Jet Collimation by Small-Scale Magnetic Fields

A popular model for jet collimation is associated with the presence of a large-scale and predominantly toroidal magnetic field originating from the central engine (a star, a black hole, or an accretion disk). Besides the problem of how such a large-scale magnetic field is generated, in this model the jet suffers from the fatal long-wave mode kink magnetohydrodynamic instability. In this paper we explore an alternative model: jet collimation by small-scale magnetic fields. These magnetic fields are assumed to be local, chaotic, tangled, but are dominated by toroidal components. Just as in the case of a large-scale toroidal magnetic field, we show that the ``hoop stress'' of the tangled toroidal magnetic fields exerts an inward force which confines and collimates the jet. The magnetic ``hoop stress'' is balanced either by the gas pressure of the jet, or by the centrifugal force if the jet is spinning. Since the length-scale of the magnetic field is small (< the cross-sectional radius of the jet << the length of the jet), in this model the jet does not suffer from the long-wave mode kink instability. Many other problems associated with the large-scale magnetic field are also eliminated or alleviated for small-scale magnetic fields. Though it remains an open question how to generate and maintain the required small-scale magnetic fields in a jet, the scenario of jet collimation by small-scale magnetic fields is favored by the current study on disk dynamo which indicates that small-scale magnetic fields are much easier to generate than large-scale magnetic fields.Comment: 14 pages, no figur

Knots, Braids and Hedgehogs from the Eikonal Equation

The complex eikonal equation in the three space dimensions is considered. We show that apart from the recently found torus knots this equation can also generate other topological configurations with a non-trivial value of the $\pi_2(S^2)$ index: braided open strings as well as hedgehogs. In particular, cylindric strings i.e. string solutions located on a cylinder with a constant radius are found. Moreover, solutions describing strings lying on an arbitrary surface topologically equivalent to cylinder are presented. We discus them in the context of the eikonal knots. The physical importance of the results originates in the fact that the eikonal knots have been recently used to approximate the Faddeev-Niemi hopfions.Comment: 15 pages, 5 figure

Using transfer ratio to evaluate EMC design of adjustable speed drive systems

This paper proposes a way to evaluate the conducted electromagnetic compatibility performance of variable speed drive systems. It is considered that the measured noise level is determined by two factors, the level of the noise source and the conversion efficiency of the propagation path from the source to the measurement equipments. They are corresponding to the two roles played by the converter. On the one hand, a converter provides the noise source and generates the noise current and voltage on the motor side with the cable and the motor. On the other hand, it acts as the propagation path with the DC bus and the rectifier to spread the noise generated on the motor side to the line side. The transfer ratio is defined as the ratio between the CM current on the motor side and the CM current on the line side. It can be used to evaluate the EMC design of a converter because it is independent of the cable and the motor. A simplified model is used to explain this characteristic. It can be measured when the converter is powered off. Verification is carried out by experimental results obtained from a 12-kVA laboratory system.\u

Wind tunnel study of coal dust release from train wagons

The paper presents a wind tunnel study to assess the coal dust released due to aeolian erosion from wagons equipped with two different shelter cover systems. A 1:25 scale model was used, and it comprises one locomotive and four train wagons with a 3.55 m maximum length. Several tests were conducted for different train configurations, and two initial load levels, with an undisturbed flow velocity of 13.4 m/s.http://www.sciencedirect.com/science/article/B6V3M-4C7DFX4-2/1/7e5214aa39411c25551eb15955a969a

A new behavioural model for performance evaluation of common mode chokes

A galvanically isolated three-phase AC/AC converter with a high-frequency AC-link has been analyzed from an EMC point of view. This is a special configuration because of a large number of switches, a high frequency transformer, and a fourwire output. The essential coupling paths are identified. Corresponding suppression remedies are given. The results, before and after measures, have been presented to demonstrate the improvement in EMC. Keywords: AC/AC converter; electromagnetic interference; galvanically isolate