Electron temperature fluctuation in the HT-7 Tokamak plasma observed by electron cyclotron emission imaging

The fluctuation of the electron temperature has been measured by using the electron cyclotron emission imaging in the Hefei Tokamak-7 (HT-7) plasma. The electron temperature fluctuation with a broadband spectrum shows that it propagates in the electron diamagnetic drift direction, and the mean poloidal wave-number $\bar{k}_{\theta}$is calculated to be about 1.58 cm^-1, or \bar{k}_θ ρ_s≈ 0.34. It indicates that the fluctuation should come from the electron drift wave turbulence. The linear global scaling of the electron temperature fluctuation with the gradient of electron temperature is consistent with the mixing length scale qualitatively. Evolution of spectrum of the fluctuation during the sawtooth oscillation phases is investigated, and the fluctuation is found to increase with the gradient of electron temperature increasing during most phases of the sawtooth oscillation. The results indicate that the electron temperature gradient is probably the driver of the fluctuation enhancement. The steady heat flux driven by electron temperature fluctuation is estimated and compared with the results from power balance estimation.     

Effect of shear equilibrium flow in Tokamak plasma on resistive wall modes

A code named LARWM with non-ideal magnetohydrodynamic equations in cylindrical model is used to describe the instability in Tokamak plasma surrounded by a conducting wall with finite resistivity. We mainly take three factors related to the shear equilibrium plasma flow into consideration to study the stabilizing effect of the shear flow on the resistive wall modes (RWMs). The three factors are the velocity amplitude of flow, the shear rate of flow on plasma surface, and the inertial energy of equilibrium plasma flow. In addition, a local shear plasma flow is also calculated by the LARWM code. Consequently, it is found that the inertial energy of the shear equilibrium plasma flow has an important role in the stabilization of the RWMs.     

High energy radiation precursors to the collapse of black holes binaries based on resonating plasma modes

The presence of well organized plasma structures around binary systems of collapsed objects [1], [2] (black holes and neutron stars) is proposed in which processes can develop [3] leading to high energy electromagnetic radiation emission immediately before the binary collapse. The formulated theoretical model supporting this argument shows that resonating plasma collective modes can be excited in the relevant magnetized plasma structure. Accordingly, the collapse of the binary approaches, with the loss of angular momentum by emission of gravitational waves [2], the resonance conditions with vertically standing plasma density and magnetic field oscillations are met. Then, secondary plasma modes propagating along the magnetic field are envisioned to be sustained with mode-particle interactions producing the particle populations responsible for the observable electromagnetic radiation emission. Weak evidence for a precursor to the binary collapse reported in Ref. [2], has been offered by the Agile X-γ-ray observatory [4] while the August 17 (2017) event, identified first by the LIGO-Virgo detection of gravitational waves and featuring the inferred collapse of a neutron star binary, improves the evidence of such a precursor. A new set of experimental observations is needed to reassess the presented theory.     

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建立了等离子体-绝缘管-真空结构的等离子体天线物理模型,推导了新的色散关系方程,通过几何诠释,给出了一种新的高精度快速收敛求解方法,计算机仿真验证了结论的正确性和有效性。利用算法研究了当电子密度按线性律减小时,圆柱等离子体天线的色散关系,并绘制了方向图。这些结果对等离子体天线的分析与设计具有重要的参考价值。     

等离子体天线色散关系的一种高精度快速算法

建立了等离子体-绝缘管-真空结构的等离子体天线物理模型,推导了新的色散关系方程,通过几何诠释,给出了一种新的高精度快速收敛求解方法,计算机仿真验证了结论的正确性和有效性。利用算法研究了当电子密度按线性律减小时,圆柱等离子体天线的色散关系,并绘制了方向图。这些结果对等离子体天线的分析与设计具有重要的参考价值。     

Near perpendicular propagation of ion cyclotron modes in a deuterium-hydrogen-oxygen fusion plasma

A dispersion relation for the near perpendicular propagation of the electromagnetic ion cyclotron wave has been derived in a fusion plasma that has deuterium as a majority species, hydrogen as a minority species and fully ionized oxygen as an impurity constituent; all being modelled by loss cone distribution functions. The wave has a frequencyω around the deuterium ion gyrofrequency-Ω_D and a wavelength much longer than its Larmor radiusγ _LD(k _⊥ γ _LD<1); the plasma itself being characterized by large ion plasma frequencies (ω _PD ² >Ω _D ² ). Two modes, a low frequency (LF) and a high frequency (HF) mode of opposite electrical energy can propagate in the plasma; the instabilities that arise are thus due to an interaction of modes of opposite energies. We find that while hydrogen tends to destabilize the plasma, the impurity oxygen ions have the reverse effect. Also the plasma is most stable when the ratios of the perpendicular components of oxygen-to-deuterium and hydrogen-to-deuterium temperatures are kept low. Detailed studies of the wave propagation characteristics and energy reveal the close resemblance of a loss cone plasma containing oxygen to a stable Maxwellian plasma in regard to wave stability, propagation and energy.     

Investigation about the modes in the cylindrical cavity of an ECR ion source

Both theoretical and numerical electromagnetic analyses about the modes inside the cylindrical cavity of superconducting electron cyclotron resonance ion source (SERSE) have been performed. Modes close to 14, 18 and 28 GHz frequencies, usually employed in SERSE operating conditions, have been calculated in vacuum and when the chamber is filled with a uniform non-collisional plasma at different electron densities. To consider the holes present in the chamber flanges, a numerical approach has been used, by means of the HFSS^TM electromagnetic simulator, for the first mode in the cavity. Modes in a plasma-filled cylindrical cavity with holed bases have full width half maximum bandwidths larger with regard to the closed cavity in vacuum, and it leads to an increased mode frequency overlap. A monochromatic electromagnetic wave feeding this cavity can, in principle, excite different modes. Further investigations about the coupling between the SERSE cavity and its feeding waveguides have to be performed.     

HT-7 Tokamak离子回旋波和低杂波等离子体逃逸电子行为研究

HT-7 Tokamak拥有离子回旋波(ICRF)和低杂波(LHW)两套加热系统.ICRF主要对加热离子有比较好的加热效果,LHW则主要是通过电子Landau阻尼加热电子.除此之外,在ICRF和LHW协同加热的条件下,可以对等离子体产生更有效的加热效果,增加等离子体的聚变反应截面,增加聚变中子产额.本文报道了LHW对改善ICRF和等离子体耦合的重要作用,ICRF和LHW加热等离子体中电子温度随时间的演化过程,计算了放电过程中电子逃逸的阈值能量,分析了逃逸电子的产生过程,以及放电过程中的中子产额.研究结果发     

HT-7U等离子体位移快控电源设计

针对HT-7U装置等离子体位移快控电源大电流快速响应要求,提出了一种基于电流型变流器的快控电源(FCPS)设计新方案。该方案采用交-直-交拓扑结构,即网侧采用电流型PWP整流器设计。以获得稳定的直流电流及网侧单位功率因数正弦波电流控制;负载侧则采用电流型H桥逆变器设计以实现负载电流的快速跟踪控制。移相PWM控制策略在实现多组变流器并联运行的同时,提高了变流器的等效开关频率控制,从而改善了电流波形品质。模拟系统实验论证了方案的正确性。     

Ion cyclotron instabilities in a mildly relativistic hydrogen-deuterium fusion plasma

A dispersion relation for the perpendicular propagation of the electromagnetic ion cyclotron wave around the second harmonic of the deuterium ion gyrofrequency in a mildly relativistic, anisotropic Maxwellian plasma with hydrogen as the majority species and deuterium as the minority component has been derived. The work has been carried out in the frame of reference of the majority hydrogen ions; to these ions the waves at 2Θ_D would be at its own gyrofrequency. Using a small quantityɛ to order all relevant parameters of the plasma, it was possible to derive the dispersion relations in a simple form. To the lowest order the relativistic factors do not enter the dispersion relation. The plasma can now support two modes—one above and the other below the hydrogen gyrofrequency in agreement with the assumptions. This was also verified numerically using a standard root solver thereby justifying the correctness of the ordering scheme. In the next higher order, the dispersion relation is a quartic equation and is sensitively dependent on the relativistic factors. The plasma can now support four modes, both above and below the hydrogen gyrofrequency and consistent with the ordering scheme used. However the modes can now coalesce resulting in complex conjugate roots to the dispersion relation thereby indicating an instability. The advantage of such a scheme is that two dispersion relations — one of which is independent of the relativistic factors and the other which is sensitively dependent on them can be separated out.     

Fundamental frequency patterns of Chinesein different speech modes

I.IntroductionTheF,patternsofspeechareimportantnotonlyforthcprosodicfeaturesbuta1soforvoicesourcecharactcristics.Nowmoreandmorespeechscientistsrecognizedthatvoiceexcitationsourceintcxt-to-spccchsystemsp1aysanimportantro1elnbothintclligibilityandnaturalnessorsynthcticspcech.Espccially,forChinese,atone1anguagewithmulti-tonesystem,thetonalpatternswhicharcmainlydcmonstratedintheF,con-tourscarry1exicalmeaning.SomecomparativestudiesoftheF,pattcrnsinbetweentonelanguage(Chinese)andstress1anguage(En…     

Recombination effects during the laser-produced plasma expansion

The precise characterization of plasmas generated by laser irradiation is needed for the development of ion sources. As the characteristic parameters of the expanding plasma vary with both the distance and the time, an experimental study of their evolution is appropriate for a deeper knowledge of the plasma. The purpose of this work is to study the same characteristics of laser plasma produced by ablation of a pure Cu target as a function of the distance from the target along the propagation axis of the plasma plume. We irradiated the target by a KrF laser and a lens of 15 cm focal length. As the diagnostic system, a small Faraday cup array and an axial Faraday cup were utilized to study the spatial variation in the total charge carried by plasma ions. Charge loss during the plasma expansion was observed due to the recombination of charged species, which occurs within a critical distance, relatively close to the target, where the plasma density is high enough. The critical distance was determined for different laser fluences; beyond the critical distance, the collisions among plasma particles are negligible and the ion charge remains frozen. It was observed that the critical distance increases as the laser fluence increases.     

射频电感耦合等离子体在横向磁场中扩散的研究

通过轴向移动朗缪尔探针,考察了外置电磁铁提供的横向磁场对射频感应耦合等离子体(ICP)延轴向扩散过程中的电子温度、离子密度的影响。实验数据表明,与无磁场条件下的结果相比,横向磁场的加入使得扩散区内的电子温度沿轴向迅速下降,研究了横向磁场对电子的过滤作用。     

Experiment and simulation of tearing mode evolution with electron cyclotron current drive in KSTAR

The tearing mode (TM) plasma instability was observed in low confinement (L-mode) plasmas when non-axisymmetric magnetic perturbation (MP) was applied using external coils during 2011 campaign of KSTAR. Based on the collected information of the magnetic island location in a plasma, a discharge was designed for suppression of a (2,1) TM mode by adjusting electron cyclotron (EC) launcher angles to the estimated island position. Here, the (m,n) notation describes the poloidal mode number and the toroidal mode number of the TM, respectively. The discharge is analysed with experimental observations and numerical simulations. Mirnov coil (MC) arrays and electron cyclotron emission (ECE) are used for analysis of the island width and the location as well as the mode number. The EC deposition and its alignment with the island are estimated by X-ray imaging crystal spectroscopy (XICS) and ECE measurements. An integrated numerical system is employed for modelling of this discharge to analyse a temporal evolution of the mode activity by integrating plasma equilibrium, transport, heating and current drive, and the magnetic island evolution, in a self-consistent way. The effect of EC current drive is discussed by comparing with another TM discharge but without ECCD. Some possibilities for classifying this mode to neoclassical tearing mode (NTM) and stabilisation effect of ECCD are suggested based on the experimental observation and the simulation results.     

Influence of Kr doping on neon soft X-rays emission in fast miniature plasma focus device

An investigation on the possibility of enhancement of soft X-ray (SXR) (900–1600 eV) emission from a fast miniature plasma focus (FMPF) device of 235 J (at 14 kV) storage energy through doping of operating gas was performed. Neon (Ne), the operating gaseous medium, was doped with krypton (Kr) in different volumetric ratios at various operating pressures ranging from 2 to 14 mbar. The 1% Kr doping increased the average optimum SXR emission efficiency from 0.47% to 0.6% without enhancing the hard X-ray (HXR) (>1600 eV) emission. The Kr doping influenced the major pinching characteristics such as focusing efficiency and time to pinch with consequential effect on X-ray emissions. Synchronous operation of the 4 pseudo-spark gap (PSG) switches was mandatory for efficient discharge current delivery to the electrodes. A drastic improvement in the pinching efficiency was obtained with replacement of old and worn out PSG switches with the new ones. Optical imaging of current sheath dynamics was performed using gated ICCD camera to verify the normal operation of the device after the PSGs replacement. A numerical simulation analysis on the 2 cm long stainless steel tapered anode, used in this study, was done to predict the maximum SXR emission efficiency and the peak operating gas pressure. An analysis on the amount of SXR fluence generated at the source position and the proportion of it reaching the target position is also reported.     

低气压条件下的铁电体离子发射特性研究

以低气压条件下的铁电体离子发射特性为研究对象,通过结合高速相机、光谱的光学诊断和二维网格质点法耦合蒙特卡洛碰撞模型(PIC-MCC)的仿真模拟获得了锆钛酸铅(PZT)铁电体离子发射的主要成分、表面放电的发光及等离子体演化过程、离子的产生及离子电流形成机制,为进一步研究铁电体离子源奠定了基础。     

非磁化等离子体中的电子碰撞频率

电子碰撞频率是非磁化等离子体的一个重要参数,它对等离子体与电磁波相互作用的性质具有较大影响。基于产生等离子体的气压条件不同,其电子碰撞频率具有较大变化范围。研究了它对非磁化等离子体吸波能力的影响,得出了当电子碰撞频率与等离子体频率和入射波频率相当时,等离子体对电磁波具有较大吸收的结论,给出了对应于最大吸收的最佳碰撞频率值。分析了通过选择放电气体来调整等离子体的电子碰撞频率。     

Characteristics of the fast electron emission produced during the cleavage of crystals

The present paper reports the fast electron emission produced during the cleavage of alkali halide crystals and models the dynamics of the process. The mechano-emission arises as a result of the ionization of surface traps at the expense of the energy which is released in the annihilation of the defects which are formed during cleavage. The slow electrons which appear upon the ionization of surface traps are subsequently accelerated in the field of negatively charged segment of the freshly cleaved surface. Considering the basic mechanism of fast electron emission, expressions are derived which are able to explain satisfactorily the temporal, thermal, charge, surface, coloration, water adsorption and other characteristics of the fast electron emission produced during the cleavage of crystals. The decay time of the charges on the newly created surfaces, and the velocity of cracks can be determined from the measurements of fast electron emission produced during the cleavage of crystals. It is shown that two types of diffusing centres are responsible for the charge relaxation and thereby for the emission of fast electrons produced during the cleavage of alkali halide crystals.     

The effects of relativistic broadening and frequency down-shift on electron-cyclotron emission measurements in EAST

This paper presents a theoretical calculation of the effects of relativistic broadening and frequency down-shift on the electron cyclotron emission measurements for a wide range of plasma parameters in the Experimental Advanced Superconducting Tokamak (EAST). The calculation is based on the radiation transfer equation, with the reabsorption and reemission processes taken into account. The broadening effect contributes to the radial resolution of the measure- ment, and the calculation results indicate that it is ～ 2 cm in the case of the central electron temperature 10 keV. A pseudo radial displacement of the obtained electron temperature profile occurs if the relativistic frequency down-shift effect is not taken into account in the determination of the emission layer position. The shift could be a few centimeters as the electron temperature increases, and this effect should be taken into account.     

Stimulated raman scattering from plasma modes in magnetoactive semiconductors

Stimulated scattering off electron plasma mode is investigated analytically for the case when the pump wave is an intense circularly polarised electromagnetic wave propagating parallel to a homogeneous dc magnetic field in an isotropic semiconductor-plasma. The threshold electric field of the pump necessary for the stimulated Raman scattering and the growth rate of the parametrically unstable mode have been obtained for two cases (i)B ₀=0 and (ii) B₀ ≠ 0. It is seen that the magnetic field does not significantly affect the threshold electric field as well as the growth rate provided the cyclotron frequency is small compared to the frequency of the pump wave. The threshold conditions are also found to be insensitive to the electron thermal velocity.