A Refined Study on ECR Wave Propagation and Absorption in the Weakly Relativistic Plasma

1. IntroductionThe Electron Cyclotron Resonance (ECR) waveheating is now a routine method for plasma heat-lng and profile control in fuslon devices and alsoin plasma applications. Theoretical study of ECRwave propagation and absorption began very earlyin l950's, and its basic theoretical work was accom-plished during the 1970~l980 s [1--8] For toroidaldevices like the tokamak, the fundamental O-modeand X-mode with a nearly--perpendicular propaga-tion were used very often. For pure O--mode…     

Electrical and optical measurements in the early hydrogen discharge of GLAST-Ⅲ

This work presents the first electrical and optical measurements of the initial phase of hydrogen discharge in the upgraded spherical tokamak GLAST-III, initiated with electron cyclotron heating(ECH). Diagnostic measurements provide insights into expected and unexpected physics issues related to the initial phase of discharge. A triple Langmuir probe(TLP) has been developed to measure time series of the floating potential, plasma electron temperature and number density over the entire discharge, allowing monitoring of the two phases of the discharge: the ECH pre-ionization phase following by the plasma current formation phase. A TLP has the ability to give time-resolved measurements of the floating potential(V_(float)), electron temperature(T_e) and ion saturation current(I_(sat)∝n_e√kT_e).sat e eThe evolution of the ECH-assisted pre-ionization and subsequent plasma current phases in one shot are well envisioned by the probe. Intense fluctuations in the plasma current phase advocate for efficient equilibrium and feedback control systems. Moreover, the emergence of some strong impurity lines in the emission spectrum, even after only a few shots, suggests a crucial need for improvements in the base vacuum level. A noticeable change in the shape of the temporal profiles of the floating potential, electron temperature, ion saturation current(I_(sat)) and light emission has been observed with changing hydrogen fill pressure and vertical magnetic field.     

Comparison of Nitric Oxide Concentrations in μs-and ns-Atmospheric Pressure Plasmas by UV Absorption Spectroscopy

In this paper,an absorption spectroscopy measurement method was applied on two atmospheric pressure plasma sources to determine their production of nitric oxide.The concentrations are essential for evaluating the plasma sources based on the principle of the Dielectric Barrier Discharge(DBD)for applications in plasma medicine.The described method is based on a setup with an electrodeless discharge lamp filled with a mixture of oxygen and nitrogen.One of the emitted wavelengths is an important resonance wavelength of nitric oxide(λ = 226.2 nm).By comparing the absorption behaviour at the minimum and maximum of the spectral absorption cross section of nitric oxide around that wavelength,and measuring the change in intensity by the absorbing plasma,the concentration of nitric oxide inside the plasma can be calculated.The produced nitric oxide concentrations depend on the pulse duration and are in the range of 180 ppm to 1400 ppm,so that a distance of about 10 cm to the respiratory tract is enough to conform to the VDI Guideline 2310.     

X-Mode Frequency Modulated Density Profile Reflectometer on EAST Tokamak

An extraordinary-mode(X-mode) frequency-modulated continuous-wave(FMCW) profile reflectometer has been built on EAST.In the reflectometer,continuous waves with frequency sweeping from 12.5 GHz to 18 GHz were generated through a Hyperabrupt Tuned-varactor Oscillator(HTO) source and a four times active multiplier was used to increase the frequency to V-band(50 GHz to 72 GHz).The polarization of horn lens antenna is perpendicular to the magnetic field line at the edge plasmas.According to the V-band frequency range and polarization,the system cover density range from 0.5 × 1019m 3to 3.0 × 1019m 3(when toroidal magnetic field is 1.8 T),with time resolution of 12.5 ~ 50 μs.The density profile could be calculated by assuming the edge profile through an empirical equation.The maximum spatial error deduced by the method is about 4 cm.This reflectometer has been successfully applied in 2010 autumn EAST campaign,the temporal evolution of density profiles was acquired during the low confinement mode to high confinement mode transition.The density pedestal of EAST Tokamak was observed and the top value and gradient of the density pedestal were estimated.     

Quasi-3D electron cyclotron emission imaging on J-TEXT

Electron cyclotron emission imaging(ECEI) can provide measurements of 2D electron temperature fluctuation with high temporal and spatial resolution in magnetic fusion plasma devices. Two ECEI systems located in different toroidal ports with 67.5 degree separation have been implemented on J-TEXT to study the 3D structure of magnetohydrodynamic(MHD) instabilities. Each system consists of 12(vertical) × 16(horizontal) = 192 channels and the image of the 2nd harmonic X-mode electron cyclotron emission can be captured continuously in the core plasma region. The field curvature adjustment lens concept is developed to control the imaging plane for receiving optics of the ECEI systems. Field curvature of the image can be controlled to match the emission layer. Consequently, a quasi-3D image of the MHD instability in the core of the plasma has been achieved.     

Present Status of the Nd:YAG Thomson Scattering System Development for Time Evolution Measurement of Plasma Profile on Heliotron J

A new high repetition rate Nd:YAG Thomson scattering system is developed for the Heliotron J helical device.A main purpose of installing the new system is the temporal evolution measurement of a plasma profile for improved confinement physics such as the edge transport barrier (H-mode) or the internal transport barrier of the helical plasma.The system has 25 spatial points with ～10mm resolution.Two high repetition Nd:YAG lasers (>550mJ@ 50Hz) realize the measurement of the time evolution of the plasma profile with ～10ms time intervals.Scattered light is collected by a large concave mirror (D=800 mm,f/2.25) with a solid angle of ～100 mstr and transferred to interference filter polychromators by optical fiber bundles in a staircase form.The signal is amplified by newly designed fast preamplifiers with DC and AC output,which reduces the low frequency background noise.The signals are digitized with a multi-event QDC,fast gated integrators.The data acquisition is performed by a VME-based system operated by the CINOS.     

Study of plasma-based stable and ultra-wideband electromagnetic wave absorption for stealth application

A plasma-based stable,ultra-wideband electromagnetic (EM) wave absorber structure is studied in this paper for stealth applications.The stability is maintained by a multi-layer structure with several plasma layers and dielectric layers distributed alternately.The plasma in each plasma layer is designed to be uniform,whereas it has a discrete nonuniform distribution from the overall view of the structure.The nonuniform distribution of the plasma is the key to obtaining ultra-wideband wave absorption.A discrete Epstein distribution model is put forward to constrain the nonuniform electron density of the plasma layers,by which the wave absorption range is extended to the ultra-wideband.Then,the scattering matrix method (SMM) is employed to analyze the electromagnetic reflection and absorption of the absorber structure.In the simulation,the validation of the proposed structure and model in ultra-wideband EM wave absorption is first illustrated by comparing the nonuniform plasma model with the uniform case.Then,the influence of various parameters on the EM wave reflection of the plasma are simulated and analyzed in detail,verifying the EM wave absorption performance of the absorber.The proposed structure and model are expected to be superior in some realistic applications,such as supersonic aircraft.     

Present Status of the KSTAR Superconducting Magnet System Development

The mission of Korea Superconducting Tokamak Advanced Research (KSTAR) project is to develop an advanced steady-state superconducting tokamak for establishing a scientific and technological basis for an attractive fusion reactor. Because one of the KSTAR mission is to achieve a steady-state operation, the use of superconducting coils is an obvious choice for the magnet system. The KSTAR superconducting magnet system consists of 16 Toroidal Field (TF) coils and 14 Poloidal Field (PF) coils. Internally-cooled Cable-In-Conduit Conductors (CICC) are put into use in both the TF and PF coil systems. The TF coil system provides a field of 3.5 T at the plasma center and the PF coil system is able to provide a flux swing of 17 V-sec. The major achievement in KSTAR magnet-system development includes the development of CICC,the development of a full-size TF model coil, the development of a coil system for background magnetic-field generation , the construction of a large-scale superconducting magnet and CICC test facility. TF and PF coils are in the stage of fabrication to pave the way for the scheduled completion of KSTAR by the end of 2006.     

Evolution of plasma parameters in an Ar-N2/He inductive plasma source with magnetic pole enhancement

Magnetic pole enhanced inductively coupled plasmas(MaPE-ICPs) are a promising source for plasma-based etching and have a wide range of material processing appUcations.In the present study Langmuir probe and optical emission spectroscopy were used to monitor the evolution of plasma parameters in a MaPE-ICP Ar-N_2/He mixture plasma.Electron density(n_e) and temperature(T_e),excitation temperature(T_(exc)),plasma potential(V_p),skin depth(δ) and the evolution of the electron energy probability function(EEPF) are reported as a function of radiofrequency(RF) power,pressure and argon concentration in the mixture.It is observed that n_e increases while T_e decreases with increase in RF power and argon concentration in the mixture.The emission intensity of the argon line at 750.4 nm is also used to monitor the variation of the 'high-energy tail' of the EEPF with RF power and gas pressure.The EEPF has a'bi-Maxwellian' distribution at low RF powers and higher pressure in a pure N_2 discharge.However,it evolves into a 'Maxwellian' distribution at RF powers greater than 70 W for pure N_2,and at 50 W for higher argon concentrations in the mixture.The effect of argon concentration on the temperatures of two electron groups in the 'bi-Maxwellian' EEPF is examined.The temperature of the low-energy electron group T_L shows a decreasing trend with argon addition until the 'thermalization' of the two temperatures occurs,while the temperature of high-energy electrons T_H decreases continuously.     

On the heating mechanism of electron cyclotron resonance thruster immerged in a non-uniform magnetic field

To study the heating mechanism of electron cyclotron resonance thruster(ECRT) immersed in a non-uniform magnetic field, experiments and simulations are performed based on an electron cyclotron resonance plasma source at ASIPP. It is found that the first harmonic of electron cyclotron resonance is essential for plasma ignition at high magnetic field(0.0875 T), while the plasma can sustain without the first and second harmonics of electron cyclotron resonance at low magnetic field(till 0.0170 T). Evidence of radial hollow density profile indicates that upper hybrid resonance, which has strong edge heating effect, is the heating mechanism of low-field ECRT. The heating mode transition from electron cyclotron resonance to upper hybrid resonance is also revealed. Interestingly, the evolutions of electron temperature and electron density with input power experience a ‘delayed' jump, which may be correlated with the different power levels required for cyclotron and ionization. Moreover, when the field strength decreased, the variation of electron density behaves in an opposite trend with that of electron temperature,implying a possible competition of power deposition between them. The present work is of great interest for understanding the plasma discharge in ECRT especially immersed in a non-uniform magnetic field, and designing efficient ECRT using low magnetic field for economic space applications.     

Spatiotemporal Evolution of Stimulated Raman Scattering in the Absolute and Convective Regimes

A three-wave interaction (3WI) code is developed to study the stimulated Raman scattering (SRS) in both absolute and convective regimes. In the simulations, the time and spatial evolutions of a plasma wave are described by temporal growth rate and spatial factor, respectively. The spatial factors in different phases and different instability regimes are investigated. It is found that the spatial factor is caused by the finite velocity of the pump wave in the first phase and by damping in the last phase. With inclusion of the spatial factor, the temporal growth rate decreases and the threshold for SRS for a finite frequency mismatch increases. Meanwhile, the effects of wave frequency mismatch on the temporal growth rate are also discussed.     

Electric Potential in Surface Produced Negative Ion Source with Magnetic Field Increasing Toward a Wall

Electric potential near a wall for plasma with the surface produced negative ions with magnetic field increasing toward a wall is investigated analytically.The potential profile is derived analytically by using a plasma-sheath equation,where negative ions produced on the plasma grid (PG) surface are considered in addition to positive ions and electrons.The potential profile depends on the amount and the temperature of the surface produced negative ions and the profile of the magnetic field.The negative potential peak is formed in the sheath region near the PG surface for the case of strong surface production of negative ions or low temperature negative ions.As the increase rate of the magnetic field near the wall becomes large,the negative potential peak becomes small.     

Analysis of electron temperature,impurity transport and MHD activity with multi-energy soft x-ray diagnostic in EAST tokamak

A new edge tangential multi-energy soft x-ray(ME-SXR) diagnostic with high temporal(≤ 0.1 ms) and spatial(~1 cm) resolution has been developed for a variety of physics topics studies in the EAST tokamak plasma. The fast edge electron temperature profile(approximately from r a~ 0.6 to the scrape-off layer) is investigated using ME-SXR diagnostic system. The data process was performed by the ideal ‘multi-foil' technique, with no priori assumptions of plasma profiles. Reconstructed ME-SXR emissivity profiles for a variety of EAST experimental scenarios are presented here for the first time. The applications of the ME-SXR for study of the effects of resonant magnetic perturbation on edge localized modes and the first time neon radiating divertor experiment in EAST are also presented in this work. It has been found that neon impurity can suppress the 2/1 tearing mode and trigger a 3/1 MHD mode.     

Time-Resolved Emission Spectroscopic Study of Laser-Induced Steel Plasmas

Laser-induced steel plasma is generated by focusing a Q-switched Nd:YAG visible laser(532 nm wavelength) with an irradiance of 1 x 109 W/cm2 on a steel sample in air at atmospheric pressure.An Echelle spectrograph coupled with a gateable intensified charge-coupled detector is used to record the plasma emissions.Using time-resolved spectroscopic measurements of the plasma emissions,the temperature and electron number density of the steel plasma are determined for many times of the detector delay.The validity of the assumption by the spectroscopic methods that the laser-induced plasma(LIP) is optically thin and is also in local thermodynamic equilibrium(LTE) has been evaluated for many delay times.From the temporal evolution of the intensity ratio of two Fe I lines and matching it with its theoretical value,the delay times where the plasma is optically thin and is also in LTE are found to be 800 ns,900 ns and 1000 ns.     

Experimental investigation of the electromagnetic effect and improvement of the plasma radial uniformity in a large-area,very-high frequency capacitive argon discharge

We performed an experimental investigation on the electromagnetic effect and the plasma radial uniformity in a larger-area, cylindrical capacitively coupled plasma reactor. By utilizing a floating hairpin probe, dependences of the plasma radial density on the driving frequency and the radio-frequency power over a wide pressure range of 5–40 Pa were presented. At a relatively low frequency(LF, e.g. 27 MHz), an evident peak generally appears near the electrode edge for all pressures investigated here due to the edge field effect, while at a very high frequency(VHF, e.g.60 or 100 MHz), the plasma density shows a sharp peak at the discharge center at lower pressures, indicating a strong standing wave effect. As the RF power increases, the center-peak structure of plasma density becomes more evident. With increasing the pressure, the standing wave effect is gradually overwhelmed by the ‘stop band' effect, resulting in a transition in the plasma density profile from a central peak to an edge peak. To improve the plasma radial uniformity, a LF source is introduced into the VHF plasma by balancing the standing wave effect with the edge effect. A much better plasma uniformity can be obtained if one chooses appropriate LF powers, pressures and other corresponding discharge parameters.     

Development of a 140 GHz Steerable Launcher for the HL-2A ECRH System

A new electron cyclotron resonance launcher system has been designed and installed on heating and current drive （ECRH/ECCD） the HL-2A tokamak to inject four beams and enable continuous millimeter-wave beam scanning independently in the toroidal and poloidal direc- tions for ECRH/ECCD experiments. The launcher is connected to four mm-wave lines capable of transmitting high power up to 3 MW with two 1 MW/140 GHz/3 s and two 0.5 MW/68 GHz/1 s beams. Based on ray tracing simulation using the TORAY-GA code, tile scanning range of wave beams is -15~~15~ in the toroidal direction and 0~~10~ in the poloidal one for 140 GHz beams, which could cover half of the cross section of plasmas and can satisfy the requirements of advanced physical experiments. The beam radii in the plasma is 17.1 mm and 20 mm for the two 140 GHz beams and 29.5 nnn for the two 68 GHz beams, respectively, allowing a very high localization of the absorbed power. The performance of the steering system was proven to be reliable and the linearity is perfect between the displacement of drive shaft and rotate angle of mirror. Addition- ally the injection performance of the wave beams was optinfized by simultaneously setting the injection angle and the polarization to realize desirable pure O- or X-mode injection.     

Directional power absorption in helicon plasma sources excited by a half-helix antenna

This paper deals with the investigation of the power absorption in helicon plasma excited through a half-helix antenna driven at 13.56 MHz. The simulations were carried out by means of a code,HELIC. They were carried out by taking into account different inhomogeneous radial density profiles and for a wide range of plasma densities, from 10~(11) cm~(-3) to 10~(13) cm~(-3). The magnetic field was 200, 400, 600 and 1000 G. A three-parameter function was used for generating various density profiles with different volume gradients, edge gradients and density widths. The density profile had a large effect on the efficient Trivelpiece–Gould(TG) and helicon mode excitation and antenna coupling to the plasma. The fraction of power deposition via the TG mode was extremely dependent on the plasma density near the plasma boundary. Interestingly, the obtained efficient parallel helicon wavelength was close to the anticipated value for Gaussian radial density profile.Power deposition was considerably asymmetric when the n/B_0 ratio was more than a specific value for a determined density width. The longitudinal power absorption was symmetric at approximately n_0 =10~(11) cm~(-3), irrespective of the magnetic field supposed. The asymmetry became more pronounced when the plasma density was 10~(12) cm~(-3). The ratio of density width to the magnetic field was an important parameter in the power coupling. At high magnetic fields, the maximum of the power absorption was reached at higher plasma density widths. There was at least one combination of the plasma density, magnetic field and density width for which the RF power deposition at both side of the tube reached its maximum value.     

Analysis of Power in an Argon Filled Pulsed Dielectric Barrier Discharge

In this paper an argon filled coaxial dielectric barrier discharge (DBD) has been studied to understand the detail of power transfer from a unipolar square pulse to plasma during discharge. A dielectric barrier discharge based diffuse pulse discharge and its electrical characteristics are investigated. A quartz coaxial DBD tube filled at different pressures is used in the experiment. A unipolar pulse voltage of different peak voltages and frequencies has been applied to the discharge electrodes for the generation of microdischarges. Two current pulses are used for two consecutive discharges per applied voltage pulse. The second discharge, which occurs at the falling flank of the voltage pulse, is induced by the charges stored on the dielectric barrier during the first discharge. It has been deduced that the power supplied to ignite the first discharge is partly stored to ignite the second discharge when the applied voltage decays. This process ultimately leads to much improved power transfer to the plasma. The knowledge obtained from dynamic processes of the DBDs in the discharge gap explains quantitatively the mechanism of ignition, development and extinction of the DBDs.     

Structural design of cryogenic component support for EAST （HT-7U）

EAST is a full superconducting tokamak with an elongated plasma cross-section. It consists of superconducting poloidal field (PF) magnet system, toroidal field (TF) magnet system, vacuum vessel with inner parts, thermal shields and cryostat vessel. The mission of the project is to widely investigate both physics and technologies of advanced tokamak operations, especially the mechanism of power and particle handling for steady-state operations. The cryogenic component is mainly composed of superconducting TF and superconducting PF coils that ensure the ability of sustaining magnetic field for plasma confinement, control and shaping in steady-state. This report describes the process of the structure design of cryogenic component support for EAST.     

Simulations of Reversed Shear Configuration in EAST

The reversed shear （RS） mode is one of the advanced configurations being considered in EAST. Predictive simulations of EAST reversed shear configuration are carried out using an 1.5D equilibrium evolution code. In order to have the desired monotonic q-profile during a tokamak discharge, a successful preparation phase is required. In our simulation, the plasma current is ramped up from 100 kA to a fiat-top maximum of 1.0 MA for four seconds. An ICRH power of 1 MW is applied until the plasma shape is formed at the moment of 4 s, and then the power is raised to 3 MW. A LHCD power of 3.5 MW is applied from ls to optimize the plasma current density profile. A series of simulations are performed to study the influence of the time of applying the auxiliary heating on the plasma parameters. Based on these simulations, a scheme is proposed and tested for the control of the safety factor profile, which is very useful in real time profile control in tokamak experiments.