J. Felsteiner

Emission properties of different cathodes at E⩽105 V/cm

We present results of the investigation of different types of cathodes operating in an electron diode powered by a high-voltage generator (300 kV, 250 ns, 84 Ω, ⩽5 Hz). The cathodes which have the same emitting area of 100 cm2 are made of metal–ceramic, carbon fibers, carbon fabric, velvet, or corduroy. We also tested carbon fibers and carbon fabric cathodes coated by CsI. It was shown that for all types of cathodes the electron emission occurs from the plasma which is formed as a result of a flashover of separate emitting centers. The amount of the emitting centers and the time delay in the electron emission were found to depend strongly on the accelerating electric field growth rate. Experimental data concerning the uniformity of the light emission from the cathode surface and divergence of the generated electron beams are presented. Data related to the general parameters of the diode, namely its impedance, power, and energy are given as well. For all the cathodes investigated the observed diode impedan...

TOTAL ABSORPTION OF AN ELECTROMAGNETIC WAVE BY AN OVERDENSE PLASMA

We show both theoretically and experimentally that an electromagnetic wave can be totally absorbed by an overdense plasma when a subwavelength diffraction grating is placed in front of the plasma surface. The absorption is due to dissipation of surface plasma waves (plasmons polaritons) that have been resonantly excited by the evanescent component of the diffracted electromagnetic wave. The developed theoretical model allows one to determine the conditions for the total absorption.

Electron/ion emission from the plasma formed on the surface of ferroelectrics. I. Studies of plasma parameters without applying an extracting voltage

We present experimental results of plasma formation on the surface of ferroelectric samples. Different poled and unpoled ferroelectric samples having a disk or tube form and made of Pb(Zr, Ti)O3 or BaTiO3 were tested. Using fast framing photography and different electric probes it was found that the application of a high-voltage driving pulse to the ferroelectric sample causes a fast surface plasma formation. This plasma formation occurs within a few nanoseconds from the start of the driving pulse for all the tested ferroelectric samples and the methods of applying the driving pulse. It was found that reversing the polarization of a ferroelectric does not play a significant role in the process of the plasma formation. Parameters of the plasma and of the neutral flow formed during the plasma formation versus the polarity and the amplitude of the driving pulse are presented.

Formation of discharge plasma on the surface of cathodes with different dielectric constants

We present a detailed study of the spatial and temporal evolution of visible light emission from plasma formed on the surface of ferroelectrics [Pb(Zr,Ti)O3 and BaTiO3] and a printed-circuit board under the application of a high-voltage pulse. For all samples studied, plasma formation occurs within the first 5 ns from the start of the high-voltage pulse. It is shown that the plasma appears at the edges of the front strip electrodes. Further, the plasma spreads along the dielectric surface covering it partially. It is found that the uniformity of the plasma and the distance of its propagation along the surface depend on the properties of the dielectric, the polarity and amplitude of the high-voltage pulse, and the vacuum conditions. A qualitative model which explains the observed experimental data is suggested.

Underwater electrical explosion of a Cu wire

Results and analysis of a microsecond time scale underwater electrical wire explosion are presented. Experiments were carried out with a Cu wire exploded by a current pulse ⩽100kA with microsecond time duration. The analysis is based on shadow and spectrally resolved streak photography which were used to monitor the evolution of the discharge channel and the shock wave. The obtained data were used for hydrodynamic calculation of the generated water flow parameters, such as pressure and flow velocity distribution between the discharge channel and the shock wave. In particular, the pressure at the discharge channel boundary and the energy transferred to the water were estimated. The results of the calculation have been verified by comparing the measured and calculated trajectories of the shock wave. Based on the results of the calculation the energy transferred to the water was estimated. In addition, the analysis shows that the energy initially deposited in the discharge channel continues to produce mechan...

Characterization of the plasma on dielectric fiber (velvet) cathodes

An investigation of the properties of the plasma and the electron beam produced by velvet cathodes in a diode powered by a ∼200kV, ∼300ns pulse is presented. Spectroscopic measurements demonstrated that the source of the electrons is surface plasma with electron density and temperature of ∼4×1014cm−3 and ∼7eV, respectively, for an electron current density of ∼50A∕cm2. At the beginning of the accelerating pulse, the plasma expands at a velocity of ∼106cm∕s towards the anode for a few millimeters, where its stoppage occurs. It was shown by optical and x-ray diagnostics that in spite of the individual character and nonuniform cross-sectional distribution of the cathode plasma sources, the uniformity of the extracted electron beam is satisfactory. A mechanism controlling the electron current-density cross-sectional uniformity is suggested. This mechanism is based on a fast radial plasma expansion towards the center due to a magnetic-field radial gradient. Finally, it was shown that the interaction of the elec...

Electron diode with a large area ferroelectric plasma cathode

The operation of a planar diode with a large area (64 cm2) ferroelectric plasma cathode based on BaTiO3 ceramics is studied. Diode parameters and parameters of the generated electron beam are investigated under accelerating voltage of 200–350 kV and pulse duration of 250 ns. Parameters of the plasma, which forms on the surface of the ferroelectric cathode due to incomplete discharge, are reported as well. It is shown that the application of this cathode allows one to produce an electron beam simultaneously with application of the accelerating pulse. The generation of an electron beam with current density up to 20 A/cm2 and divergence of less than 1° is demonstrated. It is shown that diode operation is governed by parameters of the cathode plasma which fills the anode–cathode gap prior to application of the accelerating voltage pulse. By proper adjustment of the cathode and diode parameters the operation of the diode with almost constant impedance is achieved. Measurements of the potential distribution in ...

Spectroscopy of a ferroelectric plasma cathode

Results of spectroscopic investigations of the plasma formed on the surface of a ferroelectric cathode upon the application of a driving pulse are presented. The ferroelectric plasma cathode was made of a solid solution of Sr, Ba, Ti, Nb, Pb, and O. Its front side was covered by Cu grounded strip electrodes. A driving pulse with an amplitude ≲18 kV and pulse duration of ∼400 ns was applied to the rear Cu disk electrode. A Jobin-Yvon 750M spectrometer was used for visible light dispersion. Spectral line profiles were obtained by a fast framing camera. It was shown that light is emitted from the excited ions and neutral atoms of Cu, Pb, Sr, Ba, Ti, and H within the first 50 ns after the beginning of the driving pulse. By analyzing the Doppler broadening of the observed spectral line profiles it was found that the ion and neutral atom temperature is ⩽0.8 eV. Analysis of the Stark broadening of the Hα and Hβ spectral lines showed the absence of a high (>1 kV/cm) electric field which could be developed at the ...

Pulsed plasma electron sources

There is a continuous interest in research of electron sources which can be used for generation of uniform electron beams produced at E≤105 V/cm and duration ≤10−5 s. In this review, several types of plasma electron sources will be considered, namely, passive (metal ceramic, velvet and carbon fiber with and without CsI coating, and multicapillary and multislot cathodes) and active (ferroelectric and hollow anodes) plasma sources. The operation of passive sources is governed by the formation of flashover plasma whose parameters depend on the amplitude and rise time of the accelerating electric field. In the case of ferroelectric and hollow-anode plasma sources the plasma parameters are controlled by the driving pulse and discharge current, respectively. Using different time- and space-resolved electrical, optical, spectroscopical, Thomson scattering and x-ray diagnostics, the parameters of the plasma and generated electron beam were characterized.

Electron/ion emission from the plasma formed on the surface of ferroelectrics. II. Studies of electron diode operation with a ferroelectric plasma cathode

We present experimental results of electron diode operation with an active plasma cathode. The plasma was formed by a noncomplete discharge on the surface of different unpoled ferroelectric samples having the form of disks or tubes made of BaTiO3 or Pb(Zr,Ti)O3. To produce the discharge, a positive or a negative driving pulse was applied to the front or to the rear electrode of the sample. We studied the operation of planar and coaxial electron diodes under the application of an accelerating high-voltage pulse ⩽45 kV with repetition rate ⩽5 Hz or ⩽250 kV in a single-mode operation. Electron beams with a current density of several hundreds of A/cm2 and a time duration of several hundreds of ns were generated. It was shown that the parameters of the electron beam as well as the operation of the electron diode depend strongly on the method of the plasma formation and the time delay between the beginning of the plasma formation and the application of the high-voltage pulse.