Wavelength (nm)

The stable structures and melting dynamics of clusters of identical ions bound by linear octupole radiofrequency traps are theoretically investigated by global optimization methods and molecular dynamics simulations. By varying the cluster sizes in the range of 10-1000 ions and the extent of trap anisotropy by more than one order of magnitude, we find a broad variety of stable structures based on multiple rings at small sizes evolving into tubular geometries at large sizes. The binding energy of these clusters is well represented by two contributions arising from isotropic linear and octupolar traps. The structures generally exhibit strong size effects, and chiral arrangements spontaneously emerge in many crystals. Sufficiently large clusters form nested, coaxial tubes with different thermal stabilities. As in isotropic octupolar clusters, the inner tubes melt at temperatures that are lower than the overall melting point.

In this study, the effect of insulating barriers on the breakdown voltage of air in inhomogeneous field is observed. Tests are conducted in order to measure the 50 Hz AC breakdown voltage of small air gaps (up to 50 mm) formed by hemispherically capped rod-plane electrodes with an insulating barrier between them. Energized rod and grounded plane gaps are studied with the flat insulating barriers that have three different diameters. The results of the test series show how the breakdown voltage varies with the distance between the electrodes, the size and the material of the barrier, the relative position of the electrodes and the barrier between them. As a result, all three materials show nearly same effect when they are used as a barrier, the breakdown voltage varies due to the size of the barriers and the maximum flashover voltages are observed when the barriers are positioned at the nearest point to the rod electrode and the small sized barriers become effective only in very small air gaps.

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The aim of this paper is to point out the advantages of the use of the time-frequency analysis in the digital processing of waveforms recorded in high voltage impulse tests. Impulse voltage tests are essential to inspect and test insulation integrity of high voltage apparatus. On the other hand, generated impulse currents are used for different test applications such as investigation of high current effects, electromagnetic interference (EMI) testing, etc. Obtained voltage and current waveforms usually have some sort of interferences originated from the different sources. These interferences have to be removed from the original impulse data in order to evaluate the waveform characteristics precisely. When the interference level is high enough, it might not be possible to distinguish signal parameters from the recorded data. Conventional filtering methods cannot be useful for some interference like white noise. In that case, time-frequency filtering methods might be necessary. In this study, the wavelet analysis, which is a powerful time-frequency signal processing tool, is used to recognize the noise of impulse current and voltage data. Thus, the noise sources can be determined by short time Fourier Transform, and a coherence approach is used to determine the bandwidth of noises.