基于复合结构的气体电子倍增器增益模拟和实验研究

气体电子倍增器(GEM)作为高性能的微结构气体探测器在高能物理相关领域内得到了广泛的研究和应用.其中增益是GEM探测器基本性能研究中的一个重要参数,该值的精确测量至关重要.增益的测量一般采用电流测量或者能谱测量方法,但均存在精度较低或者过程繁琐的问题,且无法精确测量低增益值.针对GEM探测器增益的精确测量,本文提出了一种由GEM探测器与微网结构气体探测器(MM)级联构成的复合结构探测器(GEM-MM).利用GEM-MM结构以相对方法实现GEM增益的精确测量.该方法既可以省去传统方法中复杂的电子学标定过程,同时不需要进行原初电离电子数的估算,保证了增益的精确测量,并且可以实现GEM低增益的测量.基于GEM-MM测量GEM增益的原理,本文首先对GEM-MM电荷输运过程进行了模拟研究,优化了合适的工作电压.比较了三种不同类型和配比工作气体下GEM增益模拟结果,并在Ar/iC_4H_(10)(95/5)气体中测量了单层GEM在3—24范围内的有效增益.不同Penning系数下GEM增益的模拟结果表明,Penning系数为0.32时GEM增益的模拟结果与实验测量结果符合得很好.由此可以确定一个大气压下的Ar/iC_4H_(10)(95/5)气体中,Penning系数为0.32±0.01.     

Free-electron laser without inversion: gain optimization and implementation scheme

We consider a scheme of two noncollinear wigglers with an intermediate magnetic drift region, constituting a free-electron laser without inversion (FELWI). Two mechanisms of phase shifts in the drift region between the wigglers owing to a series of magnetic lenses can give rise to FELWI: velocity- and angle-dependent shifts. An appropriate combination of these shifts is shown to provide the conditions for amplification without inversion. The phase shifts optimizing the gain are found. A specific scheme for the drift region is suggested.     

热电子等离子体的耗散漂移不稳定性

在热电子等离子体中,存在密度梯度驱动的低频耗散性漂移波,本文导出了包括等离子体电阻性和粘滞性的漂移波的色散关系。热电子环能降低不稳定性的增长率,减少等离子体的反常输运损失。漂移波被热电子环完全稳定的条件是β_h>4ξ/(1+2ξ)。实验中测量了漂移波的激发区域和振荡特性,观察了热电子环的稳定作用实。验结果与理论分析相符。 关键词：     

Diode Feedback Effects in Intense Relativistic Electron Beam Oscillators

The general operation of intense relativistic electron beam oscillators is described. These oscillators employ the well-known reflex triode phenomenon to plug the ends of a magnetically confined beam drift region by setting up efficient electron reflexing between the real cathode of an accelerator and a downstream virtual cathode. For a variety of magnetic traps and experimental conditions, it is shown that the inherent electron beam feedback from the drift region can be effectively utilized to reduce the electronion diode impedance and thereby permit good matching and energy transfer between the diode and a sub-ohm particle accelerator. A theoretical model is presented to interpret the observed phenomena.     

北京自由电子激光器的光学速调管结构

采用KMR方程,结合即将运转的北京自由电子激光器总体实验参数,对其光学速调管结构研究.详细分析计算输入功率、电子束能散度、色散磁场、漂移空间长度及位置、以及电子束流等参数对光学速调管增益的影响.基于北京自由电子激光器的振荡器结构,提出一组对电子束能散度要求适中的实用化光学速调管参数.并对其饱和功率、功率谱以及渐变摆动器进行分析.     

CEE-TPC中GEM读出探测器传输性能实验研究

气体电子倍增器(GEM)因其具有较好的位置分辨以及各项同性的二维结构等优点,近年来受到了广泛的关注,在HIRFL-CSR上正在建设的低温高密核物质测量谱仪(CEE)也计划使用GEM作为TPC的读出探测器。不同电场条件下GEM探测器的传输特性对探测器的有效增益及能量分辨有较大影响。文中研究了单层GEM探测器中漂移区电场及感应区电场对探测器传输特性的影响;随后研究了双层GEM探测器的电压分配及传输区电场对探测器电荷传输性能的影响。结果表明,在单层及多层GEM探测器中,漂移区电场、传输区电场及感应区电场主要通过改变电子透过率和GEM雪崩电场强度及分布影响探测器的电荷传输性能,进而影响探测器的有效增益及能量分辨。以上实验结果表明GEM探测器是CEE-TPC读出探测器的理想选择,同时测试结果也为TPC中多层级联GEM工作点的选择提供了参考依据。     

Feedback stabilization of drift cyclotron loss cone instability by modulated electron sources

It is shown that the drift cyclotron loss cone instability can be suppressed by modulating electron density within the plasma. With the feedback in +90° phase the critical density gradient needed for the onset of the drift cyclotron loss cone instability increases approximately linearly with the gain. Typically with the gain of −50Ω _i the critical density gradient can be pushed up by as much as two orders of magnitude and minimum mirror plasma radius can be brought down in the same proportion.     

环域反馈下复Ginzburg-Landau系统的螺旋波动力学行为

针对自激振荡系统的复金兹伯格-朗道(Complex Ginzbury-Landau, 简称CGL)方程, 研究圆形环域与方形环域两种反馈控制下的螺旋波动力学。结果表明: 圆形环域反馈控制下, 螺旋波波头通常经过一段过渡漂移后进入圆形吸引子, 圆形吸引子的半径以及反馈刚启动时波头的漂移方向随环域参数呈周期性变化, 过渡漂移阶段波头轨道的平缓程度与复反馈信号模的时间函数中钟形部分的陡度有关, 且反馈增益的正负与大小也会影响受控螺旋波的动力学行为。方形环域反馈控制下的螺旋波波头的吸引子更为丰富, 主要包括方形吸引子、小的极限环吸引子、菱形吸引子以及点吸引子, 点吸引子通常位于方形环域的两条对角线上, 且波头运动随环域控制参数呈现规律性变换。     

Design of a high-efficiency low-voltage axially modulatedcusp-injected second-harmonic X-band gyrotron amplifier

We present the theoretical design of a second-harmonic small-orbit gyrotron amplifier which utilizes the interactions between a 35-kV 4-A beam and a TE₀₁₁ cavity to produce over 70 kW of amplified power at 9.9 GHz in a 1.83-kG magnetic field. One of the novel features of this device is that the electron gun produces an axially streaming annular beam which is velocity modulated by a short TM_0n0 input cavity. Perpendicular energy is imparted to the beam via a nonadiabatic magnetic transition at the end of a 13-cm drift region. An electronic efficiency of 53% is predicted with a large signal gain near 20 dB by a single particle code which takes into account nonideal effects associated with finite beam thickness and finite magnetic field transition widths     

A SCHEME FOR OBTAINING HIGH GAIN-LENGTH PRODUCT IN RECOMBINATION X-RAY LASER BY USING A LOW DENSITY TARGET

A scheme for obtaining high gain-length product(GL) of recombination X-ray laser is proposed and theoretically studied, in which a thin fiber with a density less than the critical one is driven by a short pulse laser. The features of laser-produced plasma and the gain coefficients of the transition from n = 3 to n = 2 of the H-like ions for a solid fiber and a low-density cylindrical target of carbon are shown. The Sobolev escape probability in a self-similarly expanding cylindrical geometry is used to evaluate the trapping effect on the gain coefficient. According to the simulations there are three obvious advantages of the low-density target, ie., wider gain region, longer lasing duration and no drift of the gain region. In addition, only a few joules are needed for getting saturated GL value.     

Change of zonal flow spectra in the JIPP T-IIU tokamak plasmas

When Ohmically heated low-density plasmas are additionally heated by higher-harmonics ion-cyclotron-range-of frequency heating, heated by neutral beam injection, or strongly gas puffed, the intensity of zonal flows in the geodesic acoustic mode frequency range in the tokamak core plasma decreases sharply and that of low-frequency zonal flow grows drastically. This is accompanied by a damping of the drift wave propagating in the electron diamagnetic drift direction, turbulence by trapped electron mode (TEM), and the increase of the mode propagating to ion diamagnetic drift direction (ITG). In the half-radius region, TEM and high-frequency zonal flows remain intense in both OH and heated phases. ITG and low-frequency zonal flows grow in heated plasmas, suggesting a strong coupling between ITG and low-frequency zonal flow.     

The tube character of electron drift in condensed inert gases

The behavior of an excess electron in condensed inert gases in an external electric field is considered at densities and temperatures at which the mobility of a slow electron is relatively high. On the basis of experimental data and a model of a pair electron interaction with atoms, an effective potential energy surface is constructed for an excess electron inside a dense inert gas. The region available for a slow electron consists of many intersecting channels that form a Delaunay network located between atoms. A drifting electron, as a quantum object, propagates along these channels (tubes), and electron transition between intersecting potential energy tubes of different directions provides an effective electron scattering. This mechanism of electron drift and scattering differs from that in gases and crystals. Peculiarities of electron drift inside dense inert gases are analyzed within the framework of this mechanism of electron scattering, leading to a moderate change of the electron mobility upon melting.     

Nonlinear gain computation for the sheet beam crossed-fieldamplifier

When the RF amplitude in a crossed field device is much smaller than the external DC voltage, the energy exchange between an electron and the wave is given by the change in the potential energy of the electron guiding center. The shift of the beam center of charge follows the space bunching into “spokes” caused by the RF-induced drift. A nonlinear estimate for the gain is derived and applied to the linear format crossed-field amplifier fed by a continuous sheet beam. The adiabatic approximation for the guiding center trajectories in the low gain regime determines the fraction of trapped/streaming particles and the energy exchange for each group. The radiation gain equals the change in the electron potential energy resulting from the shift in the beam center of charge across the anode-cathode voltage. The drift kinetic energy is approximately conserved, opposed to other microwave devices converting kinetic energy into radiation. The theory accounts for the symmetry of the response curve relative to the frequency detuning ω-ω₀, and the flat top near resonance. The analytic predictions agree with the experimental measurements of the gain versus frequency response     

Townsend coefficient,escape curve,and fraction of runaway electrons in copper vapor

Ionization and drift characteristics of electrons in copper vapor in the presence of an external electric field are analyzed. In contrast to normal gases, in copper vapor, the excitation energy of lower states is significantly lower than the ionization potential and the excitation cross section is several times greater than the ionization cross section at the incident-electron energy on the order of the ionization energy. This can affect the characteristics of electron bunching in gas. It is demonstrated that, as in previously studied gases, the notion of the Townsend coefficient remains meaningful even in the presence of strong fields at which the electric force exceeds the electron drag force acting in gas. The dependences of the main ionization and drift characteristics on the reduced field strength, the escape curve (which separates the region of effective electron multiplication and the region where electrons leave the discharge gap without multiplication), and the curves of equal efficiency for the formation of runaway electrons are obtained. It is demonstrated that a relatively high excitation cross section of copper levels leads to a sharper peak on the dependence of the Townsend coefficient on the field strength and a narrower region of the effective electron multiplication in comparison with previously studied gases.     

随机磁场对漂移波本征模的影响

本文分析随机磁场对低频漂移波本征模的影响。计算表明,存在随机磁场可影响电子对波的响应,改变电子色散函数,在有理磁面附近(k_‖≈0处),这种变化尤大,随机场可使电子色散函数变得相当平缓,这对漂移波本征模的稳定性有很大影响。本文证明了,在足够强的(但仍是实验允许范围内的)随机磁场下,漂移波普适模可成为绝对不稳定的。     

Sheared Flow Driven Drift Instability and Vortices in Dusty Plasmas with Opposite Polarity

Low-frequency electrostatic drift waves are studied in an inhomogeneous dust magnetoplasma containing dust with components of opposite polarity. The drift waves are driven by the magnetic-field-aligned (parallel) sheared flows in the presence of electrons and ions. Due to sheared flow in the linear regime, the electrostatic dust drift waves become unstable. The conditions of mode instability, with the effects of dust streaming and opposite polarity, are studied. These are excited modes which gain large amplitudes and exhibit interactions among themselves. The interaction is governed by the Hasegawa-Mima (HM) nonlinear equation with vector nonlinearity. The stationary solutions of the HM equation in the form of a vortex chain and a dipolar vortex, including effects of dust polarity and electron (ion) temperatures, are studied. The relevance of the present work to space and laboratory four component dusty plasmas is noted.     

Poloidal rotation of main ions in the CT-6B tokamak

The poloidal rotation velocity of neutral hydrogen atoms is measured using the Doppler shift of the Hα spectral line emitted in the CT-6B tokamak. The poloidal rotation of hydrogen atoms is generated through the collisions and charge-exchanges with main ions (protons). Therefore, the rotation direction of main ions can be deduced from that of neutral hydrogen atoms. The experimental results show that the main ions rotate in the electron diamagnetic drift direction, the same as the impurity ions, in the plasma core. The neutral hydrogen atoms rotate also in the electron diamagnetic drift direction in the edge region of the plasma. However, the rotation direction of main ions in the edge region cannot be judged from the experimental result due to the long mean free path of hydrogen atoms in the edge region. An inward diffusion flux of hydrogen atoms toward the torus inside with a velocity of the same order of magnitude as their poloidal rotation is also observed.     

Nonlinear theory of gyrotwystrons with stagger-tuned cavities

A nonlinear theory is developed to study the effects of stagger tuning on the gain and the bandwidth of stagger-tuned gyrotwystrons operating in the large-signal regime. The assumption that prebunching cavities are short and therefore the electron bunching proceeds mainly in the drift regions allows us to use a simple point-gap model of the prebunching section to study this problem. Studies of gyrotwystrons with one- and two-prebunching stagger-tuned cavities have been carried out in the large-signal, saturation regime and show that two-cavity gyrotwystrons are preferred over one-cavity gyrotwystrons for significantly larger gain, bandwidth and gain-bandwidth product. The comparison of our results with a previous gyrotwystron design shows good agreement and validates our model     

Nonlinear simulations of peeling-ballooning modes with anomalous electron viscosity and their role in edge localized mode crashes

A minimum set of equations based on the peeling-ballooning (P-B) model with nonideal physics effects (diamagnetic drift, E×B drift, resistivity, and anomalous electron viscosity) is found to simulate pedestal collapse when using the new BOUT++ simulation code, developed in part from the original fluid edge code BOUT. Nonlinear simulations of P-B modes demonstrate that the P-B modes trigger magnetic reconnection, which leads to the pedestal collapse. With the addition of a model of the anomalous electron viscosity under the assumption that the electron viscosity is comparable to the anomalous electron thermal diffusivity, it is found from simulations using a realistic high-Lundquist number that the pedestal collapse is limited to the edge region and the edge localized mode (ELM) size is about 5%-10% of the pedestal stored energy. This is consistent with many observations of large ELMs.     

Study of gain in C-band deflection cavities for afrequency-doubling magnicon amplifier

An experimental study of the gain between two half-wavelength, 5.7-GHz TM₁₁₀ mode pillbox cavities, separated by a quarter-wavelength drift space, and powered by a 170-A, 500-keV electron beam immersed in an 8.1-kG magnetic field is reported. These cavities constitute the first section of a planned multicavity deflection system, whose purpose is to spin up an electron beam to high transverse momentum (α≡υ⊥/υ_z⩾1) for injection into the output cavity of a frequency-doubling magnicon amplifier. A gain of ~15 dB was observed in the preferred circular polarization, at a frequency shift of approximately -0.18%, in the opposite circular polarization, at a frequency shift of approximately +0.06%. These results are in good agreement with theory