The Jon Beam Opening Switch

Plasma opening switches (POS's) have shown excellent characteristics in pulsed power applications. Proposed POS scaling predicts that the fastest opening time for a given conducted current should occur using a high-velocity low-density plasma as the switch medium. The ion beam opening switch (IBOS) uses a charge-neutral ion beam of 100-300 kV, ? 120 A/cm2 as the switch "plasma." Its velocity of up to 600 cm/?s and density of ~1012/cm3 make this a very fast low-density plasma compared with typical 10 cm/?s and 1013/cm3 POS plasmas. The IBOS has conducted ? 70 kA flowing in a parallel-plate transmission line driven by a 4-? pulser. IBOS opening time is load dependent, being ? 4 ns into a 15-nH load and about twice as long into a 4-? electron diode load. However, switch impedance is not zero during the entire conduction time, rising to ? 3 ? by the time of peak current. Peak current conducted before opening does not vary linearly with either injected ion current or switch axial length. Instead, the conduction current scales with plasma density in the switch, and is nearly independent of switch area until the area is restricted to a narrow (~1 cm) strip.     

Plasma density evolution in plasma opening switch obtained by a time-resolved sensitive He-Ne interferometer

To understand the formation process of vacuum gap in coaxial microsecond conduction time plasma opening switch (POS), we have made measurements of the line-integrated plasma density during switch operation using a time-resolved sensitive He-Ne interferometer. The conduction current and conduction time in experiments are about 120 kA and 1 μs, respectively. As a result, more than 85% of conduction current has been transferred to an inductive load with rise time of 130 ns. The radial dependence of the density is measured by changing the radial location of the line-of-sight for shots with the same nominal POS parameters. During the conduction phase, the line-integrated plasma density in POS increases at all radial locations over the gun-only case by further ionization of material injected from the guns. The current conduction is observed to cause a radial redistribution of the switch plasma. A vacuum gap forms rapidly in the plasma at 5.5 mm from the center conductor, which is consistent with the location where magnetic pressure is the largest, allowing current to be transferred from the POS to the load.     

能量输运对柱状等离子体开关磁场穿透过程的影响

 对于高密度、导通时间为μs级的柱状等离子体开关，利用磁流体动力学理论(MHD)，对其导通阶段的磁场穿透过程进行了模拟，得到了磁场分布随时间的变化；研究了开关导通过程中能量输运导致的温度不均匀分布对磁场穿透过程的影响。模拟结果表明：对于高密度等离子体开关，磁场以远大于磁扩散速率的速度穿透到等离子体中；在磁压对等离子体产生的压缩效应和欧姆加热效应共同作用下，激波区域的等离子体温度显著升高，这进一步加速了磁场穿透；当考虑能量输运方程时，开关导通时间为0.87 μs，比等温模型的结果0.92 μs短，与实验结果0.87 μs相一致。     

Resonant laser diagnostics of a planar plasma opening switch

The plasma opening switch (POS) is an integral part of inductive store pulsed power systems. Using flashboards coated with BaO and a dye laser tuned to the 493.4 nm ground state transition of the singly-ionized barium ion, resonant laser diagnostics have been employed to image the switch plasma and provide a measurement of the plasma density during conduction and opening. Gaps open during the conduction phase with their position in the inter-electrode region depending upon the initial fill plasma. There is little axial motion of the plasma, contrary to the predictions of analytical hydro model calculations performed using the measured switch parameters. This discrepancy may be due to a finite lifetime for ions in the switch that is less than the conduction time resulting in a larger effective mass. From a functional point of view modified bipolar model calculations best fit the data     

Scaling microsecond-conduction-time plasma opening switch operationfrom 2 to 5 MA

We describe experiments in which conduction currents were successfully scaled from 2 to 5 MA for conduction times around 1 μs in a coaxial geometry plasma opening switch (POS) on the 4 MJ ACE 4 driver. Simple models of POS operation, derived from previous work, were used to scale successful results from drivers that operate at microsecond conduction times, but at currents below 1 MA. An MHD model for the conduction phase was verified in which the square root of the plasma density is approximately proportional to the product of conduction time and peak conduction current divided by the switch radius and length. For the opening phase, a model where the POS gap is approximately constant when the local plasma conditions at the beginning of the conduction phase are kept roughly the same is consistent with the observed POS opening voltages of at least 1 MV. The conduction current was increased by increasing the POS cathode radius from 6 to 20 cm while maintaining roughly the same plasma density. This increase in radius resulted in the observed increase in the conduction-current/conduction-time product from 2 to 5 MA μs while maintaining MV POS voltages     

MHD-to-PIC transition for modeling of conduction and opening in aplasma opening switch

The plasma opening switch (POS) is a critical element of some inductive-energy-storage pulsed-power generators. Detailed understanding of plasma redistribution and thinning during the POS conduction phase can be gained through magnetohydrodynamic fluid (MHD) simulations. As space-charge separation and kinetic effects become important late in the conduction phase (beginning of the opening phase), MHD methods become invalid and particle-in-cell (PIC) methods should be used. In this paper, the applicability of MHD techniques is extended into PIC-like regimes by including nonideal MHD phenomena such as the Hall effect and resistivity. The feasibility of the PIC technique is, likewise, extended into high-density, low-temperature-MHD-like regimes by using a novel numerical cooling algorithm. At an appropriate time, an MHD-to-PIC transition must be accomplished in order to accurately simulate the POS opening phase. The mechanics for converting MHD output into PIC input are introduced, as are the transition criteria determining when to perform this conversion. To establish these transition criteria, side-by-side MHD and PIC simulations are presented and compared. These separate simulations are then complemented by a proof-of-principle MHD-to-PIC transition, thereby demonstrating this MHD-to-PIC technique as a potentially viable tool for the simulation of POS plasmas. Practical limitations of the MHD-to-PIC transition method and applicability of the transition criteria to hybrid fluid-kinetic simulations are discussed     

100 kA微秒导通时间等离子体断路开关研究

 研制了最大导通电流约为100 kA的微秒导通时间等离子体断路开关,开展了该导通电流下的等离子体断路开关性能实验,得到的负载电流上升时间为54-76 ns,最高开关电压为1.38 MV,最高电压倍增系数达到4.9。建立了导通阶段开关区等离子体运动的二维雪耙模型,初步数值模拟结果表明,该模型对目前开展的实验有较好的预估能力。     

Study of the effects of the prefilled-plasma parameters on theoperation of a short-conduction plasma opening switch

Spectroscopic methods are used to determine the density, the temperature, the composition, the injection velocity, and the azimuthal uniformity of the flashboard-produced prefilled plasma in an 85-ns, 200-kA plasma opening switch (POS). The electron density is found to be an order of magnitude higher than that obtained by charge collectors, which are commonly used to determine the density in such POSs, suggesting that the density in short conduction POS's is significantly higher than is usually assumed. We also find that the plasma is mainly composed of protons. The spectroscopically measured plasma parameters are used here to calculate the conduction currents at the time of the opening predicted by various theoretical models for the POS operation. Comparison of these calculated currents to the measured currents indicates that the plasma behavior during conduction is governed either by plasma pushing or by magnetic-field penetration and less by sheath widening near the cathode, as described by existing models. Also, the conduction current mainly depends an the prefilled electron density and less on the plasma flux, which is inconsistent with the predictions of the erosion (four-phase) model for the switch operation. Another finding is that a better azimuthal uniformity of the prefilled plasma density shortens the load-current rise time     

Plasma Opening Switch Development for the Particle Beam Fusion Accelerator II (PBFA II)

We have conducted plasma opening switch (POS) experiments on Sandia National Laboratories' new Particle Beam Fusion Accelerator II (PBFA II) (12 MV, 100 TW, 50 ns), on the Supermite accelerator (2 MV, 2 TW, 50 ns) and on the Naval Research Laboratory's Gamble II accelerator (1.8 MV, 1.6 TW, 70 ns). The POS systems on the PBFA II and Supermite accelerators use a newly developed flashboard plasma source to provide the plasma necessary to conduct the large (> 1 MA) currents produced by these accelerators. In the Supermite experiments, the plasma opening switch conducted currents of up to 1 MA before opening in less than 10 ns into an electron beam load. These experiments achieved significant voltage gain relative to the voltage across a matched load. In experiments on Gamble II, power gains of up to 1.7 were achieved using a POS in a strongly coaxial geometry (router/rinner = 2) with a large magnetic field at the cathode. The POS system on PBFA II is unique because of its size and voltage. This POS system is designed to conduct over 6 MA before opening. In present experiments it has conducted currents of 4-5 MA for over 50 ns.     

不同驱动电流和电极尺寸下的等离子体断路开关性能

 开展了驱动电流为45,75和105 kA以及阴极直径分别为Φ20 mm和Φ40 mm下的等离子体断路开关性能实验研究。结果表明：随着发生器驱动电流增加,负载电流上升时间逐渐减小,最高电压倍增系数逐渐增加。与阴极直径为Φ20 mm的等离子体开关相比,阴极直径为Φ40 mm的等离子体开关导通时间和负载电流上升时间增加,开关电压和电流转换效率降低。实验获得的最高电压倍增系数和电流转换效率分别为4.9和97%,负载电流上升时间小于100 ns。     

Energetic electron and ion beam generation in plasma openingswitches

In this paper, we present measurements of ion and electron flows in a nanosecond plasma opening switch (NPOS) and a microsecond plasma opening switch (MPOS), performed using charge collectors. In both experiments, an electron flow toward the anode, followed by an ion flow, were observed to propagate downstream toward the load side of the plasma during the plasma opening switch (POS) conduction. In the MPOS, ion acceleration was observed to propagate axially through the entire plasma. These results are in satisfactory agreement with the predictions of the electron magnetohydrodynamics (EHMD) theory and the results of fluid and particle-in-cell (PIC) code simulations. At the beginning of the POS opening, a high-current density (≈2 kA/cm²) short-duration (10-30 ns) axial ion flow downstream toward the load was observed in both experiments, with an electron beam in front of it. These ions are accelerated at the load side of the plasma and are accompanied by comoving electrons. In the NPOS, the ion energy reaches 1.35 MeV, whereas in the MPOS, the ion energy does not exceed 100 keV. We suggest that in the NPOS the dominant mechanism for the axial ion acceleration is collective acceleration by the space charge of the electron beam, while in the MPOS, axial ion acceleration is probably governed by the Hall field in the current carrying plasma     

微秒级导通时间等离子体断路开关的二维雪耙模型

 给出了适合微秒级导通时间等离子体断路开关的二维雪耙模型的具体描述，建立了二维雪耙模型的基本方程及其差分格式，对方程进行了显式求解。通过对一个模型的计算，给出了雪耙阵面的传播图像，指出了断路开关的断开位置及附近点密度随时间的变化曲线，在该位置附近出现了等离子体薄化现象。     

等离子体断路开关和电感负载间的功率流特性

利用PIC (particle-in-cell) 方法,结合实验装置的几何结构和实验结果,采用动态开关模型,对微秒等离子体断路开关和电感负载间的功率流特性进行了研究。模拟得到了与实验结果符合较好的开关电压和负载电流波形,并给出了开关下游出现的稀薄等离子体的密度(约1012 cm-3)和速度(约1 cm/ns), 同时也得到了开关下游的空间电流分布。模拟结果表明,开关下游的结构应避免阻抗突变以减少电流损失,同时提高开关阻抗可有利于提高负载上的最大功率。     

Plasma opening switch research for DECADE

Plasma opening switch (POS) research for the DECADE radiation effects test facility is reviewed. This research was first performed on a half-scale generator, DECADE Prototype Module 1, and indicated the importance of the POS electrode geometry to obtain the required impedance at appropriate conduction times. It also was demonstrated that the conduction time jitter was suitable for a multimodule system. Preliminary experiments at full energy were then conducted on DECADE Module 1 that indicated significant current loss when using a bremsstrahlung load, limiting the output radiation to about half of the DECADE requirement. These results initiated an effort to thoroughly diagnose the power-flow region downstream of the switch, an effort that ultimately provided improved understanding of POS operation and improved coupling to bremsstrahlung loads. At the conclusion of this effort, it had been demonstrated that it is possible for the POS-driven system to meet the DECADE requirements     

Experimental investigation on a long-conduction time-plasma openingswitch and its application on a 0.1 TW generator for prepulsesuppression

The authors discuss their work on plasma opening switches (POSs). After characterization of the plasma created by plasma guns, physical processes occurring during long-conduction-time POS operation are investigated using diagnostics such as Rogowski coils, Faraday cups, and time-integrated and streak photographs. The importance of different parameters of the authors' system, such as the generator charging voltage, the geometry of the switch, and the plasma density, are studied. The authors' results are correlated with two theoretical models. The experiment shows that the use of an 80% transparent inner electrode can significantly increase the opening speed. The POS is applied in a short-conduction-time experiment. Results obtained in upgrading of a 0.1-TW generator (2 Ω, ~250 kA, ~50 ns) are presented. Prepulse suppression is achieved with a short-circuit load and with an aluminum vapor Z-pinch as a load. Incorporation of the POS improves the compression of the Z-pinch and increases the radiation yield     

Plasma Erosion Opening Switch Operation at Long Conduction Times

The plasma erosion opening switch (PEOS) can conduct large (~megamperes) currents for several tens of nanoseconds before opening in < 10 ns, generating megavolt-level voltages in the process. In the present experiment, the conduction time of the PEOS has been extended by almost an order of magnitude to several hundred nanoseconds. The dependence of the peak conduction current on PEOS parameters and the results of magnetic probe and load voltage measurements are all consistent with PEOS theory. These results indicate that the PEOS operating mechanisms at these long conduction times are the same as those operating in previous experiments at shorter conduction times. Translation of the switch plasma into the load region, due to j? × B? forces during the conduction phase, was not observed in this experiment.     

“强光一号”等离子体开关及负载稳定性分析

"强光一号"等离子体断路开关(POS)及负载二极管系统工作性能不够稳定,通过分析数据指出POS等离子体源参数差异性是导致系统不稳定的主要原因。POS等离子体源参数重复性测量结果表明,在开关断路时刻等离子体源瞬时发射等离子体密度重复性极差在10%左右,而开关区间累积等离子体密度极差超过100%。开关区间累积等离子体密度和阴极重粒子发射会对POS断路性能产生显著影响。计算表明开关区间累积等离子体密度差异对POS断路电流阈值影响达到200kA,与运行数据统计结果一致;在断路电流阈值相同的条件下,阴极物质逸出对二极管电压影响显著,MCNP程序计算结果表明,产生辐射剂量差别可以达到80%,与统计数据相当。     

Study of repetitive plasma opening switch generator technology

Repetitive plasma opening switch (POS) research for X-ray and electron beam generators for commercial applications is reviewed. This research was started on the RS-20 generator in 1991 with submicrosecond conduction times, 100 kA currents, and MV voltages. In the experiments wall-plug to electron beam production efficiency was increased from 10-50%. Maximum voltage obtained at POS was 3 MV starting from 0.8 MV at the Marx generator. POS operation was improved using an applied external magnetic field for the opening phase. Diode current and total generator efficiency were increased by combining the POS and diode system in one technology unit. For the opening phase of the high-impedance, repetitive POS, a set of conditions based on early POS experiments in the Kurchatov Institute were proposed. Repetitive POS generators have obtained peak X-ray dose rates of 1 MGy/s and electron dose rates of 10 GGy/s. This may be important for applications as pulsed irradiation may cause more efficient sterilization at lower doses. An X-ray generator based on repetitive POS technology at 5 MV and 200 kW in the electron beam is proposed     

Monitoring Alfvén cascades with interferometry on the JET Tokamak

A microwave interferometry technique is applied for the first time for detecting a discrete spectrum of Alfvén cascade (AC) eigenmodes excited with fast ions in reversed magnetic shear plasmas of the Joint European Torus. The interferometry measurements of plasma density perturbations associated with ACs show an unprecedented frequency and time resolution superior to that obtained with external magnetic coils. The measurements of ACs are used for monitoring the evolution of the safety factor and density of rational magnetic surfaces in the region of maximum plasma current.     

Investigation of plasma opening switch conduction and openingmechanisms

Plasma opening switch techniques have been developed for pulsed power applications to exploit the advantages of electrical energy storage in a vacuum inductor compared to conventional, capacitive-based energy storage. Experiments are described that demonstrate the successful application of these techniques in conduction time ranges from 50 ns to over 1 μs. Physics understanding of the conduction and opening mechanisms is far from complete; however, many insights have been gained from experiments and theory. Measurements of current distribution, plasma density, and ion emission indicate that conduction and opening mechanisms differ for the 50 ns and 1 μs conduction times. For the 50 ns conduction time case, switching begins at a current level close to the bipolar emission limit, and opening could occur primarily by erosion. In the 1 μs conduction time case, limited hydrodynamic plasma displacement implies far higher plasma density than is required by the bipolar emission limit. Magnetic pressure is required to augment erosion to generate the switch gap inferred from experiments