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[en] Cathode spot phenomena show many features of fractals, for example self-similar patterns in the emitted light and arc erosion traces. Although there have been hints on the fractal nature of cathode spots in the literature, the fractal approach to spot interpretation is underutilized. In this work, a brief review of spot properties is given, touching the differences between spot type 1 (on cathodes surfaces with dielectric layers) and spot type 2 (on metallic, clean surfaces) as well as the known spot fragment or cell structure. The basic properties of self-similarity, power laws, random colored noise, and fractals are introduced. Several points of evidence for the fractal nature of spots are provided. Specifically power laws are identified as signature of fractal properties, such as spectral power of noisy arc parameters (ion current, arc voltage, etc) obtained by fast Fourier transform. It is shown that fractal properties can be observed down to the cutoff by measurement resolution or occurrence of elementary steps in physical processes. Random walk models of cathode spot motion are well established: they go asymptotically to Brownian motion for infinitesimal step width. The power spectrum of the arc voltage noise falls as 1/f 2, where f is frequency, supporting a fractal spot model associated with Brownian motion
[en] Trajectories of electron beams emitted by an ion source with an anode layer and Hall electron closed drift orbits were visualized using light emission from a working gas excited by electrons. Gas discharge of magnetron type, arising in the beam drift region under the influence of an electric field of a target bias potential, was visualized
[en] A calculation of the instability threshold for an isotope separator with only one isotope species yields a threshold comparable to that for several species. The beam interacts with a neutralizing cloud of electrons. As a side issue, a scaling law is derived that predicts that the beam particle density scales as the square of the magnetic field
[en] The dual-axis radiographic hydrodynamic test (DARHT) facility uses bremsstrahlung radiation source spots produced by the focused electron beams from two linear induction accelerators (LIAs) to radiograph large hydrodynamic experiments driven by high explosives. Radiographic resolution is determined by the size of the source spot, and beam emittance is the ultimate limitation to spot size. On the DARHT-II LIA, we measure an emittance higher than predicted by theoretical simulations, and even though this accelerator produces submillimeter source spots, we are exploring ways to improve the emittance. Some of the possible causes for the discrepancy have been investigated using particle-in-cell codes. Finally, the simulations establish that the most likely source of emittance growth is a mismatch of the beam to the magnetic transport, which can cause beam halo.
[en] Collective impact ionization has been used to explain lock-on in semi-insulating GaAs under high-voltage bias. The authors have used this theory to study some of the steady state properties of lock-on current filaments. In steady state, the heat gained from the field is exactly compensated by the cooling due to phonon scattering. In the simplest approximation, the carrier distribution approaches a quasi-equilibrium Maxwell-Boltzmann distribution. In this report they examine the validity of this approximation. They find that this approximation leads to a filament carrier density which is much lower than the high density needed to achieve a quasi-equilibrium distribution. Further work on this subject is in progress
[en] A critical analysis of available experimental data and models of an anode spot formation shows their insufficiency for developing a clear-cut physical model of anode processes in a high-current vacuum arc. Based on new results of studying an anode medium- and lowpressure arc region, a qualitative physical model of an anode spot formation in a vacuum arc is proposed. The main idea of the model is that a change of the sign of the anode voltage drop (from negative to positive) is a necessary condition for an anode spot formation. Experimental data are qualitatively discussed from the point of view of the proposed model
[en] The purpose of this paper is to provide a discussion of the fundamental processes occurring in hydrogen thyratrons. Recent experimental data pertaining to electron densities, energies, and excitation processes occurring in devices during their normal operation are presented. Electron densities are observed to be lower than 2 X 1014 cm-3, corresponding to current densities of approximately 100 A/cm2. The presence of both molecular and atomic species affecting recovery and voltage reerection is reported, and mechanisms for this are discussed. Streak camera data showing a delay in breakdown of the grid-anode relative to the grid-cathode region are also presented
[en] We describe applications of the electrostatic plasma lens for manipulating and focusing moderate energy, high current, broad, heavy ion beams. Use of a plasma lens in this way has been successfully demonstrated in a series of experiments carried out collaboratively between IP NASU (Kiev) and LBNL (Berkeley) in recent years. Here we briefly review the plasma lens fundamentals, peculiarities of focusing heavy ion beams, and summarize some recent developments (experiments, computer simulations, theory). We show that there is a very narrow range of low magnetic field for which the optical properties of the lens improve markedly. This opens up some attractive possibilities for the development of a new-generation compact lens based on permanent magnets. Preliminary experimental results obtained at Kiev and Berkeley on the operation of a permanent magnet plasma lens for manipulating wide aperture high-current heavy ion beams are presented and summarized
[en] The Cygnus radiographic machine is a relatively compact low-energy (<3 MV) x-ray source with some extremely desirable features for radiographic applications. These features include small spot size critical for high-spatial resolution and high dose in a low energy range. The x-ray source is based on bremsstrahlung production in a small diameter (∼0.75 mm) tungsten rod by a high-current (∼60 kA) electron beam converging at the tip of the rod. For quantitative analysis of radiographic data, it is essential to determine the bremsstrahlung spectrum accurately. We have used the radiographic chain model that self-consistently models the diode with a two-dimensional particle-in-cell code (Merlin) which links to an electron-photon Monte Carlo code to obtain the spectrum under three different situations. These are: steady state spectrum using a voltage puise of 2.5 MV, time-integrated spectrum using a time-dependent experimental voltage pulse, and inclusion of reflexing electrons at the anode in our particle-in-cell simulation. Detailed electron dynamics have been obtained in our study. Our investigations conclude that the time integrated bremsstrahlung spectrum is significantly softer than that of the steady state. In our latest simulations, we have included the effects of reflexing electrons around the anode rod and found the spectrum to be in better agreement with experimental data.
[en] This article presents component failure rate data for use in assessment of lead lithium cooling systems. Best estimate data applicable to this liquid metal coolant is presented. Repair times for similar components are also referenced in this work. These data support probabilistic safety assessment and reliability, availability, maintainability and inspectability analyses.