Whirling frequencies and amplitudes due to deviations of generator shape

Earlier measurements in large synchronous generators indicate the existence of backward whirling motion, and also relatively large deviations of shape in both the rotor and the stator. These non-symmetric geometries produce an attraction force between the rotor and the stator, called unbalanced magnetic pull (UMP). The target of this paper is to analyse the whirling frequencies and amplitude of the response for large synchronous generators with a high number of poles, due to deviations of shape in the rotor and stator. A mathematical model is developed to describe the shapes of the rotor and stator, and the corresponding UMP is obtained by using the law of energy conservation. The UMP is analysed due to different deviations of shape. The result gives the average angular frequency and the magnitude of the UMP for certain deviations of shape. From this result, the whirling frequency and the amplitude of the corresponding response can be approximated. Simulations of the response of a Jeffcott rotor model show good agreement with the theoretical results of the UMP for some generator geometries. The conclusion is that different whirling frequencies, both backward and forward whirling, can occur in these machines due to deviations in shape of the generator. Therefore, the shape of the generator can excite resonance vibrations on several other frequencies than the rotation frequency. During maintenance of hydropower generators the shapes of the rotor and stator are frequently measured. The results from this paper can be used to evaluate such measurements and to explain the existence of complicated whirling motion.     

Pulsed laser deposition of atomically flat La1-xSrxMnO3 thin films using a novel target geometry

A new ablation target geometry is presented that was used to produce thin films of La_1-xSr_xMnO₃ grown heteroepitaxially on SrTiO₃ by pulsed reactive crossed-beam laser ablation. The films were grown in order to perform angle-resolved photoelectron spectroscopy, which demands that the surface be atomically flat. In situ and ex situ analysis shows that this condition was met, even after depositing to a thickness of over 100 nm. PACS 61.10.-i; 61.18.Bn; 68.47.Gh; 71.27.+a; 81.15.Fg     

Study of ferroelectric switching by domain-wall induced light scattering

Measurements of 90°-scattering of weak laser light are used to investigate pulsed domain switching in ferroelectrics. The studies were performed on strontium-barium niobate (SBN) single crystals. A good agreement of the switching parameters estimated from the optical measurements with those obtained by means of conventional electrical methods proves the validity of the optical method for switching studies. Due to the limited scattering volume in all three spatial dimensions, the method facilitates local probing of the switching within the crystal bulk. In particular, local specialities of the domain density can be detected. Furthermore, the excellent time resolution inherent in optical probing techniques allows for a comprehensive study of the dynamics.     

Siderophore Transport in Mycobacteria Analyzed by Mössbauer Spectroscopy: Possible Routes to Novel Antibiotics against These Organisms?

Iron uptake of the ⁵⁷Fe-siderophores ferricrocin, rhizoferrin and citrate in M. smegmatis was analyzed by in situ Mössbauer spectroscopy. Siderophore dependent uptake and metabolic utilization patterns of ⁵⁷Fe were found. Rhizoferrin is accumulated in the organism and, therefore, represents a suitable candidate for the synthesis of novel siderophore-antibiotics conjugates.     

Estimates for the Rapid Decay of Concentration Functions of n-Fold Convolutions

We estimate the concentration functions of n-fold convolutions of one-dimensional probability measures. The Kolmogorov–Rogozin inequality implies that for nondegenerate distributions these functions decrease at least as O(n ^–1/2). On the other hand, Esseen⁽³⁾ has shown that this rate is o(n ^–1/2) iff the distribution has an infinite second moment. This statement was sharpened by Morozova.⁽⁹⁾ Theorem 1 of this paper provides an improvement of Morozova's result. Moreover, we present more general estimates which imply the rates o(n ^–1/2).     

Measurement of the transverse Doppler shift using a stored relativistic7Li+ ion beam

We have performed for the first time precision spectroscopy on a coasting fast⁷Li⁺ ion beam in a storage ring. The ion beam moving with 6.4% speed of light was first electron cooled and then merged with two counterpropagating laser beams acting on two different hyperfine transitions sharing a common upper level (λ-system). One laser was frequency locked to thea ₃ ¹²⁷J₂ hfs frequency component established as a secondary frequency standard at 514 nm. The second laser was tuned over theλ-resonance, which was recorded relative to¹²⁷J₂ hfs components. This experiment is sensitive to the time dilation in fast moving frames and will lead to new limits for the verification of special relatively. The present status of the experiment and perspectives in accuracy are discussed.