Yakov E. Krasik

Ferroelectric plasma sources and their applications

We review our experimental studies of ferroelectric plasma sources and their applications. Using various diagnostics, it was shown that the source of the charged particle emission from the ferroelectric is surface plasma. This plasma is formed as a result of an incomplete discharge on the surface of the ferroelectric sample. The process of the plasma formation is accompanied by desorption of neutrals from the ferroelectric surface. It was shown that the parameters of the plasma and the neutral flow strongly depend on the polarization state of the ferroelectric material and on the parameters of the driving pulse. The lifetime of ferroelectric plasma sources was also studied. Electron beams with current amplitude of a few kiloamperes were generated with rep-rate up to 10 Hz under the application of accelerating pulses with amplitudes of 25-250 kV. Operation of the electron diode with and without plasma prefilling was demonstrated. Data concerning the uniformity of the extracted electron beam and the potential distribution in the diode are presented. In addition, we present data concerning an enhanced emission mode of the ferroelectric cathode and its application as a promising source of heavy ions. Results of applications of ferroelectric plasma sources in low-pressure high-current hollow-cathode discharge, as cathodes in relativistic magnetrons, as high-current switches and for generation of high-frequency modulated electron beams are presented as well.

Investigations of a Double-Gap Vircator at Submicrosecond Pulse Durations

The results of investigations of a double-gap vircator driven by a 20 Ω and 500-ns generator operating in the output voltage range 400-600 kV are presented. The vircator generated microwave pulses with a peak power of up to 200 MW at ~5% efficiency and the frequency varied from 2.0 to 2.3 GHz depending on the cavity geometry. The limitations on the microwave pulse duration not related to the cathode plasma expansion are addressed. On the one hand, the microwave generation is terminated because of the plasma formation at the foil separating the cavity sections, so that the virtual cathode (VC) electron space charge is neutralized by the plasma ion flux. On the other hand, if the electron beam energy deposition into the foil is reduced, a substantial delay in the start time of the microwave generation appears, which has been studied in detail. With these limiting factors, the microwave pulse full duration varied from 100 to 350 ns; the maximal full width at half maximum duration achieved in the experiments was ~180 ns. Measurements of the current transmitted through the vircator cavity indicated the existence of a VC in spite of the absence of microwave generation during the delay. The experimental dependence of the microwave generation starting current on the diode voltage is presented, and possible mechanisms behind the generation delay are discussed. Simplified numerical simulations emphasize the role of the portion of electrons that are reflected from the VC, the number of which must be sufficient for the microwave generation to occur.

Plasma Emission Sources for High-Current Electron Beam Generation

Summary form given only. Main results of recent experimental research of passive and active plasma sources for high-current electron beam generation obtained during the last years in the Plasma & Pulsed Power Laboratory will be reported. We will describe passive plasma sources (velvet cathode, slab porous dielectric) based on a flashover plasma and active plasma sources based on a ferroelectric plasma source (FPS) as well on an FPS-assisted hollow anode plasma source. Different time and space-resolved electrical, optical and spectroscopic diagnostics used in these experiments will be described as well. The main data concerning the plasma parameters (plasma density and temperature, plasma uniformity and plasma potential) and the main features of these plasma sources (plasma formation, life time, vacuum compatibility) will be discussed. Also we will discuss the obtained data concerning electron diode operation and parameters of the generated electron beam while using these plasma sources.

Underwater Electrical Explosion of Wires and Wire Arrays and Generation of Converging Shock Waves

A brief review of the results obtained in recent research of underwater electrical explosions of wires and wire arrays using microsecond-, submicrosecond-, and nanosecond-timescale high-current generators is presented. In a microsecond-timescale wire explosion, good agreement was attained between the results of experiments and the results of magnetohydrodynamic calculations coupled with equations of state (EOS) and modern conductivity models. Conversely, in a nanosecond-timescale wire explosion, the wire resistance and the EOS were modified in order to fit experimental data. In experiments with cylindrical and spherical wire arrays, generation of a converging shock wave (SW) was demonstrated allowing formation of an extreme state of water in the vicinity of either the axis or the origin of the SWs implosion. In addition, it is shown that SW convergence in superspherical geometry allows one to achieve larger values of pressure, density, and temperature of water in the vicinity of the axis of convergence than in the case of a spherical implosion. The results of experiments and numerical analysis showed that a cylindrical SW keeps its symmetry along the main path of its convergence. In addition, it is shown that underwater electrical explosion of an X-pinch wire configuration and a cone wire array allows one to generate fast jets of metal and water, respectively, without using chemical explosions.

Characterization of Different Wire Configurations in Underwater Electrical Explosion

The results of a study of shock wave (SW) generation by means of underwater electrical wire explosion with different exploding wire configurations and two high-current microsecond and submicrosecond timescale generators are presented. By using aperiodical generator discharge, a ~85% and ~15% of the stored electrical energy was transferred to the exploding wire and energy of the generated water flow, respectively. The energy of the water flow is distributed between its internal (~25%) and kinetic (~75%) energies. It was shown that the exploding wire zigzag configuration, confinement of the SW propagation region, and an increase in the rate of the energy deposition into the exploding wire allow one to increase the SW pressure ges10 times that attained with microsecond timescale straight wire explosion. The averaged thermophysical properties of nonideal and weakly degenerated plasma formed as a result of the wire explosion were obtained and summarized.

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 POSs 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.

Study of electron diodes with a ferroelectric plasma cathode

We present results of studies of time- and space-resolved energy distributions of electrons and ions emitted from the plasma formed on the surface of poled and unpoled ferroelectric samples. Results of lifetime tests of different ferroelectric cathodes are also described. We studied the operation of a planar electron diode and a relativistic magnetron, both with a ferroelectric cathode under the application of a high-voltage pulse of /spl les/300 kV with a repetition rate of /spl les/5 Hz. In the planar diode, the energy of electrons, the uniformity of the extracted electron beam, and the potential distribution were studied. The obtained experimental data agree well with the model of plasma formation on the surface of the ferroelectric. Successful repetition rate operation of the magnetron with a PZT ferroelectric cathode was demonstrated.

Effects of Different Cathode Materials on Submicrosecond Double-Gap Vircator Operation

The operation of a double-gap S-band vircator has been investigated at submicrosecond duration of a high-current electron beam generated in a planar diode. The experiments were performed at accelerating voltages of les550 kV and diode currents of up to 17 kA using a radio-frequency cavity with a wide coupling window between its two sections. Three types of cathodes have been studied, namely, metal-dielectric, carbon fiber, and velvet cathodes. The main features of the operation of the vircator using each cathode are analyzed. The microwave pulse duration with the metal-dielectric and carbon fiber cathodes reached ~250 ns at the peak power level of ~100 MW; with the velvet cathode, a duration of ~400 ns was achieved. It has been found that, in addition to the common limitations of the microwave pulse duration related to the dynamics of the diode impedance governed by the cathode plasma expansion, there is another factor, namely, the anode-cathode gap, which determines the delay at the beginning of the microwave generation. The latter effect is explained by the role of electrons oscillating between the virtual and real cathodes in the generation process. The issue of radiated microwave frequency behavior is discussed as well.

Characterization of Deposited Films and the Electron Beam Generated in the Pulsed Plasma Deposition Gun

The channel spark discharge was used as a high-current density (up to 30 kA/cm2) relatively low-energy (<20 keV) electron beam source in a pulsed plasma deposition (PPD) gun. The PPD gun was used for the deposition of thin films by pulsed ablation of different target materials, at a background gas pressure in the 10-3–10-5 Torr range. The parameters of the electron beam generated in the modified PPD gun were studied using electrical, optical, and X-ray diagnostics. It was found that a higher background pressure stimulates a denser plasma formation between the gun output and the target, that restricts the energy delivery to the beam electrons. Namely, the efficient (up to ~74%) transfer of the initially stored energy to the electron beam is realized at the background gas pressure of 10-4 Torr. Conversely, at a pressure of 10-3 Torr, only ≤10% of the stored energy is acquired by the energetic electrons. It was shown that the modified PPD gun, owing to the extremely high energy density delivered by the electrons to the target, may be applied for the deposition of a wide variety of different insulators, semiconductors, and metals. A selection of materials such as diamond-like carbon (DLC), cadmium telluride (CdTe), cadmium sulphide (CdS), zinc oxide (ZnO), tungsten, and tungsten carbide (WC) have been deposited as thin films and the properties and deposition rates of the deposited thin films are discussed.

Stabilization of the Frequency of Relativistic S-Band Magnetron With Radial Output

The results of experimental research on the stabilization and tuning of the frequency of a relativistic S-band six-resonator magnetron powered by a linear induction accelerator ( U ≈ 300 κB, I ≈ 2.5 kA, and τ ≈ 150 ns) are presented. The frequency of the microwaves was stabilized using a partial reflection of the generated microwave power P ≈ 300 MW from the output of the magnetron and a proper adjustment of the phase of the reflected microwaves.