obtained near and outside the last closed flux surface (LCFS). Analysis for two-dimensional density profile reconstruction indicates that significant amounts of surface plasma are confined outside the LCFS for NBI plasmas even though the ergodic layer is cut by the vacuum chamber wall (inboard limiter configuration). The usefullness of this new two-dimensional diagnostic in the edge region is demonstrated. [ABSTRACT FROM AUTHOR]

The close relationship between the behavior of the edge plasma characteristics and the global confinement in discharges in tokamaks has been extensively explored in recent years. A 15 keV lithium beam with 80% neutralization efficiency, using a high ion current density emitter and a modified Pierce gun design for extraction of ions, was developed for edge studies on the spherical tokamak ETE—(major radius R=0.30 m, aspect ration A=1.5, designed plasma current of 200 kA with a torodial magnetic field up to 0.4 T). Already in the calibration phase this diagnostics confirmed to be an efficient experimental tool, allowing accurate measurements of the fast varying pressure of the discharges in ETE. The measurement of the temporal evolution of the density of the plasma edge with fast neutral lithium beam is in accordance with data from Langmuir probe for the region near the limiter and the one made by the Thomson scattering for the inner region. [ABSTRACT FROM AUTHOR]

This article presents a method for density profile reconstruction using a heavy ion beam probe. A model calculation is performed to demonstrate the feasibility of the proposed method. The possibility to infer the density profile widely extends the applicability of the heavy ion beam probe by providing fine structural measurements of density profile that is, in principle, continuous in space, with a high temporal resolution of milliseconds. [ABSTRACT FROM AUTHOR]

Experimental test of the radial force balance equation was done in the Compact Helical System Heliotron/Torsatron [S. Okamura et al. Nucl. Fusion 39, 1337 (1999)]. A radial electric field is measured with a heavy ion beam probe, while plasma rotation and drift velocity of fully ionized carbon are measured with charge exchange spectroscopy. The two measurements agree with each other to within 10% of the radial electric field in a wide range of electron densities of 0.3-2.0x10[sup 19] m[sup -3]. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A fully calibrated multilayer mirror (MLM) based soft x-ray spectrometer has been developed and installed on the Compact Helical System (CHS). This MLM based spectrometer is a new approach to devising a soft x-ray spectrometer with medium time and energy resolutions. The spectrometer mainly consists of a MLM as the dispersive element and a 20-channel (p-i-n) pin diode array for the detection of the soft x-rays. Both the MLM and the pin diodes have been calibrated at the KEK (High Energy Accelerator Research Organization) Photon Factory for an energy range of 300–1200 eV. The reflectivity of the mirror was found to increase from 3% (photon energy of 335 eV) to 25% (photon energy of 1050 eV). The pin diodes were found to have almost 100% efficiency for creating electron-hole pairs in the calibrated energy range. The spectrometer was installed on the CHS with the aim of fast electron temperature measurement and study of fast magneto-hydrodynamic (MHD) events occurring in plasmas. The experiments show that in the energy range of measurement, the electron “temperature” determined from the slope of the soft x-ray spectrum is much lower than that measured by the Thomson scattering diagnostic. Analysis showed that the soft x-ray spectrum is highly contaminated by impurity emission. Therefore it may be possible to measure electron temperature with this diagnostic if we choose another energy range where we can measure the continuous spectrum. The present time resolution of the system is of the order ∼0.1 ms, which made it possible to study the behavior of the plasma during fast MHD events. Modulations in the soft x-ray intensity were observed during MHD events in CHS plasmas. Analysis of these shows that these may be due to modulations in the temperature or the impurity concentration. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Large Helical Device (LHD) is the largest helical machine with superconducting coils. Key diagnostics issues for LHD are: (a) capability for multidimensional measurements because of the nonaxisymmetric toroidal plasma; (b) measurements of the electric field; (c) cross check of fundamental parameters using different methods; (d) advanced measurements appropriate for steady-state operation; and (e) a satisfactory data acquisition system. The design and research and development of plasma diagnostics were carried out taking these issues into consideration. As a result, the present status of diagnostics is described: diagnostics for LHD operation, fundamental diagnostics for plasma performance, diagnostics for physics subjects, innovative diagnostics and diagnostics for long-pulse operation. The LHD experiment started in March, 1998. Since then, the development of diagnostics has kept pace with the experimental campaigns. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents thermal transport barrier in heliotron-type devices. Features associated with the transport barrier; Observation on the internal electron transport barrier; Effect of high shear of the radial electric field on turbulence.

The stabilization of low-frequency density fluctuations is evidenced in the Nagoya Bumpy Torus (NBT-1M) [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1985), Vol. 2, p. 551] in the presence of microwave-heated hot-electron rings. Flute-type fluctuations, which are considered to be stabilized by the charge-uncovering effect of the rings, are found to cause large plasma losses, and to affect radial density profiles in the way that the lower fluctuation level yields the steeper density gradient. The particle confinement is, therefore, improved by the hot-electron rings to some extent, but is mainly determined by the plasma convection, which is expected from the discrepancy between density and potential profiles. It is also found that fluctuations in a toroidal plasma inside the ring grow when a weak negative ambipolar potential and a steep density gradient are formed, and are reduced to a low level when a deep potential well is achieved. [ABSTRACT FROM AUTHOR]

Discusses two problems inherent with heavy ion beam probe measurements. Effect of fluctuations with rather long scale length; Conditions where the intensity fluctuation can be regarded as being local fluctuation of electron density; Contamination of the fluctuations by fluctuations along the trajectory.

Describes the active trajectory control for a heavy ion beam probe (HIBP) on the compact helical system. Measurement of the potential profiles of the toroidal helical plasma; Description of the results of the potential profile measurements.

Reports on the preliminary characterization of the second-order focusing property and the energy resolution obtained in the cylindrical energy analyzer, an electrostatic energy analyzer used in the scientific fields. Examination of the possibility that the cylindrical analyzer could have second-order focusing using theoretical analysis.

A succinct model is proposed to describe non-ideal characteristics owing to electric field penetration into the drift region in actual parallel plate energy analyzers. A good agreement has been obtained between the theoretically expected and experimentally observed focus properties of the 30° parallel plate analyzer. [ABSTRACT FROM AUTHOR]

Active control of beam trajectories using four sets of sweep plates is proposed for a heavy ion beam probe (HIBP) system on helical plasma, such as torsatron, stellarator, etc., in order to solve problems arising from the complicated beam trajectories in its nonaxisymmetrical magnetic field. By this method, a series of the probing beam trajectories giving a whole potential distribution can be managed to pass at a particular point (detector aperture) with the same angle. An equation is introduced as a guiding principle to simplify the optimization process of geometrical configuration of the HIBP system, which will be adjusted for different magnetic configurations by only changing the sweep plate voltages. [ABSTRACT FROM AUTHOR]

Presents space potential profiles directly observed in a toroidal helical plasma of the Compact Helical System, using a heavy ion beam probe. Potential profiles' exhibition of widely varied characteristics, including positive and negative polarities for electron cyclotron and neutral beam-heated plasmas.

The radial electric field is driven to positive value by off-axis second harmonic electron cyclotron heating (ECH) in the Compact Helical System [Plasma Physics and Controlled Nuclear Fusion Research 1988, Nice (International Atomic Energy Agency, Vienna, 1989), Vol. II, p. 411]. The observed positive electric field is associated with the outward particle flux enhanced with ECH. The enhanced particle flux triggered by the production of the electrons accelerated perpendicularly to the magnetic field with ECH results in the change of the electric field. [ABSTRACT FROM AUTHOR]

The radial electric field profiles derived from poloidal rotation velocity are compared with neoclassical estimates for a plateau regime plasma heated by a tangentially injected neutral beam in the Compact Helical System Heliotron/Torsatron device [Plasma Physics and Controlled Nuclear Fusion Research 1988 (IAEA, Vienna, 1989), Vol. 2, p. 411]. The measured electric field (Er) is more negative than the neoclassical prediction, especially near the plasma periphery. A gradient of radial electric field (∂Er/∂r) in this region is found to be more negative in the plasma with higher electron density. Although the corresponding poloidal rotation velocity is too small to trigger the L/H transition, these negative Er and ∂Er/∂r are associated with a reduction of thermal electron conductivity. [ABSTRACT FROM AUTHOR]

The effect of field irregularity due to the use of conducting meshes to cover holes of a lower electrode in a parallel-plate electrostatic analyzer, is experimentally and numerically investigated. Displacement of a focal point and degradation of analyzer characteristics are found in the experiment. Dependence of analyzer characteristics on wire spacing is obtained. Primary results are also confirmed by numerical analysis. Criteria for error estimation are theoretically derived and found to be consistent with the experiment. [ABSTRACT FROM AUTHOR]

Two different electron density reconstruction methods for 8-kev neutral lithium beam probing have been developed for the Compact Helical System (CHS). Density dependences on emission and ionization processes are included by using effective rate coefficients obtained from the collisionaI radiative model. Since the two methods differ in the way the local beam density in the plasma is determined, the methods have different applicable electron densities. The beam attenuation is calculated by iteration from the electron density profile in method I. In method II, the beam remainder at the observation point z is determined by integrating the Li I emission intensity from z toward the position of emission tail-off. At the emission tail-off, the fast lithium beam is completely attenuated. Selecting an appropriate method enables us to obtain edge electron density profile well inside the last closed flux surface for various ranges of plasma densities (10[sup 12]-5 × 10[sup 13] cm[sup -3]). The electron density profiles reconstructed by these two different methods are in good agreement with each other and are consistent with results from ruby laser Thomson scattering. [ABSTRACT FROM AUTHOR]

A new modification on the parallel plate analyzer for 500 keV heavy ions to eliminate the effect of the intense UV and visible radiation, is successfully implemented. Its design principle and results of numerical optimization are presented. In addition, the analyzer characteristics experimentally obtained in a high temperature plasma device, are discussed. [ABSTRACT FROM AUTHOR]

Reports that a formula including correction due to change of beam injection angle has been derived for measurements of beam energy using parallel plate energy analyzers. Potential measurements in high temperature plasma with heavy ion beam probes; Example to demonstrate the usage of the formula.

A lithium neutral beam probe has been improved for space and time-resolved measurements of plasma density in NBT-1M. A lithium neutral beam (4 keV, 10-30 µA) is injected into the plasma and photon flux emitted from the injected lithium atoms by electron impact excitation is detected. This cross section is not sensitive to the electron temperature in a wide range (10 eV < T[sub e] < 200 eV) and the photon flux intensity is proportional to the electron density. Typical spatial and temporal resolutions are 1 cm and a few milliseconds, respectively. Beam attenuation is not severe for plasmas with n1 < 10[sup 14] cm[sup -2] unless the ion temperature is very high (T[sub i] > 1 keV), where the attenuation through charge-exchange process becomes dominant. This method is not influenced by the magnetic field and can be applied to plasmas in any magnetic field configuration. [ABSTRACT FROM AUTHOR]

Proposes a tandem parallel plate analyzer that has the second-order focus at any injection angle. Analyzer model and theory; Advantages of the small injection angle; Angular aberration of the tandem parallel plate analyzer; Energy dispersion of the analyzer.

Discusses several improvements in the high-voltage heavy-ion-beam probe (HIBP). Precision slide mount of the detector plates parallel to the base electrode; Reduction of the effect of ultraviolet radiation in the analyzer with a two-staged optical trap.

A new method combining Li[sup 0]-beam probing and spectroscopic techniques has been developed to measure local magnetic fields in a bumpy torus. A collimated thermal Li[sup 0] beam is injected into the plasma. The Zeeman pattern of the lithium resonance radiation (2²S-2²p, 6708 Å) is observed with a Fabry-Perot interferometer. The strength of the local magnetic field is determined from the splitting between two π components of the 2²S[sub ½]-2²P[sub ½] transition with the spatial resolution of about 7 mm. The magnetic fields from 2 to 4 kG were measured with accuracy of ±2%. This method is applicable to plasmas of a line-integrated electron density below 5 × 10[sup 12] cm[sup -2]. [ABSTRACT FROM AUTHOR]

The radial electric field profiles are derived from poloidal rotation velocity for a plateau regime plasma heated by a neutral beam in a CHS device. The measured electric field is more negative than the neoclassical prediction especially near the plasma periphery. However, the negative radial electric field is not directly related to the reduction of thermal diffusivity. [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 multilayer mirror (MLM) soft x-ray spectrometer has been installed on the Compact Helical System. The x-ray energy spectrum from ECH and neutral beam injected heated low beta plasmas was measured as a function of radiation energy. Modulations of the x-ray intensity associated with magnetohydrodynamic instability were observed. The electron temperature measurement was also investigated using the MLM x-ray spectrometer. The data suggest the possibility of a fluctuation measurement of the electron temperature T[sub e] with fast time resolution using the MLM spectrometer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A high-brightness and low-divergence neutral beam is obtained for a Zeeman polarimetry of edge plasmas on JT-60U. The electron density and the pitch angle of the magnetic field line, thus the plasma current density distribution, can be measured by the Zeeman polarimetry using the lithium beam. A thermionic ion source heated by an electron beam is developed in order to obtain the ion beam current extraction over 10 mA. The beam optics is designed after detailed numerical simulation taking the space charge effects into account because a low-divergence angle of the neutral lithium beam leads to a narrow spectrum of the beam emission. It is also necessary to keep the beam radius small for good spatial resolution due to a long beam line of 6.5 m. The newly developed ion gun is operated on a test stand which simulates the diagnostic arrangement on JT-60U. The ion beam current of 10 mA at a beam energy of 10 keV is successfully extracted from the ion source operated at the temperature over 1300 degrees C and focused by Einzel lens. The full width at half maximum radius of the ion beam at the neutralizer is about 9 mm. A sodium vapor neutralizer neutralizes the collimated ion beam fully at the temperature of 300 degrees C. The neutral beam profiles are measured at two locations of the beam line at Z=2.3 m (beam monitor position) and Z=6.5 m (plasma region). The half-width at half maximum radius of the neutral beam of 26 mm and the equivalent beam current of 3 mA with the beam divergence angle of 0.2 deg which is the half-angle divergence are obtained. Those parameters satisfy the requirements of the Zeeman polarimetry. Furthermore, a long pulse extraction with a current of 10 mA and duration of 50 s is attained.