轴线起爆式螺线管型爆磁压缩发生器理论模型

 对轴线起爆式螺线管型爆磁压缩发生器进行了理论模型研究，建立了爆炸管的一维爆轰驱动模型、螺线管内空间磁场强度分布模型、爆炸管外表面磁压力模型和发生器系统的等效电路模型等，对此类发生器的物理过程进行系统描述。在此基础上，编制了相应的零维数值模拟程序CEMG 1.0，利用该程序分别对四种不同模型参数的发生器进行了理论计算和参数优化，并对其中一模型发生器爆炸管外表面的磁压力及其引起的剩余电感进行了计算，给出了剩余电感与初始输入条件及负载电感的关系，从而得到该模型的输出性能极限。对理论模型的正确性进行了实例验算证明。     

Reduced-order modelling of thermoacoustic instabilities in a two-heater Rijke tube

The topic of thermoacoustic instabilities in combustors is well-investigated, as it is important in the field of combustion, primarily in gas-turbine engines. In recent years, much attention has been focused on monitoring, diagnosis, prognosis, and control of high-amplitude pressure oscillations in confined combustion chambers. The Rijke tube is one of the most simple, yet very commonly used, laboratory apparatuses for emulation of thermoacoustic instabilities, which is also capable of capturing the physics of the thermally driven acoustics. A Rijke tube apparatus can be constructed with an electrical heater acting as the heat source, thus making it more flexible to operate and safer to handle than a fuel-burning Rijke tube or a fuel-fired combustor. Augmentation of the heat source of the Rijke tube with a secondary heater at a downstream location facilitates better control of thermoacoustic instabilities. Along this line, much work has been reported on the investigation of thermoacoustics by using computational fluid dynamics (CFD) modelling as well as reduced-order modelling for both single-heater and two-heater Rijke tube systems. However, since reduced-order models are often designed and built upon certain empirical relations, they may not account for the dynamic behaviour of the heater itself, which is a critical factor in the analysis and synthesis of real-time robust control systems. This issue is addressed in the current paper, where modifications have been made to existing models by incorporating heater dynamics. The model results are systematically validated with experimental data, generated from an in-house (electrically heated) Rijke tube apparatus.     

Excitation of a cylindrical cavity by a helical current and an axial electron beam current

The explicit expressions (in the Vainshtein and Markov forms) are derived for the excitation of a cylindrical cavity with perfectly conducting walls and with impedance end faces. Excitation of a cylindrical cavity and a cylindrical waveguide with a preset nonuniform axial electron-beam current and a helical current with a variable pitch, which is excited by a concentrated voltage source and is loaded by a preset pointlike matched load, is considered. For the helical current, the integro-differential equation is formulated. The traveling-wave tube (TWT) is simulated in the preset beam current approximation taking into account the nonuniform winding of the spiral coil, nonuniform electron beam, and losses.     

基于自激移相控制的行波管电源LLC变换器

为提高行波管高压电源变换器全工作状态的软开关和动态响应性能,分析了行波管负载的动态突变特性,设计了全桥LLC谐振倍压功率变换器,提出了基于自激移相控制的全桥LLC变换器的工作原理、大信号建模和设计方法,重点针对负载突变特性分析了变换器的瞬态软开关特性和控制参数设计原则。通过仿真验证了变换器在负载突变时能够实现开关管的零电压开通,说明了控制参数的自适应调整能够提高变换器的动态响应性能。通过实验分析了基于自激移相控制的全桥LLC变换器的典型波形,表明变换器具有较好的软开关特性。     

An adaptive quarter-wave tube that uses the sliding-Goertzel algorithm for estimation of phase

This paper describes an adaptive quarter wave tube used to attenuate a tone from the exhaust noise of a large diesel engine. A sliding-Goertzel algorithm was used to calculate the phase angle of the transfer function between a microphone in the adaptive quarter wave tube and in the main exhaust duct. The control system adjusted the length of the adaptive quarter wave tube until the phase angle was −90° and caused the sound pressure level at the cylinder firing frequency in the exhaust duct to be minimized. The system was able to adapt to changes in engine speed, exhaust gas temperature, and load applied to the engine. The results demonstrate that the sliding-Goertzel algorithm can be used effectively to estimate the phase angle in an adaptive–passive acoustic control system.     

Energy losses in a thin-wall tube multifilament composite superconductor carrying a transport current in a time-varying transverse magnetic field

An approach and formulae are given to calculate the hysteresis, coupling, eddy-current and Ohmic losses in a thin-wall tube multifilament composite superconductor carrying a transport current under low excitation by a homogeneous transverse time-varying magnetic field, taking into account possible filament saturation and current sharing effects. The technique can be applied to a single layer superconducting cable twisted around a thin-wall normal conducting tube.     

Retrapping of current in a vacuum transmission line upon reflection of a magnetic self-insulation wave from a load

The equations derived in the framework of the Brillouin model of an electron layer in a “hot” vacuum transmission line form the basis for analysis of reflection of an electromagnetic wave from a load, leading to a redistribution of currents in the magnetically insulated transmission line (MITL). It is shown theoretically for the first time that conversion (retrapping) of a considerable fraction of the current in the electron layer to the cathode current upon the reflection of the wave from the load upon an insignificant change in the current and voltage in the line is possible even under the conditions close to the self-consistent regime. This effect can be explained by the change in the state of the electron layer under the action of the reflected wave, which is associated with a transition of the system from the right branch of the magnetic self-insulation curve to the left branch.     

Experimental studies on isentropic efficiency of a nonequilibriumMHD disk generator

Isentropic efficiency of the nonequilibrium MHD power generator was studied by a shock tube driven disk generator. Cesium seeded helium was used as a working gas. From the measurements of Faraday current density distribution, it was possible to estimate the general tendency of Joule dissipation in the generator. The Joule dissipation did not decrease due to the occurrence of nonuniformity of the plasma when external load resistance was low, although it decreased with the decrease in the load resistance when the load resistance was high. The electrical efficiency increased with the increase in applied magnetic flux density. This fact is thought to be caused by high Hall parameter and the stabilization of the plasma due to high degree of seed ionization     

Stationary structure of a high-frequency discharge maintained by a distributed electromagnetic source in the presence of an external magnetic field

The stationary structure of an axisymmetric high-frequency discharge maintained by a given source in an external dc magnetic field is investigated. The source is assumed to be a current wave that travels over the discharge tube surface in the direction of the external magnetic field. The source current has a single azimuthal component and its frequency belongs to the lower hybrid range. The main emphasis is placed on the special case where the electron heat conduction length across the external magnetic field exceeds considerably the tube radius. The dependences of discharge plasma parameters on the current amplitude and propagation constant along the tube have been found for this case. The results of numerical calculations of the distributions of the field and the power of the Joule loss in a discharge are presented.     

Evaluation of the radiation dose to a phantom for various X-ray exposure factors performed using the dose area product in digital radiography

The aim of this study is to measure the dose area product (DAP) in digital radiography by using a DAP meter to determine the X-ray exposure. Pediatric X-ray examinations can be obtained for any radiographic examinations using the selected radiographic examination parameters (kVp and mAs), the DAP information recorded. The best peak signal-to-noise ratio (PSNR) at a fixed tube voltage of 70 kVp was obtained at tube currents of 20 and 32 mA, whereas the best PSNR at a fixed tube current of 25 mA was obtained at a tube voltage of 73 kVp. The fixed tube voltage of 70 kVp and the fixed tube current of 25 mA could help to obtain the best image quality and depict the spatial resolution of an anthropomorphic torso phantom radiographic examination. The normalized data over the DAP were provided to determine the patient dose from radiography.     

Generation of a strong magnetic field inside a dielectric tube

Experiments on compression of an axial magnetic flux by a plasma liner are conducted with the MIG high-current generator. It is shown that, when the current passing through the liner is about 1 MA, a magnetic field with a strength of up to 50 T and a duration of 300 ns (in the steady state) can be achieved in a dielectric tube 3–4 mm in diameter that is placed on the axis. The physics of magnetic field evolution in the tube is discussed.     

The influence of a transverse magnetic field on a subnormal glow discharge in air

In subnormal glow discharge under d.c. excitation at different pressure in a varying transverse magnetic field (0 to 30 G) some measurements have been carried out for various initial average tube currents. The voltage across the discharge increases and average tube current and residual current decreases in the magnetic field. With the help of Beckman’s expression [4] for the axial field and the electron density distribution in a transverse magnetic field the observed variation of current and voltage can be satisfactorily explained. The variation of axial electric field with transverse magnetic field can be represented to a fair degree of accuracy by the derived equation. The behaviour of residual current with magnetic field has been observed in these oscillations.     

Matching of an extended high-current Z-discharge to a pulsed power system

Conditions for matching an extended high-current Z-discharge to a pulsed power system are numerically investigated. The power system consists of a pulsed voltage generator and a long transmission line. Experiments are aimed at generating a highly ionized dense plasma as an active medium for an extreme ultraviolet laser on hydrogen-like ions of nitrogen (λ = 13.4 nm). Emphasis is on the distribution of the energy transmitted from a storage ring to a load among its components and on the reduction of the energy that remains in the electrical circuit by the end of the pump pulse and will inevitably dissipate in the discharge tube. The solution of this problem will make it possible to diminish the load on the walls of the discharge chamber and extend its service life. It is shown that energy deposition into the load is effective when the timeaveraged sum of the ohmic and dynamic components of the discharge resistance is roughly equal to the wave impedance of the transmission line. In this case, the wave reflected from the load carries away a minimal energy, which allows for optimization of the energy deposited into the load. The input and output energy balances for different matching conditions are calculated with an eye to designing an efficient short-wavelength extreme ultraviolet laser with a long service life of the discharge tube.     

Photogalvanic effects in heteropolar nanotubes

We show that an electrical shift current is generated when electrons are photoexcited from the valence to conduction bands on a BN nanotube. This photocurrent follows the light pulse envelope and its symmetry is controlled by the atomic structure of the nanotube. We find that the shift current has an intrinsic quantum mechanical signature in which the chiral index of the tube determines the direction of the current along the tube axis. We identify the discrete lattice effects in the tangent plane of the tube that lead to an azimuthal component of the shift current. The nanotube shift current can lead to ultrafast optoelectronic and optomechanical applications.     

封离式He-N$lt;sub$gt;2$lt;/sub$gt;-CO$lt;sub$gt;2$lt;/sub$gt;激光器横模特性的测量与分析

利用激光光束分析仪,实时在线测量了一根He-N₂-CO₂封离式激光管的横模分布特性,得到了激光模式随放电电流的变化关系.实验表明该激光器在最佳工作电流时,容易形成低阶模运转,但很难获得基模运转,通过改善对称性以及加快冷却水流速可以获得基模运转.通过激光动力学过程分析了模式变化的形成原因,为大功率激光器模式控制与改善提供了一定的理论和实验借鉴. 关键词： 2激光')" href="#">CO₂激光 横模 激光模式分析 光束质量     

紧凑型X-pinch装置探头标定

 介绍了紧凑型X-pinch脉冲功率装置的电流电压测量设计方法。根据该装置的同轴传输线结构特点,研制了一种利用金属膜连接传输线外筒与负载外筒,构成回路测量负载电流的探头。在传输线末端设计电容分压器作为测量负载电压的探头,并利用电路模拟软件对此过程进行模拟,同时这两个探头需要进行在线标定。实验研究结果表明,该探头性能稳定、响应快,是测量负载电流与电压的理想工具。     

Pressure variation by a magnetohydrodynamic method at the surface of a body placed in a supersonic flow

This study is aimed at investigating the possibility of pressure variation near the surface of a body placed in a supersonic flow as a model of an aerofoil or the nose of an aircraft by organizing a surface gas discharge in a magnetic field transverse to the flow. The flow parameters and pressure are mainly affected by the ponderomotive Lorentz force acting on the gas in the direction orthogonal to the direction of the organized discharge current and leading to the removal or compression of the gas at the surface of the body and, hence, a variation of pressure. Experimental data on the visualization of the flow and on the pressure at the surface of the body are considered for various configurations of the current and intensities of the gas discharge and magnetic field; it is demonstrated that such configurations of the current and magnetic field near the surface of the body under investigation can be organized in such a way that the pressure at the front part as well as the upper and lower surfaces of the body under investigation can be increased or decreased, thus changing the aerodynamic drag and the aerofoil lift. Such a magnetohydrodynamic control over aerodynamic parameters of the aircraft can be used during takeoff and landing as well as during steady-state flight and also during the entrance into dense atmospheric layers. This will considerably reduce the thermal load on the surface of the body in the flow.     

一种用于触发引燃管的脉冲电源

设计了一种单电源的变压器型高电压、大电流脉冲源。该电源只有一套放电电容,以晶闸管作为放电开关,单原边双副边的脉冲变压器作为传输线。利用二极管的单向导通特性,使变压器根据负载不同的工况运行在不同的状态,分时输出高电压、大电流脉冲。该设计利用变压器在空间上将高压输出回路和低压控制回路隔离,与一般的双电源设计方式相比,降低了驱动电路的成本,减少了装置的体积,有利于设备的小型化和紧凑化。试验结果表明:当原边18 μF的储能电容充电电压为700 V时,通过晶闸管开关控制电容向2 ∶2 ∶20的单原边双副边脉冲变压器放电,副边开路时输出幅值7.6 kV、上升沿432 ns的开路电压,副边短路时输出幅值690 A、半高宽15.6 μs、前沿7.0 μs的短路电流,满足NL37248引燃管的触发要求。     

黎开管热声振荡的主动控制研究

本文以黎开管内的热声耦合振荡为研究对象,设计基于主动补偿的适应性控制器抑制黎开管内的不稳定燃烧。试验以扬声器为执行机构来改变黎开管的边界条件,从而抑制黎开管内的热声耦合振荡。实时控制效果表明,本文所采用的适应性控制算法能够有效抑制因热声耦合产生的燃烧振荡,为实际动力系统燃烧振荡抑制提供了思路。     

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在J-TEXT 装置上设计了一种基于现场可编程逻辑门阵列(FPGA)的数字式峰值检测电路,该电路先对负载电流进行模数(AD)转换,再经有限冲激响应(FIR)滤波器进行数字滤波,然后用峰值算法获取电流峰值,最后经数模转换器(DA)输出电流峰值。在此基础上,完成了扰动场交流电源负载电流峰值反馈系统的设计,实验结果表明系统动态性能好、检测精度高,能很好满足交流电源负载电流反馈控制的需要。