Magnetic Flux Coupling Research Articles

Exact linearization of a voltage-controlled 3-pole active magnetic bearing system

Most magnetic radial bearings adopt the configuration of eight magnetic poles. Such a configuration usually requires four power amplifiers and has high power loss, resulting in high cost. In this study, a 3-pole active magnetic bearing (AMB) is investigated, which requires only two power amplifiers. In addition, it has lower power loss. Thus, the overall cost can be reduced. However, the 3-pole AMB system is strongly nonlinear due to magnetic flux coupling. If the actuator dynamics are also considered, the overall system will be much more complicated. Nonlinear control is thus necessary. By taking magnetic fluxes as part of the system states, the state equations of the voltage-controlled 3-pole AMB system admit a quadratic nonlinear form. It is shown that this quadratic nonlinear system is feedback linearizable. An admissible domain for the initial states such that the resulting system responses will not hit the bearing boundary (or back-up bearing, if exists) is also obtained via a Lyapunov analysis. Numerical simulations are carried out to verify the theoretical results.

Solar wind input between substorm onsets during and after the October 18–20, 1995, magnetic cloud

We study an aspect of substorm occurrence during Earth passage of the October 1995 magnetic cloud and the corotating stream overtaking it. The substorms were identified primarily by five latitudinal arrays of ground‐based magnetometers around the globe and the interplanetary observations were from the Wind spacecraft. The aspect we study is the energy and magnetic flux accumulated in the geomagnetic tail between successive substorm onsets, which we calculate by integrating different theoretical and empirical expressions for the solar wind‐magnetosphere coupling rates. These coupling functions, which depend strongly on the interplanetary magnetic field (IMF) north‐south component, Bz, have all been proposed previously as appropriate measures of solar wind input into the magnetosphere and indeed most correlate well with the cross‐polar cap potential. We contrast the effect on magnetospheric substorms of three solar wind states: A 14 hour long period of continuous and strong IMF Bz < 0 and a 16 hour long period of continuous and strong IMF Bz > 0, both states due to the magnetic cloud; and a 22 hour long period of intermittent IMF Bz polarity of large, but weaker amplitude, due to Alfvén waves in the faster stream. We find that (1) the average rate of substorm onset occurrence during the Bz < 0 phase in the cloud is higher than during the intermittent Bz polarity in the wake, by about a factor of 1.4. (2) The long Bz > 0 phase of the cloud elicited no substorm onsets. (3) The total time for which Bz is negative between substorm onsets is less variable than the interonset interval itself, but still varies between ∼ 0.5 hour and ∼ 4 hours. (4) The integral of any individual solar wind‐magnetosphere coupling function between successive substorm onsets is also variable. (5) The interonset integral varies differently for different coupling functions, even for those which measure the same quantity (energy or magnetic flux). (6) For all energy and magnetic flux coupling functions we find a positive correlation between the interonset integral and the value of the coupling function around the time of the preceding substorm onset. The correlation is best for the ε and υBs functions. This result is consistent with those substorm models in which substorm onset occurs at some fixed energy or magnetic flux threshold and where the amount of energy or magnetic flux lost following each substorm onset is proportional to the solar wind input rate at the time of onset.

Study of the magnetic coupling between a SQUID and wires integrated on a chip

Magnetic coupling between a SQUID and wires in an integrated circuit has been studied. Using test integrated circuits, the coupling is measured by varying the distance from the SQUID to each wire. The coupling between the SQUID and the wires with and without a groundplane decreases with an increase in the distance. The coupling for the wires with the groundplane is smaller than that for the wires without the groundplane at distances less than 595 /spl mu/m. However, the rate of the decrease in the coupling for the wires with the groundplane falls off for distances more than 595 /spl mu/m and the coupling converges to around 2.9 pH. From two dimensional simulations for the magnetic flux coupling, the origin of the residual inductance is found to be the coupling between the SQUID, and shielding current in the groundplane. The value of the distance, 595 /spl mu/m, does not depend on the size of the SQUID. To decrease the coupling, the use of wires with stripline structure or coplanar structure is desirable.

A probe for magnetization measurements of thin superconducting films in pulsed high magnetic fields

This paper describes an inductive magnetization sensor, designed to measure thin superconducting films (of order 3000 Å) in very strong magnetic fields. For magnetic fields in the range 250–500 kG, the sensitivity of the probe is 10−2 emu. Accuracy and high sensitivity were obtained by combining a sample extraction system, vibration isolation, and an inductive sensor with optimized flux coupling geometry. The magnetic flux coupling to the sensor was verified to exceed 70%, both by calculation and experimental calibration.