Statistical Characterization of Boundary Fluctuations in the HT—7 Tokamak

The statistical properties of plasma fluctuations are characterized in the boundary region of HT-7 tokamak. A non-Gaussian feature is observed in fluctuations of ion saturation current and floating potential in most of the scrape-off layer regions. The statistical properties of fluctuations have a clear radial dependence, showing a near-Gaussian character in the proximity of the velocity shear layer location and another region where the poloidal velocity has a trend to zero. Fluctuations show a bursty character with pulses asymmetric in time and the time asymmetry reaches the minimum around the shear layer. From the results, we can see an obvious coupling of the pulses and the poloidal flow.     

Measurements of Boundary Plasma in Synergy Discharges of IBW and LHCD on the HT-7 Tokamak

By applying ion Bernstein wave (IBW) heating into the lower hybrid current drive (LHCD) plasma, improved confinements have been obtained in the HT-7 tokamak. The central electron temperature was doubled and the storage energy was increased significantly. The core electron density and temperature were broadened and ther profiles near the edge were steepened. A transport barrier has been formed in the vicinity of the limlter radial location. An enhanced shear in poloidal phase velocity was found in the same region with reduction of the fluctuation levels and the coherences between fluctuations. The results suggest that the improved confinement in the IBW and LHCD plasma is at least partially due to the modification of shear in poloidal velocity and then the suppression of fluctuations and fluctuation induced fluxes via de-correlation effect.     

Role of Advance Refuelling and Heating on Edge Reynolds Stress—Induces Poloidal Flow in HL—1M

The radial profile of electrostatic Reynolds stress,plasma poloidal rotations,radial and poloidsal electric fields have been measured in the plasma boundary region of the HL-1M tokamak using a multiarray of Mach/Langmuir probes.In the experiments of ohmic discharge,lower hybrid current drive,supersoniuc molecular beam injection (SMBI)and multi-shot pellet injection,the correlation between the Reynolds stress and poloidal flow in the edge plasma is presented.The adial profile changes of the Reynolds stress and poloidal flow velocity Vpol with lower hybrid wave injection power and SMBI injection are obtained.The results indicate that the sheared poloidal flow can be generated in tokamak plasma due to the radially varying Reynolds stress.     

High Bootstrap Current Fraction during the Synergy of LHCD and IBW on the HT-7 Tokamak

More than 70% of the total plasma current is sustained by the bootstrap current and current drive during the synergy of lower hybrid current driving (LHCD) and ion Berstein wave (IBW) heating on the HT-7 tokamak.The lower hybrid non-inductive current source is off-axis and well localized, and more than 35% bootstrap current plasma has been obtained. The IBW in controlling electron pressure profile can be integrated into the LHCD target plasma. The largest steep gradient of the electron pressure profile in the region ρ - 0.5-0.7 mostly comes from the electron temperature profile, which may induce the large fraction bootstrap current. The large off-axis bootstrap current can help to create negative magnetic shear, and the good plasma confinement is achieved.     

Characterisation of the plasma density with two artificial neural network models

This paper establishes two artificial neural network models by using a multi layer perceptron algorithm and radial based function algorithm in order to predict the plasma density in a plasma system.In this model,the input layer is composed of five neurons:the radial position,the axial position,the gas pressure,the microwave power and the magnet coil current.The output layer is the target output neuron:the plasma density.The accuracy of prediction is tested with the experimental data obtained by the Langmuir probe.The effectiveness of two artificial neural network models are demonstrated,the results show good agreements with corresponding experimental data.The ability of the artificial neural network model to predict the plasma density accurately in an electron cyclotron resonance-plasma enhanced chemical vapour deposition system can be concluded,and the radial based function is more suitable than the multi layer perceptron in this work.     

Characterisation of the two artificial neural plasma density with network models

This paper establishes two artificial neural network models by using a multi layer perceptron algorithm and radial based function algorithm in order to predict the plasma density in a plasma system. In this model, the input layer is composed of five neurons： the radial position, the axial position, the gas pressure, the microwave power and the magnet coil current. The output layer is the target output neuron： the plasma density. The accuracy of prediction is tested with the experimental data obtained by the Langmuir probe. The effectiveness of two artificial neural network models are demonstrated, the results show good agreements with corresponding experimental data. The ability of the artificial neural network model to predict the plasma density accurately in an electron cyclotron resonance-plasma enhanced chemical vapour deposition system can be concluded, and the radial based function is more suitable than the multi layer perceptron in this work.     

Frequency Response of Near-Wall Coherent Structures to Localized Periodic Blowing and Suction in Turbulent Boundary Layer

We experimentally investigate the frequency response of near-wall coherent structures to localized periodic blow- ing and suction through a spanwise slot in a turbulent boundary layer by changing the frequency of periodic disturbance at similar velocities of free stream. The effects of blowing and suction disturbance on energy redistri- bution, turbulent intensity U^＋ rums over y^＋ and waveforms of phase-averaged velocity during sweeping process are respectively discussed under three frequencies of periodic blowing and suction in near-wall region of turbulent boundary layer, compared with those in a standard turbulent boundary layer. The most effective disturbance frequency is figured out in this system.     

Investigation of lower hybrid current drive during H-mode in EAST tokamak

H-mode discharges with lower hybrid current drive (LHCD) alone are achieved in EAST divertor plasma over a wide parameter range. These H-mode discharges are characterized by a sudden drop in D_α emission and a spontaneous rise in main plasma density. Good lower hybrid (LH) coupling during H-mode is obtained by putting the plasma close to the antenna and by injecting D₂ gas from a pipe near the grill mouse. The analysis of lower hybrid current drive properties shows that the LH deposition profile shifts off axis during H-mode, and current drive (CD) efficiency decreases due to the increase in density. Modeling results of H-mode discharges with a general ray tracing code GENRAY are reported.     

Supersonic Shock Wave with Landau Quantization in a Relativistic Degenerate Plasma

A three-dimensional(3D)BurgersJ equation adopting perturbative methodology is derived to study the evolution of a shock wave with Landau quantized magnetic field in relativistic quantum plasma.The characteristics of a shock wave in such a plasma under the influence of magnetic quantization,relativistic parameter and degenerate electron density are studied with assistance of steady state solution.The magnetic field has a noteworthy control,especially on the shock wave's amplitude in the lower range of the electron density,whereas the amplitude in the higher range of the electron density reduces remarkably.The rate of increase of shock wave potential is much higher(lower)with a magnetic Held in the lower(higher)range of electron density.With the relativistic factor,the shock wave's amplitude increases significantly and the rate of increase is higher(lower)for lower(higher)electron density.The combined effect of the increase of relativistic factor and the magnetic field on the strength of the shock wave,results in the highest value of the wave potential in the lower range of the degenerate electron density.     

EDGE REYNOLDS STRESS STRUCTURE OF LOWER HYBRID WAVE INJECTION IN THE HL—1M PLASMA

The radial profiles of electrostatic Reynolds stress,plasma poloidal rotations,radial and poloidal electric fields have been measured in the plasma boundary region on the HL-1M tokamak using a multi-array of Mach/Langmuir probes.During experiments of lower hybrid wave current drive,the variations in LHW drive power xill cause changes in the edge electric field,poloidal rotation veloity and Reynolds stress.The results indicate that sheared poloidal flow can be generated in the edge plasma due to radially varied Reynolds stress.     

Simulation on effect of poloidal power spectrum on lower hybrid wave propagation

The coupling of lower hybrid wave to the plasma is a crucial issue for efficient current drive in tokamaks. This paper establishes a new coupling model which assumes the antenna to be a curved face and the plasma to be a cylinder. Power spectrum considering the coupling between wave-guides in both poloidal and toroidal direction is simply estimated and discussed. The effect of the poloidal wave vector on wave propagation, power deposition and driven current is also investigated with the help of lower hybrid current drive code. Results show that the poloidal wave vector affects the ray tracing, and also has effect on power deposition and driven current. The effect of the poloidal wave vector on power deposition and driven current profile depends on plasma parameters. Preliminary studies suggest that it seems possible to control the current profile by adjusting the poloidal phase difference between the waveguide in poloidal direction.     

Light trapping enhancement in thin film silicon solar cells with different front and back grating periodicities

Dual-grating structure thin-film silicon solar cells with different front and back grating periodicities are designed The geometrical parameters of both gratings are investigated. The reflection is greatly reduced by the front grating, whereas the absorption in the long wavelength is increased because of the back grating. The short circuit current of the combined structure is enhanced by 16.8% for a 1μm thick c-Si layer compared with that of the conformal grating structure. The short circuit current can be further increased by creating a relative lateral displacement between the front and back gratings. The displacement results in a more remarkably enhaneed absorption when the thickness of the aetive layer is reduced, indicating its importance in the design of ultra-thin high-efficiency solar cells.     

Landau damping in a bounded magnetized plasma column

The damping decrement of Landau damping and the effect of thermal velocity on the frequency spectrum of a propagating wave in a bounded plasma column are investigated.The magnetized plasma column partially filling a cylindrical metallic tube is considered to be collisionless and non-degenerate.The Landau damping is due to the thermal motion of charge carriers and appears whenever the phase velocity of the plasma waves exceeds the thermal velocity of carriers.The analysis is based on a self-consistent kinetic theory and the solutions of the wave equation in a cylindrical plasma waveguide are presented using Vlasov and Maxwell equations.The hybrid mode dispersion equation for the cylindrical plasma waveguide is obtained through the application of appropriate boundary conditions to the plasma-vacuum interface.The dependence of Landau damping on plasma parameters and the effects of the metallic tube boundary on the dispersion characteristics of plasma and waveguide modes are investigated in detail through numerical calculations.     

Numerical simulation and experimental validation of a direct current air corona discharge under atmospheric pressure

Air corona discharge is one of the critical problems associated with high-voltage equipment. Investigating the corona mechanism plays a key role in enhancing the electrical insulation performance. An improved self-consistent multi-component two-dimensional plasma hybrid model is presented for the simulation of a direct current atmospheric pressure corona discharge in air. The model is based on plasma hydrodynamic and chemical models, and includes 12 species and 26 reactions. In addition, the photoionization effect is introduced into the model. The simulation on a bar-plate electrode configuration with an inter-electrode gap of 5.0 mm is carried out. The discharge voltage-current characteristics and the current density distribution predicted by the hybrid model agree with the experimental measurements. In addition, the dynamics of volume charged species generation, discharge current waveform, current density distribution at an electrode, charge density, electron temperature, and electric field variations are investigated in detail based on the model. The results indicate that the model can contribute valuable insights into the physics of an air plasma discharge.     

Experimental study on spectrum and multi-scale nature of wall pressure and velocity in turbulent boundary layer

When using a miniature single sensor boundary layer probe,the time sequences of the stream-wise velocity in the turbulent boundary layer(TBL) are measured by using a hot wire anemometer.Beneath the fully developed TBL,the wall pressure fluctuations are attained by a microphone mechanism with high spatial resolution.Analysis on the statistic and spectrum properties of velocity and wall pressure reveals the relationship between the wall pressure fluctuation and the energy-containing structure in the buffer layer of the TBL.Wavelet transform shows the multi-scale natures of coherent structures contained in both signals of velocity and pressure.The most intermittent wall pressure scale is associated with the coherent structure in the buffer layer.Meanwhile the most energetic scale of velocity fluctuation at y+= 14 provides a specific frequency f9≈ 147 Hz for wall actuating control with Reτ= 996.     

Observation of Radial Propagation of Electrostatic Fluctuations in Edge Plasma of the Sino United Spherical Tokamak

Radial propagation of electrostatic fluctuations in the edge plasma of Sino-United Spherical Tokamak (SUNIST) has been measured using Langmuir probes. The propagation characteristics of the floating potential fluctuations are analysed by the two-point correlation technique. The results show radially outward propagation of the turbulent fluctuations at all measured radial positions. The power-average wavenumber profile is approximately constant in plasma edge region and suddenly increases to the limiter. These results are in good agreement with the model predictions proposed by Mattor which suggests that the drift wave propagation may be a source of edge turbulence.     

Effects of trapped electrons on off-axis lower hybrid current drive in tokamaks

The effects of trapped electrons on off-axis lower hybrid current drive （LHCD） in tokamaks are studied, A computer code for solving the Fokker-Planck equation in a toroidal geometry is developed and employed. The code is suitable for various auxiliary heating and current drive schemes in tokamak plasmas. The influence of the resonance regime on the current drive efficiency as well as the influence of trapped particle fraction on the current drive efficiency are emphasized. It is shown that, as an electrostatic force, the lower hybrid wave causes some of the trapped electrons to be untrapped and lose their energy, which can cut the LHCD efficiency by about 30%. The ITER scaling law is also used to estimate the trapped electron effects.[第一段]     

Self-aligned-gate AlGaN/GaN heterostructure field-effect transistor with titanium nitride gate

Self-aligned-gate heterostructure field-effect transistor(HFET) is fabricated using a wet-etching method.Titanium nitride(TiN) is one kind of thermal stable material which can be used as the gate electrode.A Ti/Au cap layer is fixed on the gate and acts as an etching mask.Then the T-shaped gate is automatically formed through over-etching the TiN layer in 30% H_2O_2 solution at 95 ℃.After treating the ohmic region with an inductively coupled plasma(ICP) method,an Al layer is sputtered as an ohmic electrode.The ohmic contact resistance is approximately 0.3 Ω·mm after annealing at a low-temperature of 575 ℃ in N_2 ambient for 1 min.The TiN gate leakage current is only 10~(-8) A after the low-temperature ohmic process.The access region length of the self-aligned-gate(SAG) HFET was reduced from 2 μm to 0.3 μm compared with that of the gate-first HFET.The output current density and transconductance of the device which has the same gate length and width are also increased.     

The high frequency acoustic radiation from the boundary layer of an axisymmetric body

1 IntroductionWhen an edsynunetric body moves under water, a boundary layer is formed at the surfaceof the body. There are three regions in the boundary layer: lallilnar, transition from the laminarto turbulent, and turbulent region. The boundary layyer near the stagnation region remainslaminar and subsequently it goes through transition to turbulence. The acoustic radiation fromthe transition and turbulellt boundary 18y6r is the main components of self-noise of marines,and so its mechanical s…     

Comparison of nitride-based dual-wavelength light- emitting diodes with an InAlN electron-blocking layer and with p-type doped barriers

The advantages of nitride-based dual-wavelength light-emitting diodes (LEDs) with an InAlN electron blocking layer (EBL) are studied. The emission spectra,carrier concentration in the quantum wells (QWs),energy band and internal quantum efficiency (IQE) are investigated. The simulation results indicate that an LED with an InAlN EBL performs better over a conventional LED with an AlGaN EBL and an LED with p-type-doped QW barriers. All of the advantages are due to the enhancement of carrier confinement and the lower electron leakage current. The simulation results also show that the efficiency droop is markedly improved and the luminous intensity is greatly enhanced when an InAlN EBL is used.