High and low frequency relaxation oscillations in a capacitive discharge plasma

Both high and low frequency relaxation oscillations have been observed in an argon capacitive discharge connected to a peripheral grounded chamber through a slot with dielectric spacers. The oscillations, observed from time-varying optical emission of the main discharge chamber, show, for example, a high frequency （46 kHz） relaxation oscillation at 100 mTorr, with an absorbed power near the peripheral breakdown, and a low frequency （2.7-3.7 Hz） oscillation, at a higher absorbed power. The high frequency oscillation is found to ignite a plasma in the slot, but usually not in the periphery. The high frequency oscillation is interpreted by using an electromagnetic model of the slot impedance, combined with the circuit analysis of the system including a matching network. The model is further developed by using a parallel connection of variable peripheral capacitance to analyse the low frequency oscillation. The results obtained from the model are in agreement with the experimental observations and indicate that a variety of behaviours are dependent on the matching conditions.     

Instability Parameters of Optical Oscillation Frequency in Plasma Central Discharge and Periphery Region

We have observed relaxation oscillations in a capacitive discharge in Ar gas, connected to a peripheral ground chamber. The plasma oscillations observed from time-varying optical emission from the main discharge chamber show, for example, a high frequency （75.37kHz） relaxation oscillation, at lOOmTorr and 8 W absorbed power, and a low frequency （2.72Hz） relaxation oscillation, lOO mTorr and 325 W absorbed power. Time-varying optical emission intensity and plasma density are also detected with a Langmuir probe. The theoretical result agrees well with experiments.     

Low frequency longitudinal waves in argon hot cathode discharges. II

Taking into account the multistep ionization a simple hydrodynamical model for the low-frequency longitudinal waves in low-current argon discharges with a thermionic cathode at the pressure from 10 Pa to 100 Pa is presented. A conclusion is drawn that the most important instability factor in such discharges is the interaction between the low-energy electron beam (U _b12 eV) and the plasma. The calculated dipersion characteristics show good agreement with the experimental results.     

离子振荡对低压脉冲负电性放电条件的影响

为了研究该离子振荡及其对低气压负电性放电自持条件的影响，建立了一个整体模型描述低气压正负离子等离子体中离子振荡与少量电子的相互作用. 在模型中引入参数r描述电子流体与电极碰撞后的动量保存(或损失)的程度. 发现体系存在一个临界值r=r_c,它导致了两种不同性质的电子损失机理. 另一临界值r=4r_c决定了两种不同的电子密度随时间增长的阈值. 这使得该阈值随r非单调变化, 进而导致RF负电性脉冲放电主动放电阶段初期的自持放电条件参数空间中可以存在间隙. PIC-MCC 关键词： 负电性放电 脉冲放电 离子振荡     

Vibration testing based on impulse response excited by laser ablation

This paper proposes an innovative vibration testing method based on impulse response excited by laser ablation. In conventional vibration testing using an impulse hammer, high-frequency elements of over tens of kilohertz are barely present in the excitation force. A pulsed high-power YAG laser is used in this study for producing an ideal impulse force on a structural surface. Illuminating a point on a metal with the well-focused YAG laser, laser ablation is caused by generation of plasma on the metal. As a result, an ideal impulse excitation force generated by laser ablation is applied to the point on the structure. Therefore, it is possible to measure high-frequency FRFs due to the laser excitation. A water droplet overlay on the metal is used to adjust the force magnitude of laser excitation. An aluminum block that has nine natural frequencies below 40 kHz is employed as a test piece. The validity of the proposed method is verified by comparing the FRFs of the block obtained by the laser excitation, impulse hammer, and finite element analysis. Furthermore, the relationship between accuracy of FRF measurements and sensitivity of sensors is investigated.     

Spectral Investigations of a Nonstationary High-Frequency Discharge in Carbon Tetrafluoride and Sulfur Hexafluoride

The possibility of using a single-channel spectral computer complex for studying, by an actinometry method, the laws governing a change in the concentration of atomic fluorine in the nonstationary period of a high-frequency discharge in SF₆ and CF₄ gases is shown. It has been established that the dependence of the atomic fluorine concentration on the time of discharge running is influenced by the state of the discharge chamber surface and, first of all, the degree of its hydration. In particular, this dependence can be monotonic or have a maximum that owes its origin to the sedimentation of the products of the dissociation of molecules on the surfaces of the discharge chamber walls contacting a plasma.     

A magnetically stabilized coaxial laser discharge

A new magnetic discharge stabilization technique for coaxial laser systems is described. The approach utilizes crosses electric and magnetic fields to create and maintain a large and rapidly rotating plasma volume which does not experience glow-to-arc transitions. Very high cw specific discharge power loadings have been achieved even without the benefit of external gas cooling or circulation.Performance is insensitive to gas composition and pressure such that high power coaxial discharges have been run in CO₂ laser gas mixtures up to several hundred torr. Stable cw discharges have also been obtained in mixtures containing several torr of SF₆.The technique appears to be readily scalable to give very large excited volumes in systems with comparatively small overall physical dimensions.     

Low-temperature PECVD of silicon dioxide on polymeric hydrogels

In this work, a complex investigation of the film surface composition and nanoscale mechanical properties, i.e. hardness and elastic modulus, of plasma-modified and silica-coated hydrogel thin films was carried out. Plasma treatment was performed in a reactive ion etching chamber (SF₆, CHF₃) at radio frequency (rf, 13.56 MHz) and in a plasma-enhanced chemical vapor deposition chamber (SiH₄/N₂, NH₃, N₂O) at radio frequency and dual frequency (13.56 MHz/100 kHz), respectively. The use of the dual-frequency configuration comprising two power supplies and operated in a switched mode enabled the investigation of the ion-bombardment influence on the polymer properties. For the application in silicon micromachined sensors best results were obtained by using a NH₃ or SiH₄/N₂ low-pressure plasma modification and a silica coating of the sensitive hydrogel film. PACS 81.05.Lg; 81.15.Gh; 81.65.Cf; 81.70.Bt     

Low-Frequency Relaxation Oscillations in Capacitive Discharge Processes

Low-frequency （2.72-3.70 Hz） relaxation oscillations at 100 mTorr at higher absorbed power were observed from time-varying optical emission of the main discharge chamber and the periphery. We interpret the low frequency oscillations using an electromagnetic model of the slot impedance with parallel connection variational peripheral capacitance, coupled to a circuit analysis of the system including the matching network. The model results are in general agreement with the experimental observations, and indicate a variety of behaviours dependent on the matching conditions.     

硅烷低温等离子体阶跃响应的仿真(1)

了解含有负离子的低温等离子体的过渡特性，在等离子体控制过程中，尤其在选择性等离子 体腐蚀工艺和改善电荷堆积等现象中是十分重要的课题.对电源驱动频率为1356MHz，压力 为05Torr(05×10333Pa)状态下的硅烷（SiH₄）低温等离子体的阶跃响应 进行仿真. 当电源电压振幅从550V阶跃减小到350V时，硅烷低温等离子体表现出以数千RF周期为周期的振荡现象，等离子体中的带电粒子的运动变化决定了振荡现象的产生和振荡周期等特性. 关键词： 迁移率 扩散系数 阶跃响应 等离子体振荡     

Scrape-off layer turbulence modulated by Mirnov oscillations

We investigate plasma turbulence at the scrape-off layer of TCABR tokamak. We apply a power spectral analysis to the magnetic Mirnov oscillations and electrostatic fluctuations, to quantify statistical properties and to estimate the turbulence-driven radial-particle flux. A distinctive peculiarity is the modulation of electrostatic turbulence by the Mirnov oscillations shown by the partial superposition of the frequency power spectra of these two oscillations. This characteristic allows us to investigate any possible influence of the Mirnov oscillations on particle transport. In fact, a significant part of this transport occurs at the Mirnov frequencies. The effect of this modulation is also analyzed for discharges modified by external perturbations, a DC biased electrode or an ergodic magnetic limiter.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

Electroacoustic oscillations of aluminum oxide particles in the thermal plasma

The frequencies of natural electroacoustic oscillations of aluminum oxide particles in a laminar disperse aluminum flame are determined experimentally using the capacitive method. A computational model is proposed for estimating the natural frequency of oscillations of charged particles in the smoky plasma taking into account the Doppler effect. It is shown that, for a natural frequency of oscillations of 51 kHz, two measured maxima at frequencies of 30 and 60 kHz in the oscillation spectrum correspond to the Doppler frequencies.     

Surface tension measurement of liquid metal with inelastic light-scattering spectroscopy of a thermally excited capillary wave

Thermally excited capillary waves propagate everywhere on a liquid surface. The relationship between the frequency and wavelength of the capillary wave reflects the surface tension of the liquid, and the lifetime of the capillary wave reflects the shear viscosity of the liquid. By observing a capillary wave with inelastic light-scattering spectroscopy, we can obtain the surface tension and the viscosity from the frequency modulation of the light and the spectral width of the scattered light. In this study, we observed capillary waves on liquid metal using high-resolution spectroscopy, and successfully measured their properties in a non-contact manner. In the experiment, values were obtained at a frequency of about 20 kHz. The values were equivalent to those at 0 Hz, since the properties of liquid metals are constant at least in the frequency region from 0 to several tens of kilohertz.     

Multiwavelength Erbium doped fiber ring lasers

We demonstrate a multiple wavelength laser, employing wavelength dependent cavity losses, with up to four lasing wavelengths on the ITU-grid in the C-band and report on its stability characteristics. The linewidth of each wavelength was estimated to be less than 4 pm and we observed antiphase oscillations around 3 kHz and relaxation oscillations around 25 kHz. The antiphase oscillations in the multiwavelength laser lie on a limit cycle in a relative phase diagram. We present an analytic lumped model to determine the output power and relaxation oscillation frequency of the multiple lasing wavelengths in the Erbium doped fiber. The model is validated against our implementation of a multiwavelength ring laser.     

高频脉冲功率源初级单元的设计

针对半导体断路开关型、数十kHz高频脉冲功率源设计了初级单元。该单元在低频电路的基础上进行了改进和优化,放电主开关采用IGBT串并联组件,解决了同步触发问题,增加了过流保护系统,设计了高频脉冲触发器。研究发现,初级单元的放电电容放电后有残余电压存在,这会降低脉冲功率源的输出稳定性。该初级电路加强了高频率脉冲功率源的稳定性和可靠性,成功应用于数十kHz高频脉冲功率源。从波形上看,初级充电电源工作电压约为1 kV,放电电流约1.5 kA,在10 kHz条件下可以稳定工作。     

Manipulation of prenatal hormones and dietary phytoestrogens during adulthood alter the sexually dimorphic expression of visual spatial memory

Background

Recently a hyperthermic rat hippocampal slice model system has been used to investigate febrile seizure pathophysiology. Our previous data indicates that heating immature rat hippocampal slices from 34 to 41°C in an interface chamber induced epileptiform-like population spikes accompanied by a spreading depression (SD). This may serve as an in vitro model of febrile seizures.

Results

In this study, we further investigate cellular mechanisms of hyperthermia-induced initial population spike activity. We hypothesized that GABA_A receptor-mediated 30–100 Hz γ oscillations underlie some aspects of the hyperthermic population spike activity. In 24 rat hippocampal slices, the hyperthermic population spike activity occurred at an average frequency of 45.9 ± 14.9 Hz (Mean ± SE, range = 21–79 Hz, n = 24), which does not differ significantly from the frequency of post-tetanic γ oscillations (47.1 ± 14.9 Hz, n = 34) in the same system. High intensity tetanic stimulation induces hippocampal neuronal discharges followed by a slow SD that has the magnitude and time course of the SD, which resembles hyperthermic responses. Both post-tetanic γ oscillations and hyperthermic population spike activity can be blocked completely by a specific GABA_A receptor blocker, bicuculline (5–20 μM). Bath-apply kynurenic acid (7 mM) blocks synaptic transmission, but fails to prevent hyperthermic population spikes, while intracellular diffusion of QX-314 (30 mM) abolishes spikes and produces a smooth depolarization in intracellular recording.

Conclusion

These results suggest that the GABA_A receptor-governed γ oscillations underlie the hyperthermic population spike activity in immature hippocampal slices.     

气流对微秒脉冲滑动放电特性的影响

脉冲电源驱动的滑动放电能够在大气压下产生高能量、高功率密度的低温等离子体. 为了研究微秒脉冲电源在针-针电极结构中产生滑动放电的特征, 本文采用电压幅值为0–30 kV, 脉冲宽度约8 μs, 脉冲重复频率为1–3000 Hz的微秒脉冲电源, 通过测量电压、电流波形和拍摄放电图像, 研究了微秒脉冲滑动放电的电特性. 实验结果表明, 随着施加电压的增加微秒脉冲滑动放电存在三种典型的放电模式: 电晕放电、弥散放电和类滑动放电. 不同放电模式的电压、电流波形和放电图像之间差异显著. 脉冲重复频率对微秒脉冲滑动放电特性有影响, 表现为当气体流量较小(2 L/min)时, 类滑动放电的放电通道随着脉冲重复频率的增大逐渐集中, 而当气体流量较大(16 L/min)时, 类滑动放电的放电通道随着脉冲重复频率的增大逐渐分散. 不同气流下重复频率对滑动放电特性的影响与放电中粒子的记忆效应和气流的状态有关.     

基于遗传算法的CO2激光器三气体组份优化

应用遗传算法和数值求解CO₂激光动力学方程,以输出激光功率为目标函数,优化了典型封离型CO₂激光器三种主要工作气体(CO₂、N₂、He)的气体压强.对1.2m长谐振腔,在放电电压15kV条件下,三种气体的优化压强分别为PCO₂=1.15×133.3Pa、PN₂=7.36×133.3Pa、PH₂=13.33×133.3Pa.优化后激光功率可提高0.96倍.     

Self-pulsation and optical chaos in self-pumped photorefractive BaTiO3

Sinusoidal intensity oscillations of the phase conjugated wave reflected from a self-pumped photorefractive BaTiO₃ crystal have been observed. The oscillation amplitude is shown to be increased by providing optical feedback of the light scattered during grating formation. The oscillation frequency ? can be tuned from 0.024 Hz to 2 Hz by changing the light intensity I₀ of the object beam from 0.7 W/cm² to 100 W/cm². A power law ? ∝ I^β₀ ∝ σ with β = 0.89 was observed over this range of intensities (σ = photoconductivity). Transitions to a chaotically oscillating or a stable reflecting state have been observed at higher power levels or with increased optical feedback. An additional pump beam near the entrance face of the signal beam can decrease the build-up time for the self-pumping process by a factor of five or more.     

Investigation of the plasma potential oscillations in the range of geodesic acoustic mode frequencies by heavy ion beam probing in tokamaks

Specific oscillations within a range of 20 kHz (20 kHz-mode) were investigated on the T-10 and TEXT tokamaks using Heavy Ion Beam Probe (HIBP) diagnostic. Regimes with ohmic heating on both machines, and with off-axis ECRH in T-10 were studied. It was shown that 20 kHz-modes are mainly the potential oscillations. The power spectrum of the oscillations has the form of a solitary quasi-monochromatic peak with a contrast range of (3–5). They are the manifestation of torsional plasma oscillations with poloidal wavenumber m = 0, called zonal flows. It was shown that in TEXT the radial electric field oscillations exist in a limited radial range of 0.65 > < 0.95. The frequency of 20 kHz-mode is varied in the region of observation; it diminishes to the plasma edge. In T-10, after ECRH switch-on, the frequency increases, correlating with the growth of the electron temperature T _e. In both machines the frequency of the 20 kHz-mode varies with local T _e: f T _e ^1/2 , which is consistent with theoretical scaling for geodesic acoustic modes (GAM): f _GAM c _s/R T _e ^1/2 , where c _s is the speed of sound. The absolute frequencies are close to GAM values within a factor of unity.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.