Experimental Analysis of a Low Frequency Instability in a Magnetized Discharge Plasma

In the paper we present the results of an investigation of a low frequency (30-100 kHz) instability in a weakly magnetized discharge plasma. The instability is triggered by a disc electrode which terminates the magnetized plasma column and is biased above the plasma potential. Frequency dependence on various parameters, e.g. electrode diameter, electrode bias, neutral gas pressure and plasma density is measured. Space and time dependence of the plasma potential and density in the perturbed region during one period of the electrode current oscillation are measured. During the phase of the current decrease a potential structure moves in axial direction from the edge of the perturbed region towards the electrode. During the phase of current saturation the motion of ions is mostly radial. The observed phenomenon is approximately modeled as a two dimensional potential relaxation instability (PRI).     

A SOGICON type instability in silicon

A current oscillation phenomenon in SOGICON type Si device fabricated by the use of a planar process is studied. The oscillation is generated in the region between anode and notch which is located at the center on the sample. A considerably high electric field is observed in the notch region. Considering the hot-electron effect due to the high electric field, the mechanism underlying the current oscillation is discussed.     

Observation of reflex klystron oscillation phenomena in a plasma

Reflex klystron electron oscillation, occurring in a plasma potential well formed in a system consisting of plasma and two electrodes (filaments and a mesh grid which is at floating potential), was observed in a very simple device with only filaments and a mesh grid. This oscillation mechanism consists of three elements: 1) an acceleration region on the side in which filaments are located, which accelerates primary electron beams emitted from filaments; 2) a deceleration region on the side in which the mesh grid is located, which causes the reflection of the beams; and 3) a plasma region. In addition, the velocity modulation of primary electron beams is given by the electron plasma oscillation at the presheath on the filament side. The maximum amplitude and frequency of an oscillation obtained by this mechanism were V_pp=210 mV 210 mV and f=200 MHz, respectively. These values can be controlled by the discharge potential     

Investigation of Impurity Ion Transport with Laser Blow-off in HL-2A Tokamak

Non-recycling impurities are injected into ohmic HL-2A plasma for the first time. The impurities of titanium and aluminium are injected in the discharges with varying plasma density and current. The convection and diffusion process of the injected impurity ions during the inward phase are qualitatively investigated. The results show that the transport of impurities is much slower in the central region of the plasma than outside of it and that it is greatly enhanced during sawtooth crashes.     

激光等离子体光丝中太赫兹频谱的调控

研究了双色激光场激发空气成丝产生太赫兹辐射频谱的变化规律.实验观察到随驱动光功率和光丝长度增加,太赫兹光谱主要发生红移的现象.分析表明,由于等离子体密度的增加,太赫兹辐射的趋肤深度减小,等离子体吸收主导了红移的发生.在光丝足够短的条件下,趋肤深度远大于光丝长度,从而产生等离子体振荡主导的太赫兹辐射光谱蓝移.本研究为超快宽带太赫兹辐射的频谱调控提供了新思路.     

On the effect of a longitudinal magnetic field on oscillatory characteristics of a subnormal region in discharge in argon

Oscillating nature of current pulses under d.c. excitation in subnormal region with longitudinal magnetic field at pressure range 0.20 torr to 0.85 torr have been studied. The frequency, bandwidth, peak-peak voltage, cut-off current and rise time of the current pulses have been observed with pressure, average tube current and magnetic field. A study of these oscillograms in magnetic field, average tube current and pressure are presented. The probable mechanism for the generation of oscillation based on space-charge field modification with magnetic field is discussed     

Current Oscillation and dc-Voltage-Controlled Chaotic Dynamics in Semiconductor Superlattices

We report a detailed theoretical study of current oscillation and dc-voltage-controlled chaotic dynamics in doped GaAs/AlAs resonant tunneling superlattices under crossed electric and magnetic fields. When the superlattice is biased at the negative differential velocity region, current self-oscillation is observed with proper doping concentration. The current oscillation mode and oscillation frequency can be affected by the dc voltage bias, doping density, and magnetic field. When an ac electric field with fixed amplitude and frequency is also applied to the system, different nonlinear properties show up in the external circuit with the change of dc voltage bias. We carefully study these nonlinear properties with different chaos-detecting methods.     

Review of relaxation oscillations in plasma processing discharges

Relaxation oscillations due to plasma instabilities at frequencies ranging from a few Hz to tens of kHz have been observed in various types of plasma processing discharges. Relaxation oscillations have been observed in electropositive capacitive discharges between a powered anode and a metallic chamber whose periphery is grounded through a slot with dielectric spacers. The oscillations of time-varying optical emission from the main discharge chamber show, for example, a high-frequency (\sim 40~kHz) relaxation oscillation at 13.33Pa, with an absorbed power being nearly the peripheral breakdown power, and a low-frequency ( \sim 3 Hz) oscillation, with an even higher absorbed power. The high-frequency oscillation is found to ignite plasma in the slot, but usually not in the peripheral chamber. The kilohertz oscillations are modelled using an electromagnetic model of the slot impedance, 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. In low-pressure inductive discharges, oscillations appear in the transition between low-density capacitively driven and high-density inductively driven discharges when attaching gases such as SF₆ and Ar/SF₆ mixtures are used. Oscillations of charged particles, plasma potential, and light, at frequencies ranging from a few Hz to tens of kHz, are seen for gas pressures between 0.133 Pa and 13.33 Pa and discharge powers in a range of 75--1200 W. The region of instability increases as the plasma becomes more electronegative, and the frequency of plasma oscillation increases as the power, pressure, and gas flow rate increase. A volume-averaged (global) model of the kilohertz instability has been developed; the results obtained from the model agree well with the experimental observations.     

Low-frequency current oscillations in a linear hot-cathode discharge

Low-frequency (ω ⪡ ω_pi) plasma oscillations in the transition regime between the high and the low current mode of a thermionic hot-cathode discharge are investigated experimentally. This type of current oscillation often shows chaotic dynamics. The current oscillations are related to nonlinear short wavelength potential structures which are identified as ion bunches formed by a fluctuating ionization front. These ion bunches are separated by ion holes and move at ion thermal speed rather than ion acoustic speed. By entering the negative space charge region of the cathode sheath, the ion bunches trigger electron current fluctuations that provide the required feedback mechanism for the observed wave train formation.     

Suppression of laser noise in CW 337μm HCN lasers

In a d.c.-discharge-excited 337 μm HCN laser, amplitude laser noise has been observed at the frequency range of 30 kHz to 1 MHz. The laser noise is classified into three types; oscillation noise having sharp peaks, broadly distributed noise having a center frequency and 1/f-noise. Characteristics of the laser noise have been studied experimentally. The laser noise is caused by small oscillation which appears on d.c.-discharge current and by random fluctuations of current and plasma density. The laser noise has been suppressed more than 30 dB by adjusting the laser parameters such as discharge current, pressure and flow rates of mixed gas (CH4₄ + N₂ + He) and added He.     

相对论电子束虚阴极静电振荡的粒子模拟

本文运用有限大小粒子模型,用等离子体粒子模拟方法,对相对论电子束的虚阴极振荡作了一维静电粒子模拟。结果表明,在注入电流小于空间电荷极限电流时,电子束能稳定传输并能全部通过。当注入电流超过空间电荷极限电流时,传输是不稳定的,将有部分电子反射,部分通过。空间电荷形成的虚阴极的位置和电势周期性地振荡。电流越大,虚阴极位置越靠近注入面且变化范围越小,虚阴极电势越低且振幅越大,振荡频率越高且大于电子束等离子体频率。     

Sawtooth-like electron density oscillations under laser blow-off and lower hybrid current drive in HL-1M tokamak

By using of a six-channel HCN laser interferometer, the sawtooth-like density oscillations without being exhibited in the ohmic phase plasma have been observed during lower hybrid current drive (LHCD) discharges in the HL-1M device. There are often accompanied by injected impurity with laser blow-off. Analyses show that this kind of density sawtooth-like oscillation comes from the mutual effect of LHCD and impurity. An available mechanism is the loss of superthermal electrons by the magnetic perturbation, which is caused by the crash of the central accumulated impurities.     

Metastable state of sliding CDW in K0.3MoO3

The narrow band noise and the transient voltage oscillation were investigated in a k_0.3MoO₃ sample, which showed different I–V characteristics in the non-linear conductivity region for dc and pulse methods. For repeated current pulses, after a sufficiently long duration of dc current, the voltage response showed relaxation behavior with a relaxation time of about 30 min at 77 K. Similar relaxation was also observed for the opposite case of a dc current applied after repeated pulses. The slope of frequency of voltage oscillation against CDW current was constant through these relaxation process in both dc and pulse cases. But the peak of narrow band noise is larger and sharper after repeated pulses than in the dc stationary state. This result was understood as an enhancement of the coherent-phase region for voltage oscillation in the case of repeated pulses.     

Reversed current structure in a Z-pinch plasma

The current profile of a Z-pinch plasma is investigated using a one-dimensional magnetohydrodynamic code. Simulation results reveal the formation of a reversed current profile, its propagation, and an ejection of plasma at boundary region, which have been observed in previous experiments. A new physical mechanism is proposed to account for such phenomena. The physical mechanism involves the propagation of a shock wave. It is found that a reversed current profile appears when a shock wave reflected at axis expands in a compressing plasma column.     

Measurements of the Drift Current Oscillations in Thrusters with Closed Electron Drift

An induction method used to measure the drift current in thrusters with anode layer (TAL) has been applied for measuring the integral distortion of the magnetic field in the channel of a low-power stationary plasma thruster (SPT). Experiments are described in which the integral level of magnetic field oscillations in the narrow channel of a thruster with closed electron drift was determined. Experimental data show that when a discharge is ignited in an SPT, the drift current completely demagnetizes the discharge gap (the ratio between the self-magnetic and external fields reaches 90%). The same effect has been observed in TALs. In thrusters of both types, the total discharge current is one and a half order of magnitude higher than that typical of the stationary discharge and the plasma glow is observed not only inside but also outside the discharge channel. Drift current oscillations have been measured under steady-state conditions. It has been shown, in particular, that when the oscillations are intense (in weak magnetic fields on the order of 100 G), the drift-current-induced distortion of the magnetic field may reach 33%. Under rated operating conditions (200 G), the distortion does not exceed 8%. Beyond optimal operating conditions, the type of oscillation in SPTs and TALs and their performances differ. For example, in SPTs, a critical magnetic field at which the “turbulent” regime sets in is absent. In general, the discharge current is weakly dependent on the magnetic field.     

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.     

Low-frequency current oscillation in high-resistivity n-InP

Low-frequency current oscillation due to travelling high-field domains and current saturation have been observed in high-resistivity Cu-doped n-InP below room temperature under illumination. The features of the oscillation depende on applied voltage, illumination, and temperature. The origin of the phenomenon seems to be a bulk negative resistance due to electron transfer and trapping, which is similar to that in high-resistivity GaAs.     

Resonant escape over an oscillating barrier in underdamped Josephson tunnel junctions

The escape from a metastable state over an oscillating barrier of an underdamped Josephson tunnel junction has been experimentally investigated with oscillation frequency well separated from the plasma frequency of the junction. The resonant escape, namely, a minimum of the average escape time as a function of the oscillation frequency, was observed. For the oscillation frequency much smaller than the "resonant frequency," the average escape time is the average of the times required to cross over each of the barriers. On the other hand, for the oscillation frequency much greater than the "resonant frequency," the average escape time is that required to cross the average barrier.     

Effect of resonant magnetic perturbations on edge plasma parameters in the STOR-M tokamak

Edge plasma properties have been modified in the Saskatchewan Torus-Modified tokamak by means of resonant magnetic perturbations (RMP). It has been found that the radial profiles of ion saturation current and floating potential in the edge region can be modified by an externally applied static (m?=?2, n?=?1) RMP field. An increase in the pedestal plasma density (n) and more negative electric field (E_r) have been observed in the plasma edge region. It is believed that the RMP field altered the plasma transport in the edge and scrape-off layer regions, leading to a higher density pedestal and a potential drop in some cases. During the enhanced confinement phase, it is possible to identify a region where intermittent transport events, the so-called blobs, are created and the holes of lower density left behind.     

行波管放大器中的电子混沌现象

用自洽方程模拟了波-粒相互作用过程中的电子混沌行为.结果表明：随着电流的增大，电子在相空间的运动轨道将变得混沌，混沌轨道受失谐量的影响.在时间上，电子混沌比场的极限环和混沌振荡出现要早.与场出现极限环振荡的电流阈值相比,出现电子混沌的电流阈值要小；在场呈极限环状态的“软”非线性区域，电子的混沌轨道占据大部分相空间；而在场混沌的“硬”非线性区域，混沌轨道则弥漫在整个相空间.当电流一定时,电子的混沌运动图样是不变的；在一定的电流范围内， 场的极限环和混沌振荡特征是确定的， 但它们的输出功率是不确定的. 关键词： 行波管放大器 电子混沌 相空间轨道 非线性相互作用