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[en] The magnetohydrodynamic stability of an ordinary compressible hollow cylinder pervaded by an oblique varying magnetic field, under the influence of capillary, inertia and Lorentz forces, has been developed. The problem is modelized. The basic equations are formulated, solved and upon applying appropriate boundary conditions the singular solutions are excluded. The eigenvalue relation has been derived and discussed. The capillary force has destabilizing influence only for long wavelengths in the axisymmetric perturbation but it is stabilizing in the rest and also so in the non-axisymmetric perturbations. The compressibility increases the stable domains and simultaneously decreases those of instability. The electromagnetic force has different effects due to the axial uniform fields and varying transverse one. The axial fields are stabilizing for all wavelengths in all kinds of perturbations. The transverse field is stabilizing or not according to restrictions. Here the high compressibility increases rapidly the magneto dynamic stable domains and leads to shrinking those of instability.
[en] The Lorentz force can dynamically detune pulsed Superconducting RF cavities. Considerable additional RF power can be required to maintain the accelerating gradient if no effort is made to compensate for this detuning. Compensation systems using piezo actuators have been used successfully at DESY and elsewhere to control Lorentz Force Detuning (LFD). Recently, Fermilab has developed an adaptive compensation system for cavities in the Horizontal Test Stand, in the SRF Accelerator Test Facility, and for the proposed Project X.
[en] A degradation phenomenon in RE–Ba–Cu–O (REBCO) coated conductors caused by the screening current was observed at 4.2 K. Even without transport current, delamination of the superconducting layer from the buffer layer occurred because of the cleavage force. The Lorentz force between an induced screening current and an external magnetic field is considered to generate the cleavage force. In this paper, we report the degradation of REBCO conductors due to the screening current and discuss the cleavage force using simple models. The degradation of the REBCO conductor was observed for inclination angles of 0°, 10°, 20°, 45°, 60° and 90°. We found serious degradation at 45°. In practical REBCO coils, especially feed through parts, degradation of the REBCO conductor by the cleavage force due to the induced screening current should be prevented. (paper)
[en] The unipolar generator has a long history of confusing physicists, dating back to Faraday’s original experiment in 1831. Even today it is sometimes referred to as an example where the flux rule for induction fails, and students are often told that they must instead use the Lorentz force law to find the correct answer. In actuality, neither the flux rule nor the Lorentz force law ‘fail’ to describe the unipolar generator. The correct application of these laws is summarised here, along with a new technique that uses Gauss’ law for magnetism to account for unipolar generators with non-uniform magnetic fields. A history of the confusion from Faraday to present day is presented along with a recommendation for preventing confusion in the future. (paper)
[en] A large mass of electron-hole liquid (γ drop) formed in a strain-induced potential well in Ge is known to distort its shape significantly in a magnetic field B > or approx. = 1 kG. It is shown in this paper that the shape change can be understood in detail as due to a ''recombination current'' of electron-hole pairs needed to replace those pairs which recombine in the drop volume. The Lorentz force deflects this current and produces a macroscopic dipole current loop inside the drop. The drop then changes shape to minimize its total energy, including magnetic, strain, and surface energies. While the drop usually flattens along the field direction, both para- and diamagnetic effects (elongated drops) are found to be possible, depending on excitation conditions, in accord with experiment. Similar effects are predicted to occur in small drops in unstrained Ge. This paper presents a magnetohydrodynamic theory of the magnetostriction which takes into account density variations which occur in the strain well and in high magnetic fields. A simpler theory is given for the special case in which the drop may be considered incompressible (small drops and moderate fields). Effects of carrier mass anisotropy and fluid viscosity are taken into consideration
[en] The Super-FRS is a large acceptance superconducting fragment separator with three branches serving different experimental areas. Low Energy Branch of the main separator delivers secondary beams of low energy beams and thus the range of straggling of hot fragments is reduced. The quadrupole magnets of the Low Energy Branch is a super-ferric magnet having a field gradient of 8.0 T/m. It has usable aperture of ±190 mm in horizontal direction and ±120 mm in vertical dimension. It has a pole tip radius of 250 mm. The Lorentz forces on the coil are non-uniform and must be supported to prevent movement and subsequent quench. Moreover the magnets will be assembled at room temperature and will undergo thermal contraction during operation at liquid helium temperature Therefore it is very important to perform detailed stress analysis of the magnet to evaluate thermal as well as electromagnetic stresses and deformation developed in the coil, coil support structures, and iron during cool down and coil excitation. This paper describes the three dimensional stress analysis of the magnet assembly. (author)