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A pseudo-3D modeling approach, based on a particle-in-cell (PIC)-Monte Carlo collisions algorithm, has been developed for the study of large- and short-scale organization of the plasma in a planar magnetron. This extension of conventional PIC modeling permits the observation of spontaneous organization of the magnetron plasma, under the influence of crossed electric and magnetic fields, into the well-known, large-scale regions of enhanced ionization and density known as spokes. The nature of complex three-dimensional electron trajectories around such structures, and non-uniform ionization within them, is revealed. This modeling provides direct numerical evidence for the existence of high-amplitude internal spoke electric fields, proposed in earlier works. A 3D phenomenological model, consistent with numerical results, is proposed. Electron density fluctuations in the megahertz range, with characteristics similar to the electron cyclotron drift instability experimentally identified in a recent Letter, are also found. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

The recent developments in nanosecond pulsed power supplies facilitate the emission of high density electron bursts but their safe operation demands avoiding breakdowns. Using the theoretical and numerical modeling of the electron emission phenomena from a tip (micro-protrusion), the breakdown threshold (pre-breakdown) is analyzed considering it as the highest value of the voltage preserving the system out of the thermo-emission instability regime. However, the space charge that builds up in front of the tip limits the performance of these electron sources by decreasing the local electric field and consequently the thermo-field emission as well as the temperature of the emissive surface. Hence, it is found that the system can safely hold higher voltages (without breakdown) in the presence of dense space charge. In direct current, for a titanium elliptic tip, the highest operation voltage increases by about 15%, whereas for a tungsten hyperbolic tip, it increases by 70%. Remarkably, the emitted current close to the pre-breakdown voltage stays unchanged with or without taking into account the space charge. Surprisingly, when very short pulses (3 ns) are applied to a tungsten hyperbolic tip, the pre-breakdown voltage additionally increases by 30%, and the Coulomb screening, very effective in front of the tip apex, enlarges the electron emission area by 60%, releasing about 1.3 times more electrons compared to vacuum emission (without the space charge). Moreover, the ring effect, experimentally discovered by Dyke and Trolan [Phys. Rev. 89, 799 (1953)] on the radial electron density distribution, can be microscopically observed and understood with your model. [ABSTRACT FROM AUTHOR]

A simple and reproducible method for nanoaperture opening of metal film covered tapered optical fibers is reported as the last step of the manufacturing process for near-field scanning optical microscopy (NSOM) probes. It uses a microspark between the biased tip and another plane electrode working as a coronalike discharge in point-to-plane configuration. The resultant high electric field is enhanced at the extremity of the metallized tip self-focusing the discharge ions. The opening process is performed in situ in a specially designed plasma reactor which also allows, for the optical fiber tips, their surface cleaning and silver coating. Nanoaperture diameter can be controlled between 30 and 100 nm. These probes are able to generate subwavelength resolved images and are appropriate for high resolution NSOM. [ABSTRACT FROM AUTHOR]

The lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared with direct current magnetron sputtering for the same average power is often reported as a drawback. The often invoked reason is back-attraction of ionized sputtered material to the target due to a substantial negative potential profile, sometimes called an extended presheath, from the location of ionization toward the cathode. Recent studies in HiPIMS devices, using floating-emitting and swept-Langmuir probes, show that such extended potential profiles do exist, and that the electric fields Ez directed toward the target can be strong enough to seriously reduce ion transport to the substrate. However, they also show that the potential drops involved can vary by up to an order of magnitude from case to case. There is a clear need to understand the underlying mechanisms and identify the key discharge variables that can be used for minimizing the back-attraction. We here present a combined theoretical and experimental analysis of the problem of electric fields Ez in the ionization region part of HiPIMS discharges, and their effect on the transport of ionized sputtered material. In particular, we have investigated the possibility of a 'sweet spot' in parameter space in which the back-attraction of ionized sputtered material is low. It is concluded that a sweet spot might possibly exist for some carefully optimized discharges, but probably in a rather narrow window of parameters. As a measure of how far a discharge is from such a window, a Townsend product ΠTownsend is proposed. A parametric analysis of ΠTownsend shows that the search for a sweet spot is complicated by the fact that contradictory demands appear for several of the externally controllable parameters such as high/low working gas pressure, short/long pulse length, high/low pulse power and high/low magnetic field strength. [ABSTRACT FROM AUTHOR]