Plasma Opening Switch Operation Research Articles

Simulation and experimental results of plasma opening switch operation with inductive and electron beam diode loads in different plasma regimes

This paper reports the experimental results of a plasma opening switch (POS) that operates at hydrogen and carbon plasma regimes confirmed based on the conduction bipolar limit of the plasma species and the availability of plasma species during the conduction based on the density profiles. The generator is the compact capacitor bank developed using the synchronization of four field distortion spark gap switches. The plasma source is coaxial cable plasma guns characterized using the Faraday cup plasma diagnostic. The experimental results are compared with the time domain simulation code in which the POS is represented as the dynamic resistance governed by the physical equations of the bipolar model. The experiments and simulations are compared for inductive load and dynamic load of accelerating gap of 5 mm between the anode and cathode at the load end. From the results, POS can be operated based on the availability of plasma species with proper synchronized triggering between the plasma gun firing and generator firing that can operate at low and high values of currents.

Some issues of the operation of plasma opening switches

The paper analyzes some processes in a plasma opening switch with a conduction time of ~ 100 ns. Relations are derived for assessing the evolution of the ion current to its space charge-limited state and for estimating its maximum possible density. It is shown that the ion current density reaches its extremum before electron magnetization. Estimates for the electromagnetic energy delivered to the switch are also presented demonstrating that this energy is spent equally for creating a magnetic field in the plasma volume and for its ohmic heating.

PLASMA DYNAMICS IN ACCELERATOR WITH PLASMA OPENING SWITCH

Plasma propagation in the pulse electron accelerator chamber with a plasma opening switch is investigated. Output beam and X-ray radiation parameters are shown to vary depending on the plasma channel location at the moment of current breaking. Experiments showing plasma compression into channels during the conductive phase of the plasma opening switch operation have been performed.

Dynamics of plasma filling the gap of a module of the plasma opening switch of the MOL facility

The Baikal program aimed at the creation of a superhigh-power generator for experiments in the field of inertial thermonuclear fusion includes the development of a MOL facility for modeling the main principles of the generator structure. It is suggested to use a six-module plasma opening switch (POS) in an external magnetic field as the output power sharpener. The scheme of upgraded plasma guns of the module of the MOL facility’s POS and the results of studying the dynamics of filling the POS gap with plasma transversely to an external magnetic field are presented. It is shown that, in the presence of a long (∼40 μs) prepulse in this module, a POS cathode-anode closure with plasma along the force lines of the external magnetic field occurs within a time interval between the first turning-on of the plasma guns and the POS operation, which is hazardous for its operation. To eliminate this threat, it is proposed to increase the POS length to ∼70 cm. On the basis of the results obtained, a simpler and more reliable solution of the prepulse problem is proposed.

Plasma opening switch for the MOL generator

Problems related to creating a superpower pulsed current generator (Baikal project, ≈10 MV, ≈50 MA, ≈150 ns) for inertial confinement fusion are considered. In order to test the circuit design of the generator, the MOL unit (3.7 MA, 5 MV, 150 ns), in which a plasma opening switch (POS) is proposed for use as an output power sharpener, is now under construction. Results are presented from experimental testing of a number of proposals enabling the POS operation under the MOL conditions: (i) plasma guns were employed to connect the POS to the inductive storage circuit, (ii) a separating discharger was used to avoid a POS reclosure and to switch the current to a load whose impedance was initially low and increased with time, and (iii) a programmable filling of the POS gap with plasma was implemented in order for the POS to pass a long current prepulse with parameters close to those in actual systems.

Numerical study of a plasma opening switch conduction phase

Dynamics of a Plasma Opening Switch (POS) during a conduction phase is numerically studied within the frame of a two-dimensional two-fluid system of magnetohydrodynamic (MHD) equations. The initial conditions for simulations are chosen to ensure the fast magnetic field propagation to a load plasma edge near the cathode. Simulations demonstrate that the POS dynamics depends on two plasma parameters: number of ions near the cathode and electron Hall parameter. It is shown that with respect to these parameters one of the three following regimes of POS operation can occur: dominant Hall effect, dominant MHD effects, and intermediate regime with equal influence of Hall and MHD effects. Two sets of simulations are carried out to derive the numerical scalings for conduction current and for conduction time of each of these regimes. The numerical scalings correlate with scalings proposed for the HAWK generator.

Design of a command-triggered plasma opening switch for terawatt applications

Inductive energy storage systems can have high-energy density, leading to smaller, less-expensive systems. The crucial element of an inductive energy storage system is the opening switch. This switch must conduct current while energy is stored in an inductor, then open quickly to transfer this energy to a load. Plasma can perform this function. The plasma opening switch (POS) has been studied for more than two decades. Though advances have been made, success with the conventional plasma opening switch has been limited. The plasma opening switch concept described here has the potential to increase the power achieved with inductive energy storage pulsed-power systems. This system is also command-triggered. Major improvements in performance and command-triggering both require control of the plasma conductor. This paper shows the design of a system that uses magnetic fields to control the plasma. An externally applied magnetic field clears plasma from the transmission line, For maximum power, efficiency, and accurate pulse shaping, the plasma must be cleared from most of the transmission line conductor spacing. The goal of the system described here is to clear more than 60% of the gap. This is much more than present performance levels. This paper discusses the operation of plasma opening switches, and the ways this switch design overcomes some of the limitations. Because of the importance of magnetic field penetration into the plasma, a model for field penetration is presented. The design of a terawatt-level system is shown, and the mechanism to reduce the trigger power is described. Particle-in-cell simulations of the triggered opening are shown.

Microsecond conduction time POS experiments

The plasma opening switch operation (POS) was studied by optical diagnostics, such as laser interferometry and spectroscopy. The characterization of the plasma source (cable plasma gun) allowed us to determine initial plasma conditions for the switch. The process of the vacuum gap formation in a microsecond plasma opening switch was also investigated. Time- and spatially-resolved density measurements are performed in the POS, showing the density drop during switch-opening almost in the whole interelectrode gap. The density diagrams are compared to major switch characteristics, such as conduction time and load current d I/d t. The study of spectral line intensities from the cable gun plasma showed that CII line intensity corresponds in time with plasma density measured by an interferometer. The intensities of carbon spectral lines tend to drop during switch-opening that substantiates interferometry measurements.

Observation of nonthermal turbulent electric fields in a nanosecond plasma opening switch experiment

Nonthermal turbulent electric fields due to plasma instabilities were studied in a 100-ns duration plasma opening switch using observations of hydrogen line spectral profiles. The ${\mathrm{H}}_{\ensuremath{\alpha}}$ and ${\mathrm{H}}_{\ensuremath{\beta}}$ widths were seen to rise by 2--3 times during the current pulse, shown to result from the presence of nonthermal electric fields in the plasma. The spectral profiles are analyzed using two recently developed methods based on short and intermediate time behaviors of the line profile Fourier transforms. One method gives the mean amplitude of the nonthermal fields with no dependence on their frequencies. The second method uses calculations of the autocorrelation functions for various field amplitudes and frequencies to yield bounds on these two parameters. The field amplitude is determined to be 14.5\ifmmode\pm\else\textpm\fi{}2.5 kV/cm, and the fluctuation frequency is found to be of the order of the electron plasma frequency. Based on their high frequency, the oscillations probably result from Langmuir waves, driven by the voltage drop on the plasma opening switch (POS). The waves have no significant effect on the POS operation, since they do not give rise to anomalous resistivity, and therefore have no effect on the magnetic-field evolution. We obtain an upper limit for the amplitude of possible low-frequency fields (ion-acoustic waves), that may give rise to anomalous resistivity, and estimate the resulting diffusion velocity and current channel width. Both quantities are found to be much lower than the values observed in the experiment, and the low-frequency field amplitude is much lower than the saturation limit predicted by previous theoretical treatments. This implies that in our experiment possible low-frequency waves have little influence on the magnetic-field distribution.

Study of the effects of the prefilled-plasma parameters on the operation 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 Using Plasmas Produced by a Pulsed CO2 Laser

Plasma opening switches were studied using laser-produced plasmas. In this paper, a CO2 laser (5 J/pulse, 5.5 MW peak power and 10.6 µm wavelength) was used as the plasma source of a plasma opening switch (POS). The plasma parameters were measured using a Faraday cup, a Langmuir probe and a monochromator. It was found that ions with fast flow velocity are required to obtain a short opening time, and thermalized plasma with low velocity lengthens the opening time. However, POS operation was improved by switch modification despite the existence of thermalized plasmas.

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.

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

We describe experiments in which conduction currents were successfully scaled from 2 to 5 MA for conduction times around 1 /spl mu/s 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 /spl mu/s while maintaining MV POS voltages.

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.

Observations and implications of gap closure in plasma opening switch operation

Plasma opening switches are believed to develop voltage through opening and magnetically insulating a gap formed in plasma injected between two conducting electrodes. Data will be presented to show that a relatively simple assumption about the behavior of this gap suffices to describe the overall operation of the switch. Specifically, the observed increase, then decrease, of operational voltage as a function of conduction time can be accounted for by the switch gap linearly decreasing with conduction time, while the conducted current increases. In addition to presentation of data to demonstrate this collapse, analysis is performed to couple the gap history to the voltage predicted for the switch, and to optimize this latter quantity.

Improved performance of a plasma opening switch using a novel anode configuration

The performance of a plasma opening switch (POS), as measured by the peak opening voltage developed, has been improved from /spl ap/1.2 to /spl ap/2.3 MV through a simple modification of the anode electrode. This modification was motivated by observation of electron damage to the anode in the POS region, and consisted of the removal of any metallic surfaces that exhibited significant damage and were not essential to the integrity of the switch. Because present analytic theories and PIC simulations of the switch have not yet addressed the effect modifications to the anode may have to the opening phase in depth, it is hoped that in addition to providing a technique for improving POS operation, the work reported here may also provide incentive for further theoretical investigation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Characterization of a microsecond-conduction-time plasma opening switch

This paper presents data and analyses from which emerges a physical picture of microsecond-conduction-time plasma opening switch operation. During conduction, a broad current channel penetrates axially through the plasma, moving it toward the load. Opening occurs when the current channel reaches the load end of the plasma, far from the load. During conduction, the axial line density in the interelectrode region is reduced from its value with no current conduction as a result of radial hydrodynamic forces associated with the current channel. A factor of 20 reduction is observed at opening in a small, localized region between the electrodes. When open, the switch plasma behaves like a section of magnetically insulated transmission line with an effective gap of 2 to 3 mm. Increasing the magnetic field in this gap by 50% results in an improvement of 50% in the peak load voltage and load current rise time, to 1.2 MV and 20 nsec, respectively. An erosion opening mechanism explains the inferred gap growth rate using the reduced line density at opening. Improved switch performance results when the maximum gap size is increased by using a rising load impedance.

Plasma opening switch studies of an applied B z ion diode

The light ion accelerator (1.5 MV, 4 Ω) at Cornell University is being used to study the characteristics of an applied Bz , or ‘‘barrel,’’ diode. We report here the results of a series of experiments utilizing a plasma opening switch. With a magnetically insulated ion diode load, the peak diode voltage increased from 1.5 to 1.8 MV and the ion power increased from 50 to 80 GW when a plasma opening switch was used. Our results are compared with existing theories of plasma opening switch operation.