Structure and dynamics of the plasma armature of railgunmacroparticle accelerators

Results of the optical diagnostics of the plasma armature in railgun macroparticle accelerators are summarized. Determining the structure of the plasma armature and measuring its brightness and temperature at an initial stage of high-current discharge are the focus of attention. Experiments show that the plasma induced by the electric explosion of a copper foil undergoes complex variations that include plasma stratification. The plasma structure continuously changes, being of a complicated, irregular character. The lifetime of an individual stratum changes within 1 to 10 μs. In experiments on solid-body acceleration, the arc burns for a long time at the foil site, where a strong electrode erosion takes place. As the body passes through the railgun channel, the plasma armature continuously increases its length so that it gradually occupies the entire accelerator channel. the current is observed to redistribute itself from the front towards the end face of the plasma armature. The maximum brightness temperature measured in the experiments is 26000±2600 K. As is shown by the experiments, it is arguable that a magnetic-compressed discharge may be realized in a railgun solid-body accelerator with a plasma armature     

Scaling study for the performance of railgun armatures

A model is developed for investigating the performance of solid, plasma, hybrid, and transitioning armatures as a function of railgun geometry and gun operating conditions. The two figures of merit used in the calculation are the armature efficiency and the maximum velocity. Effects investigated include armature parasitic mass, armature resistance, friction, ablation drag, and, for the hybrid armature, gap growth. The model is applied to study how armature performance scales with projectile mass, or correspondingly bore dimension, and with gun current per unit rail height in the hypervelocity regime from 7 to 15 km/s. The model indicates that armature efficiency generally increases with projectile mass, whereas the maximum velocity for plasma and transitioning armatures is relatively insensitive to projectile mass. Calculations are also performed to determine the sensitivity of the model's predictions to uncertainty in key parameters, such as the ablation entrainment fraction, the skin friction coefficient, and the contact potential     

分布储能式电磁轨道炮效率分析

分布储能式电磁轨道炮在长导轨发射中具备高发射效率优势,为实现分布储能式电磁轨道炮的恒流特点,建立可供发射器参数、结构设计参考的仿真模型尤为重要。针对口径为60 mm×80 mm的矩形轨道炮,根据电流波形的平稳性要求,沿导轨方向设置电流馈入点,诊断电枢位置并分时序触发各馈入点电源,以测试分布储能式电磁轨道炮的工作性能。在COMSOL三维磁场中建立矩形导轨-电枢模型,基于电流和磁场的多物理场耦合有限元分析得到磁场和电流的分布,并利用电磁场仿真结果实现电流趋肤效应下轨道电阻梯度计算。基于MATLAB SIMULINK平台对电容储能型脉冲功率电源模块建立电气电路;分析分布储能式电磁轨道炮非线性时变的动态特性并建立轨道及电枢阻抗模型,计算正向电磁力、滑动摩擦力构造电枢的运动方程,并使用信号电路建立电枢-导轨模块,通过Simulink测量模块连接两个隔离的网络,仿真计算得到导轨电流及电枢的出膛速度。设计了总储能为4.16 MJ的分布式储能轨道炮,结果显示,电容预充电压为10.8 kV时,导轨长为3 m的分布式电磁轨道炮可将1 kg的弹丸加速至1.4 km/s,与炮尾集中式电磁轨道炮相比,系统发射效率可提升约3%。     

On the effect of conductivity of a shock-compressed gas on interferometric recording of parameters of motion of a liner

We report on the results of a study of the acceleration dynamics of an aluminum liner to a velocity of 5.5 km/s using continuous recording of velocity (velocity interferometer system for any reflector (VISAR) and Fabry-Perot interferometer) and motion trajectory (radiointerferometer and resistive transducer) in air and in a helium atmosphere. It is found that for liner velocities exceeding 4.0 and 5.0 km/s, the displacement of the shock wave front is recorded by the radiointerferometer in air and helium, respectively. At these velocities, the conductivities of air and helium behind the shock wave front are estimated.     

Plasma-armature railgun launcher simulations

The authors review three popular loss models currently used at CEM-UT (Center for Electromechanics at the University of Texas at Austin) in modeling EM (electromagnetic) launchers: friction, ablation, and armature drag. In experiments at currents below 500 kA using existing railgun design, the friction model alone was acceptable in predicting performance. In an experiment incorporating a railgun structure modified for higher stiffness and a measured peak railgun current of 700 kA, the effects of each of the loss models were compared to the measured results, and the greatest success at predicting the final projectile velocity and exit time occurred using the velocity-dependent friction model. It is believed that reducing frictional losses and plasma leakage will be instrumental in achieving velocities greater than 6 km/s     

Parameters of an erosion carbon plasma in the channel of a railgun

The working current dependences of the thermodynamic and electrophysical parameters of a free plasma piston moving with a near-maximal velocity in the channel of an electromagnetic rail launcher with graphite electrodes are obtained. The composition and weight of the plasma depend on the degree of electrode erosion due to discharge current passage (i = 40–80 kA). It is shown that the mean temperature of the plasma piston only slightly depends on the plasma mean pressure and plasma piston weight and increases with current by a near-power law. The measured values of the maximal velocity of the plasma piston front are compared with the calculated value of the sound velocity inside the piston. With the working current and cross-sectional area of the channel fixed, the initial gas density in the channel is found to influence the ratio of the piston maximal velocity to the sound velocity in the plasma. If the initial gas density is low (lower than some critical value), the maximal velocity of the plasma piston front exceeds the sound velocity in the plasma.     

Railgun experiments with Lexan insulators

A series of railgun experiments has been performed using Lexan insulators in both round and square bores, and with closed-breech and open-breech/injected configurations. Measured armature lengths have been roughly constant at 5-10 cm in a 1-cm bore for all Lexan insulator shots, indicating that the ablated Lexan is not swept up. Projectiles have been observed to reach peak velocity of 5.65 km/s with clean armature structures: i.e. no separated secondary arc or restrike. However, in most cases a secondary arc does occur with Lexan and limits the achievable velocity. Occasionally, stationary secondary arcs have also been observed for a particularly leaky gun assembly. The effect of insulator ablation on performance is discussed, indicating that Lexan may be useful at up to 8-10 km/s for well-sealed railguns     

电枢膛内高速运动控制仿真与试验

电磁轨道发射的过程中,电枢在膛内高速运动时会受到电磁力、电枢初始正压力、摩擦力、空气阻力、烧蚀阻力等多种因素影响,电枢的出口速度呈现出在一定范围内波动的特征。为了提高电枢的出口速度精度,针对膛内电枢与轨道摩擦不均衡性和烧蚀程度不确定的特性,综合考虑脉冲成形网络的电路模型与电枢的动力学特征,建立了电枢在膛内的运动开环控制仿真模型。通过仿真,得出了脉冲电源模块触发时刻与电枢出口速度之间的关系,提出了电枢出口速度闭环控制模型,探究了电枢出口速度控制可行方案。结果表明：应用闭环控制算法,可实现对电枢出口速度的精确控制。     

On wave and rheidity properties of the Earth’s crust

The properties of the Earth’s solid crust have been studied on the assumption that this crust has a block structure. According to the rotation model, the motion of such a medium (geomedium) follows the angular momentum conservation law and can be described in the scope of the classical elasticity theory with a symmetric stress tensor. A geomedium motion is characterized by two types of rotation waves with shortand long-range actions. The first type includes slow solitons with velocities of 0 ≤ V_sol ≤ c0, max = 1–10 cm s^–1; the second type, fast excitons with V₀ ≤ V_ex ≤ V_S–V_P. The exciton minimal velocity (V₀ = 0) depends on the energy of the collective excitation of all seismically active belt blocks proportional to the Earth’s pole vibration frequency (the Chandler vibration frequency). The exciton maximal velocity depends on the velocities of S (V_S ≈ 4 km s^–1) and/or P (V_P ≈ 8 km s^–1) seismic (acoustic) waves. According to the rotation model, a geomedium is characterized by the property physically close to the corpuscular–wave interaction between blocks that compose this medium. The possible collective wave motion of geomedium blocks can be responsible for the geomedium rheidity property, i.e., a superplastic volume flow. A superplastic motion of a quantum fluid can be the physical analog of the geomedium rheid motion.     

Passage of the discharge current through the plasma–electrode interface in the electromagnetic rail accelerator channel

We investigate the phenomena that accompany the acceleration of a free plasma piston (without a striker) in the electromagnetic rail accelerator channel filled with different gases (argon, helium). An intense glow appears in the shock-compressed layer (SCL) in the case of strong shock waves that produce a high electron concentration (~10¹⁷–10¹⁸ cm^–3) behind the front. We have proposed that explosive electron emission (EEE) ensures the high-intensity emission of electrons, the passage of a part of the discharge current through the SCL, and the glow of the SCL. The velocity of a shock wave for which the strong electric field in the Debye layer at the cathode causes EEE from its surface and the passage of the current in the SCL has been determined. It has been concluded that, for high velocities of the plasma, the EEE is a universal mechanism that ensure the passage of a strong current through the interface between the cold electrode and the plasma.     

Arc motion in railgun plasma armatures

H.H. Maecker's theory (1971) for arc motion and displacement is extended to the pressure-dependent arc conditions found in railgun armatures. It is shown that for the railgun plasma armature there are thermal mechanisms, which do not exist for solid armatures, that tend to move the peak of the current density forward toward the base of the projectile, thereby mitigating the tendency of the velocity skin effect to concentrate the current density toward the rear. This effect may explain the variance between some theoretically predicted current density profiles in plasma armatures and those measured by B-dot probes     

串联型双轨增强电磁轨道炮电路模拟

建立了包括脉冲电源模块和电磁轨道炮的全电路模型,实现了从电源放电至电磁轨道炮发射过程的全电路模拟。根据电源放电与洛伦兹力加速的耦合方程组,在电路模型内将电源的基本单元封装成子电路模块、弹丸受力运动模型转化为电路解耦模块,建立了针对24个基本单元组成的电源网络驱动串联型双轨增强电磁轨道炮的电路模型。将模拟结果与实验结果进行对比,电流模拟结果偏差2.6%,电枢出膛速度模拟结果偏差9.8%,模拟与实验结果基本一致,验证了模型方程和模拟方法的可靠性和合理性。     

Study of a railgun plasma armature

An experimental and theoretical study of a railgun plasma is presented. The experimental results have been obtained by recording the voltage and current variations as well as the spectral emission of the plasma. The gas pressure, helium, or air in contact with the exploding foil has been varied from 0.1 to 2 MPa. The apparatus consisted of a test chamber simulating a railgun facility. To complete the study, some measurements of the plasma characteristics in the 500-kJ EMA 1 railgun system have been recorded. It is shown that the resistance of the plasma, which is 15 mΩ at 10 kA, decreases to 0.2 mΩ at 500 kA. These results indicate the effect of the energy flow to the walls and that of the ionization potential of the gas in contact with the exploding foil at lower currents     

Specific features of the excitation spectrum of helium II in a plasma initiated by an electron beam

The emission of a low-pressure helium plasma (P≤2 Torr) initiated by a monochromatic electron beam is investigated. It is found that an increase in the current leads to a drastic increase in the rate of charge exchange of doubly charged helium ions. The assumption is made that inelastic collisions of He⁺⁺ ions with metastable helium atoms provide the main channel of charge exchange of these ions due to the reaction He⁺⁺+Hem→ He⁺*+He ₀ ⁺ .     

Drag of ballistic electrons by an ion beam

Drag of electrons of a one-dimensional ballistic nanowire by a nearby one-dimensional beam of ions is considered. We assume that the ion beam is represented by an ensemble of heavy ions of the same velocity V. The ratio of the drag current to the primary current carried by the ion beam is calculated. The drag current turns out to be a nonmonotonic function of velocity V. It has a sharp maximum for V near v _nF/2, where n is the number of the uppermost electron miniband (channel) taking part in conduction and v _nF is the corresponding Fermi velocity. This means that the phenomenon of ion beam drag can be used for investigation of the electron spectra of ballistic nanostructures. We note that whereas observation of the Coulomb drag between two parallel quantum wires may in general be complicated by phenomena such as tunneling and phonon drag, the Coulomb drag of electrons of a one-dimensional ballistic nanowire by an ion beam is free of such spurious effects.     

电磁轨道炮高速滑动接触电阻的定量表征

从描述电磁轨道炮炮口电压波形的场路模型出发,构建了电枢/轨道高速滑动接触电阻与轨道电流波形、炮口电压波形、电枢膛内速度曲线和轨道结构参数之间的关系,依据此关系可定量表征电磁轨道炮高速滑动接触电阻。实例计算表明,电枢/轨道高速滑动接触电阻的变化依赖于轨道电流变化,对应电流上升段、平顶段和电流下降段。在平顶段接触电阻最小约0.2mΩ,在电流上升段和电流下降段,接触电阻达3mΩ。     

电磁轨道炮高速滑动接触电阻的定量表征

从描述电磁轨道炮炮口电压波形的场路模型出发,构建了电枢/轨道高速滑动接触电阻与轨道电流波形、炮口电压波形、电枢膛内速度曲线和轨道结构参数之间的关系,依据此关系可定量表征电磁轨道炮高速滑动接触电阻。实例计算表明,电枢/轨道高速滑动接触电阻的变化依赖于轨道电流变化,对应电流上升段、平顶段和电流下降段。在平顶段接触电阻最小约0.2 m,在电流上升段和电流下降段,接触电阻达3 m。     

重复推进滑动电接触的试验研究

重点研究了低速条件下重复推进对电枢与导轨的接触状态、初速等的影响。试验采用6路时序脉冲功率电源、10 mm方口径装置和U型单体铝电枢,利用建立的试验系统进行重复推进试验。试验结果表明：随着重复次数增加,电枢和导轨的接触状态趋于良好,电枢启动时间提前,初速增大。说明在电枢低速运动条件下,重复试验中导轨表面遗留富含铝的堆积物对下一次试验的滑动电接触有积极作用。     

Acceleration of magnetic dipoles by a sequence of current-carrying turns

Magnetic dipoles are accelerated by a running gradient of the magnetic field that is produced by sequentially energizing current-carrying turns. Magnetic dipoles d _sh = 60 mm in diameter and l _tot = 1 m in length are gasdynamically preaccelerated to velocity V _in = 1 km/s, with which they are injected into the main accelerator. The turnover of the dipoles in the field of an accelerating pulse is prevented and focusing of dipoles is provided by directing the dipoles into a titanium tube. The weight of the dipoles is m = 2 kg, and they acquire final velocity V _fin = 5 km/s over acceleration length L _acc = 300 m.