Study of Current Sheath Velocity and Its Distribution Using Tridimensional Magnetic Probe in Sahand Plasma Focus

The current sheath velocity in 0.25 Torr gas pressure of Filippov type plasma focus is studied experimentally.By using two tridimensional magnetic probes on top of the anode surface,the current sheath velocity is measured for argon,oxygen and nitrogen.Additionally,the effect of charging voltage on the current sheath velocity is studied in both axial and radial phases.We found that,the maximum current sheath velocities at both radial and axial phases are respectively 4.33 ± 0.28(cm/μs) and 3.92 ± 0.75(cm/μs) with argon as the working gas at 17 kV.Also,the minimum values of current sheath velocity are 1.48 ± 0.15(cm/μs) at the radial phase and 1.14 ± 0.09(cm/μs) at the axial phase with oxygen at 12 kV.The current sheath velocity at the radial phase is higher than that at the axial phase for all gases and voltages.In this study,variation of the full width half maximum(FWHM) of magnetic probe signals with voltage is investigated for different gases at radial and axial phases.     

密封中子管氘-氘产额及二次电子抑制

中子管的工作参数是影响中子产额的重要因素。为了更准确地调控D-D中子管的中子产额,对中子管的工作参数与产额关系进行了研究,同时为了提高中子管束流品质及寿命,对中子管的二次电子抑制进行实验。采用控制参数变量的方法分别研究了D-D中子管的热子电流、阳极高压、靶极高压对中子产额的影响,以及二次电子抑制电阻阻值与靶极电流之间的关系。结果表明：中子产额随着热子电流的增加而增加,当靶极高压为-80 kV、阳极高压为2.6 kV时,热子电流的最佳调控范围为290~305 mA;阳极高压与中子产额呈非线性关系,最佳阳极高压需要高于2.6 kV;靶极高压升高,中子产额随之增加,而且高压越高产额增加越快,靶极高压最佳工作范围为-120~-100 kV;D-D中子管二次电子抑制电阻阻值为8.7 MΩ或者抑制电压为403 V时,便可以完全抑制住二次电子。中子管的工作参数与中子产额的关系为今后中子管产额稳定性自调节可提供参考,二次电子抑制实验为抑制二次电子电流的产生提供依据。     

基于脉冲步进调制技术的ECRH全固态阳极高压电源控制系统设计

电子回旋共振管是产生高功率毫米微波的真空电子器件,在可控热核聚变研究、雷达等领域中有重要的应用。针对可控热核聚变研究中1 MW/105 GHz回旋管加热系统阳极电源幅度可调且调制的要求,使用高频开关电源技术和脉冲步进调制技术（PSM）研制了全固态阳极高压电源。重点阐述了阳极高压电源实现稳压、调制、前沿时间可调功能的软件控制算法,并通过实验对设计进行了验证。该阳极高压电源具有单脉冲、多脉冲调制和六电平预置波形等3种模式输出功能;输出参数达到35 kV/200 mA,波形前沿3 ms内可调,最大调制频率为1 kHz,调节精度在100 V以内。设计的控制方法也可应用于其他大功率微波源。     

Research of a fractional-turn ratio saturable pulse transformer and its application in a microsecond-range pulse modulator

As a combination device for a step-up pulse transformer and a magnetic switch,the saturable pulse transformer is widely used in pulsed-power and plasma technology.A fractional-turn ratio saturable pulse transformer is constructed and analyzed in this paper.Preliminary experimental results show that if the primary energy storage capacitors are charged to 300 V,an output voltage of about 19 kV can be obtained across the capacitor connected to the secondary windings of a fractional-tum ratio saturable pulse transformer.Theoretical and experimental results reveal that this kind of pulse transformer is not only able to integrate a step-up transformer and a magnetic switch into one device,but can also lower the saturable inductance of its secondary windings,thus leading to the relatively high step-up ratio of the pulse transformer.Meanwhile,the application of the fractional-turn ratio saturable pulse transformer in a μs range pulse modulator as a voltage step-up device and main switch is also included in this paper.The demonstrated experiments display that an output voltage with an amplitude of about 29 kV,and a 1.6 μs pulse width can be obtained across a 3500 Ω resistive load,based on a pulse modulator,if the primary energy storage capacitors are charged to 300 V.This compact fractional-turn ratio saturable pulse transformer can be applied in many other fields such as surface treatment,corona plasma generation and dielectric barrier discharge.     

Development of a new X-ray source using backscattered X-ray with the use of a cold cathode

The development of an intense X-ray source using backscattered X-ray produced using an advanced electrode configuration is described. The electrodes were composed of field emitters deposited on a wire mounted on a perforated plate as the cathode and a copper plate as the anode. Electrons from these emitters collided with the copper plate and X-ray was generated at collision points. The backscattered X-ray in the direction normal to the electron trajectory through a hole in the anode escaped from the vacuum chamber through a beryllium window. Continuous and characteristic X-rays were detected at an applied voltage lower than that of a conventional X-ray source from 3.0 to 9.4 kV, respectively. Moreover, the X-ray dosage measured with a survey meter reached 0.95 mSv/h at 5.0 kV of applied voltage. The transmission images of three types of material used as an X-ray source for the X-ray imaging system indicate three advantages; low power consumption, focal point controllable by adjusting applied voltage, and photographable motion picture of X-ray transmission.     

Experimental Study of Electron Emission Characteristics of a Surface Flashover Trigger in a Low Pressure Environment

Characteristics of electron emission induced by a surface flashover trigger device in a low-pressure trigger switch were investigated. A test method to measure the emitted charges from the trigger device was developed, and the factors affecting the emitted charges were analyzed. The results indicated that the major emitted charges from the trigger device were induced by surface plasma generated by surface flashover occurring on the trigger dielectric material. The emitted charges and the peak emission current increased linearly with the change in the trigger voltage and bias voltage. The emitted charges collected from the anode were affected by the gap distance. However, the emitted charges were less affected by the anode diameter. Furthermore, the emitted charges and the peak emission current decreased rapidly with the increase in gas pressure in a range from 0 to 100 Pa, and then remained stable or changed slightly when the increase in gas pressure up to 2400 Pa.     

Effect of the Discharge Voltage on the Performance of the Hall Thruster

In this paper,a two-dimensional physical model is established according to the discharging process in the Hall thruster discharge channel using the particle-in-cell method.The influences of discharge voltage on the distributions of potential,ion radial flow,and discharge current are investigated in a fixed magnetic field configuration.It is found that,with the increase of discharge voltage,especially during 250-650 V,the ion radial flow and the collision frequency between ions and the wall are decreased,but the discharge current is increased.The electron temperature saturation is observed between 400-450 V and the maximal value decreases during this region.When the discharge voltage reaches 700 V,the potential distribution in the axis direction expands to the anode significantly,the ionization region becomes close to the anode,and the acceleration region grows longer.Besides,ion radial flow and the collision frequency between ions and the wall are also increased when the discharge voltage exceeds 650 V.     

Temporal electric field of a helium plasma jet by electric field induced second harmonic (E-FISH) method

Electric field is an important parameter of plasma,which is related to electron temperature,electron density,excited species density,and so on.In this work,the electric field of an atmospheric pressure plasma jet is diagnosed by the electric field induced second harmonic(E-FISH) method,and the time-resolved electric field under different conditions is investigated.When positive pulse voltage is applied,the electric field has a peak of about 25 kV cm^-1 at the rising edge of the voltage...     

Design and Experimental Demonstration of a 94-GHz Gyrotron with Complex Cavity Operating at the Second Harmonic

A systematic theoretical and experimental study on 94-GHz second-harmonic gyrotron with a superconducting magnet system is presented in this paper. A complex cavity with gradual transition is employed in the gyrotron. A self-consistent field nonlinear theory is used and numerical simulation results for electron beam interaction with TE₀₂–TE₀₃ modes are given. A pulse output power of 235.2 kW is obtained by simulation at a beam voltage of 60 kV with beam current of 10 A and magnetic field 17 kG, corresponding to an efficiency of 39.2 %.     

A 20 kV, 5 A, 1 ns Risetime Pulsed Electron Beam Source

A 20 kV, 1 ns risetime pulsed electron beam source was developed using an extremely small gap （0.1 mm） diode driven by a sub-nanosecond risetime, 10 kV rectangular pulse generator. A beam current of 5 A was detected by using a fast response Faraday cup at a distance of 2 cm away from a grid anode. The shot to shot variation of the electron beam pulse was less than 10%.     

Current Sheath Dynamics and its Evolution Studies in Sahand Filippov Type Plasma Focus

One of the most important factors for optimizing the plasma focus device operation is the dynamics of the plasma. In this paper, we investigated the profile and dynamics of the current sheath by measuring the velocity and distribution of current sheath in Sahand as a Filippov type plasma focus device. For this purpose, the discharge is produced in pure neon gas with capacitor bank stored energies in the range of 14–50 kJ. The current sheath is monitored using two sets of magnetic probes, one with four and other with three equi-distant probe coils. These probes, installed in both radial and axial directions near the edge of the interior electrode (anode), are used for monitoring the distributions and dynamics of the current sheath. The maximum current sheath velocities at radial and axial phase are 4 ± 0.13 and 3.51 ± 0.22 (cm/μs) respectively for 0.25 Torr. The decreasing of CS velocities in going move away from anode surface is one of the our results in this paper. In this paper we conclude that the current sheath velocity at radial phase in Sahand is greater than axial phase. The effect of the neon working gas pressure and working voltage on the current sheath dynamics and its spatial evolution is investigated and presented.     

Dynamic Internal Electromagnetic Pulse Calculations in Three Spatial Dimensions

When a photon pulse impinges on a cavity, it causes electrons to be emitted from the cavity walls, thereby producing electromagnetic fields in the cavity. This constitutes what is known as Internal Electromagnetic Pulse (IEMP). The produced fields for a given incident photon pulse and spectrum (in the case where transient space charge effects are important) is calculated by means of a particle-in-cell numerical simulation. The calculations are three-dimensional and fully electromagnetic. The calculated fields for pressures of 0.4? and 720? show good quantitative agreement with recent underground test results. Both the calculations and the experimental data show that the presence of background gas in the cavity greatly affects the magnitude of the fields in the cavity. At 0.4? the emitted current is heavily space charge limited and the magnetic fields are comparatively low. At 720?, the emitted currents ionize the background gas and the secondary currents neutralize the space charge barrier. Then, almost the full emission current traverses the cavity causing much higher magnetic fields. The space charge neutralization process causes the risetime of the fields to be shorter than that of the emitted currents. System implications of these phenomena are discussed.     

Study on the influences of ionization region material arrangement on Hall thruster channel discharge characteristics

There exists strong interaction between the plasma and channel wall in the Hall thruster,which greatly affects the discharge performance of the thruster.In this paper,a two-dimensional physical model is established based on the actual size of an Aton P70 Hall thruster discharge channel.The particle-in-cell simulation method is applied to study the influences of segmented low emissive graphite electrode biased with anode voltage on the discharge characteristics of the Hall thruster channel.The influences of segmented electrode placed at the ionization region on electric potential,ion number density,electron temperature,ionization rate,discharge current and specific impulse are discussed.The results show that,when segmented electrode is placed at the ionization region,the axial length of the acceleration region is shortened,the equipotential lines tend to be vertical with wall at the acceleration region,thus radial velocity of ions is reduced along with the wall corrosion.The axial position of the maximal electron temperature moves towards the exit with the expansion of ionization region.Furthermore,the electron-wall collision frequency and ionization rate also increase,the discharge current decreases and the specific impulse of the Hall thruster is slightly enhanced.     

Experimental Characterization of the Plasma Synthetic Jet Actuator

The plasma synthetic jet is a novel active flow control method because of advantages such as fast response,high frequency and non-moving parts,and it has received more attention recently,especially regarding its application to high-speed flow control.In this paper,the experimental characterization of the plasma synthetic jet actuator is investigated.The actuator consists of a copper anode,a tungsten cathode and a ceramic shell,and with these three parts a cavity can be formed inside the actuator.A pulsed-DC power supply was adopted to generate the arc plasma between the electrodes,through which the gas inside was heated and expanded from the orifice.Discharge parameters such as voltage and current were recorded,respectively,by voltage and current probes.The schlieren system was used for flow visualization,and jet velocities with different discharge parameters were measured.The schlieren images showed that the strength of plasma jets in a series of pulses varies from each other.Through velocity measurement,it is found that at a fixed frequency,the jet velocity hardly increases when the discharge voltage ranges from 16 kV to 20 kV.However,with the discharge voltage fixed,the jet velocity suddenly decreases when the pulse frequency rises above 500 Hz,whereas at other testing frequencies no such decrease was observed.The maximum jet velocity measured in the experiment was up to110 m/s,which is believed to be effective for high-speed flow control.     

Development of a radiographic method for measuring the discrete spectrum of the electron beam from a plasma focus device

An indirect method is proposed for measuring the relative energy spectrum of the pulsed electron beam of a plasma focus device. The Bremsstrahlung x-ray, generated by the collision of electrons against the anode surface, was measured behind lead filters with various thicknesses using a radiographic film system. A matrix equation was considered in order to explain the relation between the x-ray dose and the spectral amplitudes of the electron beam. The electron spectrum of the device was measured at 0.6 mbar argon and 22 k V charging voltage, in four discrete energy intervals extending up to 500 ke V. The results of the experiments show that most of the electrons are emitted in the 125–375 ke V energy range and the spectral amplitude becomes negligible beyond 375 ke V.?     

The VEBA Relativistic Electron Accelerator

The VEBA high-current, relativistic electron accelerator has been designed and constructed at NRL for application in the study of high-power microwave sources. To meet the requirements of this study, the accelerator was designed for operation in either a short (60 nsec) or long (2.2 ?sec) pulse mode. The short-pulse mode has been in operation for nearly two years and has proven to be an extremely reliable design. The design of the long-pulse mode is now complete and component fabrication will soon be underway. The pulse-forming network in the short-pulse mode is an unbalanced, water Blumlein with an output impedance of 9.2? The Blumlein is pulse charged by a 17 stage Marx generator which has a series capacitance of 29.4 nF. By transmission along a tapered coaxial line, the output pulse is transformed to 20 ? and the voltage developed across a matched load increased to a maximum of 2.5 MV. The proposed conversion to the long-pulse mode will require that the Blumlein and transformer sections be removed and the diode assembly be attached directly to the oversized Marx tank. The direct coupling between the Marx and the Blumlein will then be replaced by two, nested, water capacitors which are shunted by spiral inductors. When coupled in series with the Marx, this output filter will form a three-section, voltage-fed, Guillemin (type A), pulse-forming network with a characteristic impedance of 40 ? and a maximum output voltage of 0.9 MV.     

E-beam generation in discharges initiated by voltage pulses with a rise time of 200 ns at an air pressure of 12.5–100 kPa

The effect of air pressure (12.5, 25, 50, and 100 kPa) on the generation of runaway electron beams in a non-uniform electric field when applying voltage pulses (≈35 kV) with a rise time of ≈200 ns has been studied. The results show that the discharge has various stages: streamer, diffuse, and spark. Initially, a wide streamer develops in the gap and a diffuse discharge is formed. A spark is formed ≈100 ns after the breakdown. The current pulse of a supershort avalanche electron beam (SAEB) was measured with a collector at various pressures of air. Experiments show that there are two modes of generation of runaway electrons. At an air pressure of 25–100 kPa, a single SAEB current pulse with a full width at half-maximum (FWHM) of 120–140 ps is observed. At the air pressure of 12.5 kPa, two current pulses of the electron beam are observed. FWHM of the first and second current pulses are ≈140 ps and ≈300 ps, respectively. The current pulse amplitude of the second electron beam is higher than that of the first one, but the electron energy is less.     

两种深空核爆炸辐射电磁脉冲模型的比较

深空核爆炸通过电偶极子和磁偶极子两种辐射机制产生电磁脉冲。本文具体分析了两种辐射模型产生电磁脉冲的物理机制,并估算了远处观察点的辐射电场。计算表明,电子以高斯波形出射时,百吨TNT当量爆炸在km量级范围内产生的电偶极子和磁偶极子辐射电场强度分别为kV/m和10 V/m量级。深空中地磁感应强度较弱,电子的角向运动相比初始方向运动是小量,因而磁偶极子的辐射强度远小于电偶极子的。     

基于电子直线加速器的空间电子环境地面模拟用电子源设计与实现

空间电子环境地面模拟装置由1台电子直线加速器提供能量1~5 MeV范围内的电子,后续束流传输系统将电子束进行扩束处理。较大的能量范围对加速器的设计与运行条件提出了较高要求。本文主要阐述了该加速器的设计与实现过程,综合考虑了能量开关技术和束流负载效应,通过研究不同条件下的耦合度参数特性确定了加速管耦合度,分析提出了磁控管输出参数并进行了实验研究。加速器实验测试结果表明,电子束能量参数达到指标要求,为模拟装置提供了有效可靠的电子源。     

Approximate Relativistic Solutions for One-Dimensional Cylindrical Coaxial Diode

Two approximate analytical relativistic solutions for one-dimensional,space-chargelimited cylindrical coaxial diode are derived and utilized to compose best-fitting approximate solutions.Comparison of the best-fitting solutions with the nuerical one demonstrates an error of about 11% for cathode-inside arrangement and 12% in the cathode-outside case for ratios of larger to smaller electrode radius from 1.2 to 10 and a voltage above 0.5MV up to 5MV,With these solutions the diode lengths for critical self-magnetic bending and for the condition under which the parapotential model validates are calculated to be longer than 1 cm up to more than 100cm depending on voltage,radial dimensions and electrode arrangement.The influence of ion flow from the anode on the relativistic electron-only solution is numerically computed,indicating an enhancement factor of total diode current of 1.85 to 4.19 related to voltage,radial dimension and electrode arrangement.