W. D. Liu

Experimental observation of ion-acoustic waves in an inhomogeneous dusty plasma

The propagation of a dust-ion-acoustic wave (DIAW) wave down the steep density gradient in an inhomogeneous diffusive dusty plasma is experimentally studied. It is observed that the presence of the dust enhances the plasma inhomogeneity. The phase velocity of the DIAW increases rapidly with distance and becomes supersonic. The interplay between the effects of the density nonuniformity and collisional damping results in the continuous transition of the relative amplitude of the DIAW from damping to growth. The experimental data are in agreement with theoretical prediction and confirm the importance of density-dependent ion–dust collisions.

First results from H-mode plasmas generated by ICRF heating in the EAST

Deuterium high-confinement (H-mode) plasmas, lasting up to 3.45 s, have been generated in the EAST by ion cyclotron range of frequency (ICRF) heating. H-mode access was achieved by coating the molybdenum-tiled first wall with lithium to reduce the hydrogen recycling from the wall. H-mode plasmas with plasma currents between 0.4 and 0.6 MA and axial toroidal magnetic fields between 1.85 and 1.95 T were generated by 27 MHz ICRF heating of deuterium plasma with hydrogen minority. The ICRF input power required to access the H-mode was 1.6–1.8 MW. The line-averaged density was in the range (1.83–2.3) × 1019 m−3. 200–500 Hz type-III edge localized mode activity was observed during the H-mode phase. The H-mode confinement factor, H98IPB(y, 2), was ~0.7.

Crack-free fully epitaxial nitride microcavity with AlGaN∕GaN distributed Bragg reflectors and InGaN∕GaN quantum wells

Using metal-organic chemical vapor deposition, we have fabricated fully epitaxial nitride microcavties with AlGaN∕GaN distributed Bragg reflectors and InGaN quantum wells as the light emitter. To solve the problem of cracking, a thin AlN anticracking layer was used. The samples were characterized using transmission electron microscope, reflectivity spectroscopy, and photoluminescence spectroscopy. A cavity quality factor of 200 was obtained and the spontaneous emission of cavity mode was measured from a 1λ GaN microcavity, with 40-pair Al0.24Ga0.76N∕GaN distributed Bragg reflectors as the bottom and top reflectors and three period In0.10Ga0.90N∕GaN quantum wells in the GaN cavity layer.

Ambipolar diffusion in an inhomogeneous dusty plasma

The ambipolar diffusion of plasma particles in a column of inhomogeneous plasma containing dust grains is investigated analytically as well as numerically. The grains act as charge sinks and behave like an immobile but charge-varying negative background. They can lead to significant depletion of the electron density and decrease the diffusion scale length. The theoretically obtained election density distribution agrees well with that from experiment.

Effect of high temperature and interface treatments on photoluminescence from InGaN/GaN multiple quantum wells with green light emissions

InGaN/GaN multiple quantum wells (MQWs) with emission peaks in the green light region were grown by metalorganic chemical vapor deposition. Photoluminescence (PL) measurements on these samples showed extra peaks in the spectrum beside the green emission due to reactor temperature ramping up to 1000 °C after the MQWs growth. It was found that growth interruption at the upper interface of MQWs combining with interface treatment by H2 flow could eliminate these extra peaks. Our analysis of the PL spectra suggested that these extra emission peaks originate due to the occurrence of In segregation in the upper part of the quantum wells induced by partial strain relaxation in the processes of high temperature treatment, and the interface treatment with H2 flow can suppress this phase separation phenomenon.InGaN/GaN multiple quantum wells (MQWs) with emission peaks in the green light region were grown by metalorganic chemical vapor deposition. Photoluminescence (PL) measurements on these samples showed extra peaks in the spectrum beside the green emission due to reactor temperature ramping up to 1000 °C after the MQWs growth. It was found that growth interruption at the upper interface of MQWs combining with interface treatment by H2 flow could eliminate these extra peaks. Our analysis of the PL spectra suggested that these extra emission peaks originate due to the occurrence of In segregation in the upper part of the quantum wells induced by partial strain relaxation in the processes of high temperature treatment, and the interface treatment with H2 flow can suppress this phase separation phenomenon.

Microwave Doppler reflectometer system in the Experimental Advanced Superconducting Tokamak.

A Doppler reflectometer system has recently been installed in the Experimental Advanced Superconducting (EAST) Tokamak. It includes two separated systems, one for Q-band (33-50 GHz) and the other for V-band (50-75 GHz). The optical system consists of a flat mirror and a parabolic mirror which are optimized to improve the spectral resolution. A synthesizer is used as the source and a 20 MHz single band frequency modulator is used to get a differential frequency for heterodyne detection. Ray tracing simulations are used to calculate the scattering location and the perpendicular wave number. In EAST last experimental campaign, the Doppler shifted signals have been obtained and the radial profiles of the perpendicular propagation velocity during L-mode and H-mode are calculated.

Theory of the special Smith-Purcell radiation from a rectangular grating

The recently uncovered special Smith-Purcell radiation (S-SPR) from the rectangular grating has significantly higher intensity than the ordinary Smith-Purcell radiation (SPR). Its monochromaticity and directivity are also much better. Here we explored the mechanism of the S-SPR by applying the fundamental electromagnetic theory and simulations. We have confirmed that the S-SPR is exactly from the radiating eigen modes of the grating. Its frequency and direction are well correlated with the beam velocity and structure parameters, which indicates its promising applications in tunable wave generation and beam diagnostic.

The influence of V defects on luminescence properties of AlInGaN quaternary alloys

In a certain growth mode, V pits have been observed in AlyInxGa1−x−yN grown by metal–organic chemical vapour deposition. Room temperature photoluminescence (PL) spectra from these quaternary alloys show strong band-to-band emission with equally intense yellow luminescence (YL). Simultaneous measurements of the PL intensity and topography using ultraviolet–visible near field scanning optical microscopy show that the enhanced YL intensity originates from the V pits whereas the intensity of the band edge PL decreases in the vicinity of pits. The strong YL emission can be related to the defect clusters associated with impurity decoration at dislocation lines and sidewalls of the pits. A higher degree of Al incorporation at partially filled V pits leads to the multiple near band edge PL peaks.

Structural Properties of ZnO Grown on GaN ∕ Sapphire Templates The Transition from Nanorods to Thin Films

ZnO nanorods were synthesized on GaN/sapphire substrates using a modified thermal-evaporation process. The as-synthesized ZnO nanorods and thin films were characterized using scanning electron microscopy, micro-Raman, and X-ray diffraction techniques. The morphology of the ZnO changes from nanorods to continuous thin films when the growth temperature increases to 800°C. Further increase in the growth temperature leads to a lower growth rate of ZnO along the (0001) direction. Micro-photoluminescence measurements show ultraviolet band-edge emission peaks around 378 nm from both nanorods and thin films. Realization of such ZnO structures may be useful for the fabrication of hybrid ZnO/GaN optoelectronic devices.

Optics design for J-TEXT ECE imaging with field curvature adjustment lens.

Significant progress has been made in the imaging and visualization of magnetohydrodynamic and microturbulence phenomena in magnetic fusion plasmas. Of particular importance has been microwave electron cyclotron emission imaging (ECEI) for imaging Te fluctuations. Key to the success of ECEI is a large Gaussian optics system constituting a major portion of the focusing of the microwave radiation from the plasma to the detector array. Both the spatial resolution and observation range are dependent upon the imaging optics system performance. In particular, it is critical that the field curvature on the image plane is reduced to decrease crosstalk between vertical channels. The receiver optics systems for two ECEI on the J-TEXT device have been designed to ameliorate these problems and provide good performance with additional field curvature adjustment lenses with a meniscus shape to correct the aberrations from several spherical surfaces.