速调管离子噪声特性的模拟分析

 离子噪声已成为影响现代微波管性能的一个重要因素，采用一维混合模型研究了速调管的离子噪声，用自编的1维粒子模拟程序对速调管的离子噪声特性进行了分析。采用小信号近似，从理论上推导出了速调管离子噪声与相位畸变关系的表达式，表明微波管相位噪声直接来源于管内离子量的变化。模拟了有离子噪声时速调管的相位特性，对模拟过程做了离子诊断并与实验结果进行比较，证明了模拟过程的正确性。探讨了电子束电流、电压以及聚焦磁场对离子噪声的影响规律，束电流与束电压改变后，离子噪声的周期与大小相应改变，增大束电流，离子噪声幅度会下降，并趋于稳定，而在束电流不变的情况下，离子噪声存在一个最小值。束电流与束电压确定，存在最佳的磁场使离子噪声幅度最小。     

微波管中离子张弛振荡的混沌现象

微波管中离子张弛振荡引起的噪声对其工作性能有很大影响，已成为微波管领域研究的焦点之一.用包络方程描述电子束特性，用离散的宏粒子模型描述离子特性，在此基础上编写了一维粒子模拟程序，对行波管与速调管模拟，得到张弛振荡的时间序列；将振荡看作是一个复杂非线性动力学系统的响应，分析了离子张弛振荡时间序列的功率谱、重构相图及Lyapunov指数，指出离子张弛振荡具有混沌性质，为研究离子张弛振荡的控制与带有离子振荡噪声信号的处理提供了参考. 关键词： 张弛振荡 粒子模拟 微波管 混沌     

强流脉冲驱动的X波段多注相对论速调管相位特性

相位特性是目前制约多注相对论速调管放大器进一步拓展应用的关键参数之一,为了有效提高器件输出微波相位的稳定性,利用一维非线性理论对X波段强流多注速调管放大器开展了理论研究,得到由强流脉冲特性引起的腔体杂频以及电子束运动速度变化率是造成输出微波相位波动的部分主要原因,同时基于18注实心电子束构成的X波段多注相对论速调管放大器开展了强流脉冲特性对输出微波频率和相位影响的数值计算,最后利用粒子模拟手段对理论结果进行验证。理论和模拟结果一致表明:强流脉冲的前沿和波动都将导致器件内实际工作频率的偏移,并引起相位波动;在脉冲前沿段,脉冲前沿长度越短,器件内实际工作频率偏移越大,相位波动幅度越大;在脉冲平顶段,脉冲波动导致的频率偏移与电压变化率相关,与电压的幅值无关,而脉冲电压波动导致的输出微波相位波动由电压变化率及其变化幅度两者共同决定。     

扇形多注强流相对论电子束的产生与传输研究

多注相对论速调管相对于常规相对论速调管, 每注电子束具有更低的导流系数和更低的空间电荷力, 却具有更高的束波转换效率. 本文基于这方面的需求, 通过三维软件模拟与实验研究了扇形多注强流相对论电子束的产生与传输. 通过建立电子枪的三维模型, 分析了阴极端面静电场的分布及其对电子束产生的影响; 通过粒子模拟获得了发射束流, 然后通过粒子跟踪仿真, 得到了电子束在空心漂移管和多扇形孔漂移管中传输的束斑图, 并对其进行了理论分析与解释. 模拟和实验结果表明, 电子束在空心漂移管传输过程中不仅绕束自身中心旋转, 还绕系统的中心旋转, 通过旋转多扇形孔漂移管实现对中的方法可提高传输效率.     

动态追踪算法在速调管输出段计算中的应用

在速调管的输出段中电子与输出腔电场相互作用向外界交出能量，电子速度出现很大的零散，采用传统的时间积分算法无法对超越粒子进行有效跟踪. 说明了动态追踪算法的原理，并将基于该方法编写的一维圆盘模型计算程序用于速调管输出段的分析，与传统算法相比取得了更加准确的数值结果. 关键词： 动态追踪算法 速调管 输出段     

导引磁场控制相对论速调管放大器电子束收集的方法

 通过改变磁场位形,利用粒子模拟方法,研究了相对论速调管放大器（RKA）中电子束收集位置对器件效率和工作稳定性的影响。合适的电子束收集位置对增加输出腔区束波作用强度、减小输出腔区强流电子束的空间电荷势能以及减少RKA中反射电子数量非常有利。对一个工作频率2.85 GHz的RKA的电子束收集方式进行了改进,在电子束参数为510 keV和8.1 kA,注入微波功率500 kW和导引磁场1.5 T时,模拟得到了1.4 GW的微波输出,效率33.7%,增益33.8 dB,改进电子束收集方式之前的模拟结果为输出功率1.1 GW,效率为26.3%。利用Surperfish设计了改进收集方式后所需的磁场位形,并导入粒子模拟程序进行了模拟,实现了对电子束收集位置的有效控制,输出效率为32%。     

充中性气体相对论返波振荡器的粒子模拟研究

 用PIC粒子模拟方法研究了充中性气体相对论返波管的物理机制，成功模拟了电子束碰撞充入返波管中的中性气体电离产生等离子体的过程，在电子束传输的路径上形成离子通道，有效中和电子束径向空间电荷力，有利于电子束的传输及束波相互作用产生微波。增加中性气体密度，返波管的输出频率明显上移，其辐射的功率和效率比相同的真空器件也有明显的提高。     

强流电子束碰撞电离背景气体研究

利用数值计算与粒子模拟两种方法,结合实际的实验数据,对高功率微波二极管中相对论电子束与背景气体相互作用碰撞产生的等离子体密度进行了研究.研究结果表明：碰撞产生的等离子体密度数值计算结果与粒子模拟结果基本一致,背景气压在0.01 Pa—0.05 Pa时,碰撞产生的等离子体密度在4—12×10⁹cm^-3,即便在考虑电子离子复合的情况下,数值计算结果与粒子模拟结果依然符合得很好.另外,粒子模拟结果表明：随着气压的增加,等离子体密度呈现先增大再减小然后又逐渐增大的过程, 关键词： 相对论电子束 等离子体 数值计算 粒子模拟     

S波段相对论速调管放大器同轴输出腔的数值模拟

 介绍了S波段强流相对论速调管放大器（RKA）同轴输出腔内束波转换效率和腔主要参数的计算、微波提取的粒子模拟和优化以及实验。模拟计算时，采用单间隙的同轴输出腔，束压580 kV、束流4 kA的环行电子束，基波调制深度为80%，利用3维粒子程序得到约500 MW的微波输出功率，效率21.5%。将该模拟结果应用于实验的设计，实验中采用束压550 kV、束流4 kA的电子束得到功率500 MW、脉宽120 ns的输出微波，束波转换效率22.7%，实验结果与模拟结果吻合较好。     

相对论速调管放大器的相位特性研究

理论分析了影响相对论速调管放大器(RKA)输出微波相位的相关因素,同时采用粒子模拟程序分析了RKA输出微波相位随电压、束流、电子束尺寸、电子束前沿和延迟时间等电子束参数以及腔体和漂移管长度等几何参数的变化,另外还开展了RKA相位特性的初步实验研究. 研究结果表明,电压、束流和电子束尺寸的改变,会造成RKA相移的改变,引导磁场、电子束前沿和延迟时间以及注入微波功率在适当范围内改变不会造成明显的RKA相移改变. RKA的相位灵敏度为2.6°,相位抖动小于20°. 关键词： 相对论速调管放大器 相位特性 功率合成 高功率微波     

Feedback method of the noise suppression in wave-mixing amplifiers based on non-linear materials with photorefractive response in a reflection grating configuration

In the photorefractive wave-mixing system, fluctuation in the signal beam intensity of the photorefractive output with a reflection grating has been analyzed by employing pump feedback method. In this method, fluctuations of the photorefractive wave-mixing process not only induce the intensity fluctuation of the mixing waves but also induce phase fluctuation of the mixing waves. Thus, the phase of the pump and signal beams at the output surface fluctuates in time around a mean value. Using such a positive feedback method of a pump beams, the relative fluctuation in the photorefractive output signal beam intensity with respect to its mean intensity can be minimized significantly without reducing its mean intensity. The factors that control the fluctuation in the signal beam intensity, such as the phase fluctuation of the output pump beam, absorption strength of the material and the feedback reflectivity of the cavity mirrors, on the relative fluctuation of the output signal intensity in the photorefractive wave-mixing systems have been studied in detail. It has been found that the fluctuation of the output signal intensity relative to its mean intensity in the photorefractive wave-mixing system can be suppressed to larger extent by taking lower value of feedback reflectivity of the cavity mirrors which could exist at a higher value of absorption strength of the photorefractive materials.     

基于二次电子发射的离子束剖面测量

介绍了利用二次电子发射测量束流强度分布的基本原理。针对能量为几十keV的低能离子束,制作了一个基于印刷线路板工艺的离子束剖面测量系统模型。模型采用宽度1.8 mm的金属条作为收集电极,相邻两条之间的间距为2 mm。利用电子回旋共振源对束流剖面测量系统进行了性能测试,考察了网栅电压对信号收集的影响以及系统的线性响应和成像特性。结果表明,探测器输出信号与束流强度之间具有较好的线性关系,系统能得到离子束的一维横向强度分布,位置分辨率2 mm。     

一种新型双环路光电振荡器

利用偏振分束器(PBS)和合束器(PBC)在不增加有源器件的基础上设计了一种双环路光电振荡器。该振荡器可以同时产生光、电两种输出。由于没有增加有源器件,可以避免附加电噪声的引入。此外,光载波在光域中是正交耦合,消除了随机干涉和拍噪声对振荡频率的影响。通过理论分析,双环结构的光电振荡器可以有效抑制每一单环路中起振的边模。经过实验对比,采用双环结构可以把边模抑制比提高30~70 dB。最后,利用该方案和普通商用器件得到了中心频率为12 GHz的振荡,测得边模抑制比为50~70 dB,相位噪声为-93 dBc/Hz at10kHz,Q值达到1010。     

5MW中性束弧电源DC/DC变换器的设计与实现

中性束离子源弧放电具有气体放电等离子体的非线性特性,工作时还会受到气体压强、外磁场、阴极状态等因素的影响,采用晶闸管相控调压技术的弧电源很难实现对这种大功率电弧的稳定的闭环控制。为此,提出了一种多相多重的大电流DC/DC变换器,具有响应速度快、电流上升时间短、电流纹波小等特点,大幅提高了离子源弧放电闭环控制的稳定性。设计了滤波电感能量回馈电路,弧电源可以根据中性束系统的需要使弧电流快速减小0%~100%(可调),然后根据控制信号迅速恢复正常弧电流输出,形成一个弧电流凹坑。电源还采用超级电容储能技术,使电源体积减小了2/3,电网容量小于10kV·A。离子源放电时不会受到电网波动的影响,弧放电更加稳定。实验数据显示:该电源最大输出为220kW/1500A,电流纹波在1%以内,电流上升时间约100μs,最大超调量小于3%,可以满足5 MW中性束离子源及系统的要求。     

Minimization of the fluctuation in the signal beam intensity of a nonlinear optical medium with a transmission grating

Fluctuation in the intensity of the output signal beam of a photorefractive optical resonator can be reduced significantly by employing pump beam positive feedback. The fractional intensity transfer which is independent of time in the absence of fluctuation becomes time dependent in the presence of fluctuation of the optical wave-mixing process. The influence of various controlling parameters on the relative fluctuation of the output signal intensity has also been studied.     

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建立了包含两种正离子的碰撞等离子体鞘层的流体模型,通过四阶龙格-库塔法模拟了碰撞对含有两种正离子的等离子体鞘层中的离子密度和速度分布产生的影响。结果表明,对于两种正离子的等离子体来说,鞘层中无论哪种离子与中性粒子碰撞频率的增加,该种离子的密度和速度分布都将呈现波动变化,密度是先增加后降低,速度是先降低后增加;而另一种离子的密度和速度呈单调变化。鞘边正离子的含量越少,受自身与中性粒子的碰撞频率增加,鞘层中该种离子密度先增加后降低的变化位置就越远离等离子体的鞘层边界。同时发现该种离子密度分布受自身碰撞频率增加,降低的幅度变化非常小。另外发现电子碰撞器壁产生的二次电子发射系数对质量较轻的离子影响要大一些。     

The statistical fluctuation of a single-mode laser system driven by coloured pump noise with signal modulation and the quantum noise with cross-correlation between its real and imaginary parts

Using the linear approximation method, this paper studies the statistical property of a single-mode laser driven by both coloured pump noise with signal modulation and the quantum noise with cross-correlation between its real and imaginary parts, and calculates the steady-state mean normalized intensity fluctuation and intensity correlation time. It analyses the influences of the modulation signal, the net gain coefficient, the noise and its correlation form on the statistical fluctuation of the laser system respectively. It is found that the coloured pump noise modulated by the signal has a great suppressing action on the statistical fluctuation of the laser system; the pump noise self-correlation time and the specific frequency of modulation signal have the result that the statistical fluctuation tends to zero. Furthermore, the `colour' correlation of pump noise has much influences on the statistical fluctuation of the laser system. Increasing the intensity of pump noise will augment the statistical fluctuation of the laser system, but the intensity of quantum noise and the coefficient of cross-correlation between its real and imaginary parts have less influence on the statistical fluctuation of the laser system. Therefore, from the conclusions of this paper the statistical property can be known and a theoretical basis for steady operation and output of the laser system can be provided.     

Visualization of aerodynamic noise source around a rotating fan blade

The purpose of this study is to understand the aerodynamic noise source distribution around a rotating fan blade by measuring the noise signal and velocity field around the blade. The local noise-level distribution over the fan blade is measured by microphone arrays, and the flow field is visualized by smoke and phase-averaged PIV measurement. The noise source distribution is examined by cross-correlation analysis between noise signal and velocity fluctuation. It is found that the noise source is located near the rotating fan blade, especially around leading and trailing edges. The separation and reattachment of flow are observed near the leading edge, and the tip vortices and vortex shedding are found near the trailing edge. The cross-correlation distribution of the noise signal and the radial velocity fluctuation shows large magnitude in the correlated regions, which indicates the noise generation by the formation of vortex structure around the blade.     

具有非线性阻尼涨落的线性谐振子的随机共振

较之于线性噪声, 非线性噪声更广泛地存在于实际系统中, 但其研究远不能满足实际情况的需要. 针对作为非线性阻尼涨落噪声基本构成成分的二次阻尼涨落噪声, 本文考虑了周期信号与之共同作用下的线性谐振子, 关注这类具有基本意义的阻尼涨落噪声的非线性对系统共振行为的影响. 利用Shapiro-Loginov公式和Laplace变换推导了系统稳态响应振幅的解析表达式, 并分析了稳态响应振幅的共振行为, 且以数值仿真验证了理论分析的有效性. 研究发现: 系统稳态响应振幅关于非线性阻尼涨落噪声系数具有非单调依赖关系, 特别是非线性阻尼涨落噪声比线性阻尼涨落噪声更有助于增强系统对外部周期信号的响应程度; 而且, 非线性阻尼涨落噪声比线性阻尼涨落噪声使得稳态响应振幅关于噪声强度具有更为丰富的共振行为; 同时, 二次阻尼涨落噪声使得稳态响应振幅关于系统频率出现真正的共振现象; 而在这些现象和性质中, 非线性噪声项的非线性性质对共振行为起着关键的作用. 显然, 以二次阻尼涨落作为基本形式引入的非线性阻尼涨落噪声, 可以有助于提高微弱周期信号检测的灵敏度和实现对周期信号的频率估计.     

Stochastic resonance in thermally activated reactions: Application to biological ion channels

At the molecular level many thermally activated reactions can be viewed as Poisson trains of events whose instantaneous rates are defined by the reaction activation barrier height and an effective collision frequency. When the barrier height depends on an external parameter, variation in this parameter induces variation in the event rate. Extending our previous work, we offer a detailed theoretical analysis of signal transduction properties of these reactions considering the external parameter as an input signal and the train of resulting events as an output signal. The addition of noise to the system input facilitates signal transduction in two ways. First, for a linear relationship between the barrier height and the external parameter the output signal power grows exponentially with the mean square fluctuation of the noise. Second, for noise of a sufficiently high bandwidth, its addition increases output signal quality measured as the signal-to-noise ratio (SNR). The output SNR reaches a maximum at optimal noise intensity defined by the reaction sensitivity to the external parameter, reaction initial rate, and the noise bandwidth. We apply this theory to ion channels of excitable biological membranes. Based on classical results of Hodgkin and Huxley we show that open/closed transitions of voltage-gated ion channels can be treated as thermally activated reactions whose activation barriers change linearly with applied transmembrane voltage. As an experimental example we discuss our recent results obtained with polypeptide alamethicin incorporated into planar lipid bilayers.(c) 1998 American Institute of Physics.