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     

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.     

A Self-Similar Model for Conduction in the Plasma Erosion Opening Switch

The conduction phase of the plasma erosion opening switch (PEOS) is characterized by combining a 1-D fluid model for plasma hydrodynamics, Maxwell's equations, and a 2-D electron-orbit analysis. A self-similar approximation for the plasma and field variables permits analytic expressions for their space and time variations to be derived. It is shown that a combination of axial MHD compression and magnetic insulation of high-energy electrons emitted from the switch cathode can control the character of switch conduction. The analysis highlights the need to include additional phenomena for accurate fluid modeling of PEOS conduction.     

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     

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。     

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.     

Interferometry of flashboard and cable-gun plasma opening switcheson Hawk

Interferometry of plasma opening switch (POS) plasmas on the Hawk generator has shown many important features of the plasma evolution during conduction and opening. Opening occurs when a low-density region forms at a radial location determined by plasma redistribution during the conduction phase, consistent with J×B forces and the measured plasma distributions produced by the sources alone. High neutral densities have been detected in the POS region during conduction. Low-density plasma appears between the POS and the load at the time current appears in the load, and high-density plasmas appear there later in time. There are two important differences between the density evolution of POS's utilizing flashboard and cable-gun plasma sources. 1) There is a substantial (two-three times) increase in the electron inventory during conduction using cable guns that is not detected using flashboards. This is attributed, primarily, to ionization of ions and neutrals for the cable-gun case. 2) The conduction scaling with plasma density implies that the cable-gun POS has an effective ion mass/charge ratio about double that for the flashboard POS     

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.     

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     

高密度等离子体融断开关融蚀现象的粒子模拟研究

 利用自行研制的2.5维全电磁柱坐标粒子模拟程序对高密度等离子体融断开关融断区域中的融蚀现象进行了模拟研究,详细地介绍了计算模型的建立以及复杂边界的算法处理。模拟结果表明在融断开关导通电流的最后阶段,由于磁压力、磁场渗透作用和非中性静电融蚀作用,在融断区域的阴极附近会形成一定宽度的真空鞘层。由于等离子体密度的下降以及初始真空鞘层的存在,使得即使只有较小的离子电流,融蚀机制也完全可以导致PEOS最终断开。     

Experimental investigation of the conduction phase of themicrosecond plasma opening switch

Results of experimental studies of the conduction phase of the microsecond plasma opening switch are presented. It is shown that during this phase, translation of the current channel in the plasma near the anode takes place with anomalously high velocity. The ion component of the current reaches 25-30% of the total value, and the current streamlines late in the conduction phase acquire a considerable slant in the axial direction. The ion current behind the current channel makes up more than 30% of the total current there. The ion current density reaches a maximum during the conduction phase and decreases slowly during switching     

长导通等离子体开关断路过程的粒子模拟

 利用PIC(particle-in-cell) 粒子模拟方法对长导通等离子体断路开关的断路过程进行了模拟研究。介绍了计算模型的建立和边界的处理。模拟结果揭示了断路间隙的形成过程和机制，并据此对等离子体断路开关断路阶段的现有模型进行了定性的修正，认为断路过程中Hall融蚀和静电融蚀机制将同时存在，静电融蚀在开关最终的断路中占主导作用，且静电融蚀是由于磁场排斥引起的。     

低密度等离子体融断开关的粒子模拟研究

 采用2.5维柱坐标粒子模拟程序研究了低密度等离子体融断开关（PEOS）工作过程中的物理现象,介绍了计算模型的建立和复杂边界的算法处理。模拟结果表明,在PEOS导通电流的过程中,电流通道最初在等离子体的发生器端形成,并且随着导通时间的增大而向负载端漂移。离子的空间分布并没有明显的变化,当PEOS发生断路时,等离子体离子的密度会迅速降低,并最终导致PEOS阴极附近的等离子体的密度已接近为零,此时,阴极电子完全受磁场箍缩作用而不能到达阳极,PEOS完全断开。     

Experimental Investigation of Voltage Scaling in a Plasma Switch

Russian Physics Journal - Voltage scaling in a plasma opening switch is theoretically generalized depending on the conduction switch current. The scaling has been verified experimentally using the...     

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。     

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     

A long conduction time compact torus plasma opening switch

Experiments to form and accelerate compact toroid (CT) plasmas have been performed on the 0.4-MJ Shiva Star fast capacitor bank at Phillips Laboratory. Theoretical investigations of employing a CT as a very fast opening switch are reported. A particular axisymmetric, geometrically complex switch design is studied with the help of 2-1/2-dimensional magnetohydrodynamic computer simulations. This design, called a magnetically-confined-plasma opening switch (McPOS), accumulates magnetic energy in an inductive store. Because of its intrinsic stability, the switch can conduct current for ten or more microseconds and can open in less than 100 ns-substantially less than the risetime of the capacitively produced electric current. A long conduction time compact torus plasma opening switch     

High-Power Opening-Closing Switches Using Grid-Controlled Plasmas

Two switches are described with the capability of rapidly interrupting high-power circuits: a vacuum triode with a large-area plasma cathode, and a grid-controlled plasma conduction switch. Theoretical models for the vacuum triode imply that it could control voltages in the range ?100 kV at current density ?2 × 104 A/m2. The vacuum switch has the advantage of rapid switching at the expense of reduced efficiency because of its significant anode-cathode voltage drop. In contrast, the plasma switch has almost zero voltage in the conducting state. The theoretical models presented indicate that the plasma switch could conduct current densities in the range 10 × 104 A/m2 with open-circuit voltage ? 100 kV. Although the closing time is long (~1 ?s), the predicted opening time is short (~20 ns). Initial experiments demonstrating the principle of operation of the plasma switch are reported.     

Concerning microsecond megaampere-current plasma opening switches

The operation mechanism of a microsecond megaampere-current plasma opening switch is considered. The magnetic field penetrates into the plasma via near-electrode diffusion. The increase in the degree of plasma magnetization due to electron heating results in an increase in plasma resistivity and current break. The problem of calculating a plasma opening switch is mathematically formulated. The problem reduces to simultaneously solving one-fluid two-temperature MHD equations with allowance for the Hall current and two-dimensional electric circuit equations. To analyze the solution obtained, one-dimensional equations are derived based on the assumption that the size of the electrode region in which the plasma is strongly magnetized is much smaller that the plasma column length. In this approximation, the operating modes of a plasma opening switch are studied numerically. On long time scales (≥2–3 μs), the operation is limited by plasma ejection from the interelectrode gap. On short time scales (≤1 μs), the dominant process is the penetration of the magnetic field with the current velocity. The results of the calculations are compared with the available experimental data. The developed concept and numerical procedure are used to optimize the scheme for an explosion experiment on breaking megaampere currents under conditions similar to those in the EMIR complex.     

Hall MHD model of a microsecond plasma opening switch and its application to experiments on a pulsed current generator installation

The necessity of considering the motion of electrons and ions in the plasma of a microsecond switch in the conduction and cutoff stages (Hall MHD model) is substantiated experimentally. We give the Hall MHD model relations that describe the main parameters of the plasma opening switch as an element of the electrical circuit of a pulsed current generator. Comparison of the deductions of the theory with the experimental results obtained in pulsed current generator (PCG) installations indicates that the theory and experiment at least are not in contradiction with each other. An improvement of the POS design from the standpoint of the Hall MHD model is proposed. Institute of High-Current Electronics, Siberian Branch of the Russian Academy of Sciences. Laboratory of the Physics of Ionized Media. école Polytechnique, France. Translated from Izvestiya Uchebnykh Zavedenii, Fizika, No. 12, pp. 56–66, December, 1997.