The Plasma Erosion Opening Switch

FAST INDUCTIVELY COUPLED SUPERCONDUCTING OPENING SWITCH TRIGGERED BY SHORT LASER PULSES

Inertial confinement fusion using light ion beams requires fast rise time, high power drivers. One technique to enhance a conventional pulsed power generator is to use a plasma erosion opening switch (PEOS) between the generator and the ion diode. Ideally, the PEOS conducts current until a current threshold is reached, then rapidly opens and delivers the inductively stored energy to the load on a short time scale. Experiments at the Naval Research Laboratory (NRL) used a PEOS to improve the Gamble II current rise time from 60 ns to 10 ns at 1 MA. Load voltages over 4 MV have been obtained (>2× the matched load voltage).Based on these experiments and theoretical predictions, a PEOS system has been incorporated into the PBFA II experiment at Sandia National Laboratories (SNL). The PEOS on PBFA II is ultimately to provide a 6 to 8-MA, 30-MV pulse with 10-ns rise time into an “applied B” ion diode. This is a large extrapolation of present experiments. The PEOS system on PBFA II is described here and theoretical predictions are given.

Vacuum inductive energy storage by plasma erosion opening switches (PEOS's) offers the possibility of enhancing pulsed power generator efficiency for bremsstrahlung production. Different laboratories, mainly the Naval Research Laboratory (NRL), had largely developed this technology some years ago. More recently, we have done experiments and numerical simulation for a 1-TW pulsed generator. We give a comparison between simulation and experiments, and we have extended this simulation to a larger number of plasma guns. Furthermore, inductive energy techniques have been investigated theoretically to heat a dense Z-pinch plasma (DZP) and to obtain a 14-MeV neutron burst. Results, using a simple heating model to treat the plasma, indicate the feasibility of these experiments if fast opening reduces instabilities.

Plasma Erosion Opening Switch (PEOS) experiments are described covering a conduction time range from 35 ns on Gamble I to 2300 ns on Pawn. A microwave interferometer is used to estimate the plasma density for some of the experimental cases. The conduction current, conduction time, geometrical configuration and density are used to calculate the plasma center-of-mass displacement from JxB forces and to compare the conduction current with the bipolar limit of PEOS theory. At short conduction times, the conduction current agrees with the bipolar limit. As the conduction time increases, the conduction current is orders-of-magnitude less than the bipolar value, possibly the result of plasma displacement and/or compression by JxB forces. Eventually, the PEOS does not open; instead, the current is convected toward the load as in a plasma flow switch. The transition to conduction current much less than bipolar occurs when the center-of-mass displacement is a significant fraction of the plasma length.

A plasma erosion opening switch (PEOS) is used as the opening switch for a vacuum inductive storage system driven by a 1.8-MV, 1.6-TW pulsed power generator. A 135-nH vacuum inductor is current charged to ∼750 kA in 50 ns through the closed PEOS which then opens in <10 ns into an inverse ion diode load. Electrical diagnostics and nuclear activations from ions accelerated in the diode yield a peak load voltage (4.25 MV) and peak load power (2.8 TW) that are 2.4 and 1.8 times greater than ideal matched load values for the same generator values.

The article presents the experimental study of discharges in a conical cavity, filled with Ar at pressure 80 Pa. The electrical current driver (inductive storage with plasma erosion opening switch) supplies to the load electrical current pulse with growth rate about 1012 A s−1 and maximal value 30–40 kA. The convergent conical shock wave starts from the inner surface of the discharge cavity and collapses in ‘zippering’ mode. The pin hole camera imaging with MCP detector (time resolution 5 ns) have demonstrated the appearance of effectively fast moving compact plasma with visible velocity v = (1.5 ± 0.14) × 107 cm s−1. Plasma emits narrow band radiation in the spectral range of Rydberg series transitions of Ar VII, Ar VIII with quantum number up to n = 9 (wavelength about 11 nm). The intensity of radiation is comparable with the total plasma emission in the range 10–50 nm. Charge exchange between multiply charged Ar ions and cold Ar atoms of working gas is proposed as the possible mechanism of the origin of the radiation.

In the present report, plasma formed on the surface of metals and dielectric materials under the action of XeCl and KrCl lasers is studied. Plasma formation thresholds and plasma expansion velocities are measured. It was shown that long-lived plasma objects are formed near a target in the atmosphere of Xe or air at pressure 0.1 - 1 atm. Duration of luminescence of the plasma objects was maximal in xenon. Non-symmetric current passing through the laser plasma and the current interruption were obtained. The interruption time was as short as 10 ns while the plasma resistance increases with the rate up to 10<SUP>10</SUP> Ohm/s. It is shown that the interruption rate increases when plasma of elements with small atomic weight is used and at higher rice-time of the current before its interruption. It was found that the plasma ionization degree is saturated before the interruption. The interruption effect was used for development of generators with inductive energy storages. Generator with a plasma erosion opening switch with opening current of 10 kA operating at pulse repetition rate up to several Hz was used for excitation of pulsed lasers. Using the generator, laser action on nitrogen and XeCl molecules and atomic transition of Ne was obtained.

Pawn is a prototype inductive storage pulsed power generator with two switching stages. The first stage is a wire fuse array driven by a low voltage (/spl ap/40 kV) capacitor bank, and the second stage is a fast opening vacuum switch such as a plasma erosion opening switch. A triggered vacuum flashover switch isolates the fuse stage from the second stage during fuse conduction. The switch consists of an insulating ring sandwiched between electrodes. Ultraviolet light and plasma from small spark discharges initiate a flashover of the insulator surface to close the switch. The trigger command is sent by fiber optic link. The switch can be command triggered after holding off voltages for /spl ap/15 /spl mu/s; peak holdoff voltage is /spl ap/85 kV. The time to closure decreases from 240 ns at a closing voltage of 15 kV to 140 ns at 50 kv with a typical jitter of /spl plusmn/50 ns. Typical fuse stage output current is /spl ap/1 MA with an average risetime of 1/spl mu/s. Current density in the switch is approximately 25 kA/cm/sup 2/. The risetime of this current pulse can be varied by a factor of two by triggering the switch at different times (closing voltages). Switch design, operation, timing jitter characteristics and multiple shot use will be discussed.

The conduction phase of the plasma erosion opening switch (PEOS) is studied using a 1½-D electromagnetic two-fluid code. The focus of this work is on understanding how two effects, a current-limiting model of electron emission, and the magnetic insulation of electrons at the cathode, determine current conduction in the plasma. Simulations are performed in the parameter regimes of the Gamble I, POP, and PBFA II pulsed power generators, and previous low-density, short-rise time simulations of the PEOS. Fluid code results are compared to a 1-D analytic theory and to the Gamble I and POP experiments. Good agreement between theory and simulation, but mixed agreement between simulation and experiment is found. Experimental B-field measurements on POP show weaker j × B compression than the simulation. Current penetration and plasma current channels qualitatively similar to experimental observation are found in the Gamble I regime. However, magnetic insulation of electrons emitted from the cathode bunches the electron flow into narrower current channels than observed experimentally. In several cases, the presence of an electron-scattering or energy-loss mechanism near the cathode must be invoked to overcome magnetic insulation and widen the current channels.

Aim. The aim of this study is to compare the performance of toroidal and solenoidal configurations of a superconducting inductive energy storage device using CORC® and racetrack cables made from high-temperature superconducting tapes. Methods. A numerical multiphysics analysis of inductive energy storage device was performed using the finite element method in the Comsol Multiphysics engineering modeling environment. Results. The analysis revealed that the CORC® cable in a solenoid configuration, with a transport current density to critical current density ratio of 0.7 at the boiling point of liquid nitrogen, was the most suitable for inductive energy storage. Conclusion. The developed numerical model allows to calculate energy capacity and energy losses in superconducting inductive energy storage devices configured as solenoids or toroids. This model can be applied to the development of inductive storage devices made from HTS composites.

Summary form only given, as follows. Plasma opening switches (POS) are the key components of inductive energy store (IES) pulsed power systems. In the closed phase an IES system stores magnetic energy in the inductance of a vacuum transmission line. As the POS opens the stored magnetic energy flows past the POS and into a load (for example an electron or ion diode). To achieve power multiplication the opening time should be much shorter than the conduction time. There are three proposed mechanisms that control the conduction phase of the POS: 1) sheath effects at the electrodes, 2) rapid penetration of magnetic field into the body of the plasma due to EMH effects, and 3) displacement and distortion of the plasma. due to J/spl times/B (MHD) forces. In this paper, the PIC code, MAGIC, is used to study EMH and MHD effects in a POS. These simulations were performed in negative polarity with a strong radial density gradient. In regimes where EMH is expected to be dominant, the simulations show rapid axial penetration of magnetic field into the plasma in a narrow radial channel along the positive density gradient. No field penetration is observed along the negative portion of the density gradient. In positive polarity, field penetration occurs along the negative gradient and no field penetration is observed along the positive density gradient. This agrees with EMH theory of shock penetration of magnetic field. However, the speed of penetration is somewhat slower than predicted by fluid treatments. The PIC simulations also show the formation of vortices in the electron flow behind the EMH shock front.

誘導性エネルギー蓄積パルスパワーシステムの高速しゃ断素子制御によるパルス整形と形成

The parameters of laser radiation and kinetic processes of plasma formation and production of inverse population are studied. Electrophysical processes proceeding in a feed circuit containing a semiconductor opening switch and intermediate inductive energy storage are also considered. It is shown that the semiconductor opening switch controls the amount of energy in the inductive storage, and a high-voltage short prepulse with a peak pump power density of ∼1 MW cm-3 appearing during the current interruption can produce a volume-discharge plasma and create the inverse population for the time ∼10 ns, providing the conditions for efficient pumping of the active medium from the main capacitor. The time dependences of the pump and emission powers, the concentrations of electrons, excited xenon atoms, HCl(v) molecules in excited vibrational states and of the ionisation, recombination, and attachment rates are calculated. Kinetic processes determining the parameters of laser radiation are analysed. The output energy up to 1.0 J is obtained for 90-ns (FWHM) pulses with the efficiency up to 4.0% with respect to the pump energy. The maximum output power was 11 MW for the lasing efficiency of 4.7% with respect to the pump power. The calculated and experimental time dependences of the voltage across the laser gap, discharge current, and output power are in good agreement.

This paper describes experiments that use a fast capacitor bank to provide data for the production of simple but accurate empirical models for exploding copper foil fuses. It is demonstrated that foils of different thicknesses behave quite differently, a phenomenon previously noted only for exploding aluminium foils. Empirical models for both 17 /spl mu/m and 25.4 /spl mu/m foils were developed from the experimental data, and used successfully in computer programs which predict in detail the behaviour of the foils in a variety of switching circuits. The computer models were used to optimise the design of an exploding foil used as an opening pulsed power switch in a capacitor driven circuit, that transfers a sharp current pulse to a plasma erosion opening switch (PEOS). It was predicted that voltages exceeding six times the initial bank voltages could be produced, and experiments are described that provide evidence for this while avoiding the possibility of damage to the bank.

An inductive energy storage switch system for the destruction of solid materials is reported. This is based on creating a pulsed electric breakdown in the solid dielectric, which then propagates in the specimen. This scheme provides a higher destruction effectiveness compared to a capacitive energy storage system. The higher energy efficiency is attributed to a different discharge behavior during the discharge build-up in the solid material. A higher applied voltage causes a breakdown of a larger number of voids in a heterogeneous solid dielectric. The energy transfer to partial discharges, when using the inductive storage system, is faster than for the capacitive energy storage system, due to a shorter risetime of the discharge current.