Magnetic relaxation in pulse-magnetized high-temperature superconductors

Abstract

Abstract. A magnetic filed relaxation at the center of a pulse-magnetized single-domain Y-Ba Introduction.Textured high-temperature superconductors (HTS) of Re(Y)-Ba-Cu-O family appear to be very promising materials for manufacturing superconducting magnets with very large magnitudes of trapped magnetic fields [1,2]. For example, trapping a field as high as 17 T between two 26.5-mm diameter Y-Ba-Cu-O disks at 29K has been demonstrated recently [3]. A comprehensive review of synthesis, characteristics and applications of high-quality HTS suitable for applications in superconducting magnets can be found in [4]. Recently significant research efforts have been directed towards using a pulse magnetization of HTS instead of an isothermal one [5]. An obvious advantage of the pulse magnetization with a typical pulse duration in order of 1 to 100 msec is a smaller amount of energy needed to create a magnetic field of a desired magnitude. There is, however, a complication that needs to be considered when using this method: an electromagnetic heating of a superconductor during the pulse magnetization plays much bigger role than during a slow magnetization because there is not enough time for the temperature to reach an equilibrium distribution within the superconducting volume. For example, a thermal time constant of a HTS sample at 78K with characteristic dimensions in order of 10 mm would be in order of a few seconds, which is much large than the pulse duration. This circumstance allows one to analyze the pulse magnetization as an adiabatic process. A negative effect of the heating during a single-pulse magnetization can be reduced by using a multi-pulse method. Using a combination of the multi-pulse magnetization and a step-by-step temperature decrease has been shown to allow trapping magnetic fields as high as 4.5T at 30K [6].One of the key performance characteristics of any magnet is a long-term stability of its magnetic field. Here superconducting magnets have a fundamental limitation due to a phenomenon known as a thermally-activated magnetic flux creep. According to Kim-Andersen theory a thermally-activated magnetic flux creep in type II superconductors [7], supercurrents and magnetization decrease logarithmically in time as

Pulse Heating and Negative Magnetic Relaxation on Bulk HTS Annuli

“…It is wellknown that at lowering temperatures the relaxation rate suppressed exponentially [6]. In particular, when the temperature of the meltgrown samples in a liquid nitrogen changes by an amount θ , the relaxation rate varies as S = S 0 · e (−c·θ) (5) where S refers to the initial temperature, and c = 1.2 K −1 [13]. Actually, S depends weakly on the temperature; therefore, we assumed that it is equal to the isothermal relaxation measured at the liquid nitrogen temperature, so S 0 = S 78 .…”

“…Moreover, it was found that the PFM method allows one to control not only the S magnitude, but also its sign. In some cases the increase of the trapper field over time was observed after the pulse magnetization [5]. It can be called "negative relaxation," according to the generally accepted definition of the relaxation [6].…”

Relaxation Properties of the Trapped Flux of Bulk High-Temperature Superconductors at Different Magnetization Levels

“…Although the relaxation decay is mostly found to show a logarithmic time dependence [8], any decay may degrade the performance of superconducting magnetic devices, and finally involve potential influences on the operation stability. From this viewpoint, the magnetic relaxation phenomenon has aroused great attention of worldwide researchers and formed a crucial and intense topic accompanying the engineering applications of bulk HTS [8][9][10][11][12][13].…”

Импульсное Намагничивание Короткозамкнутой Катушки Из ВТСП

“…(1) Импульсное намагничивание массивного сверхпроводника обычно протекает в адиабатических условиях, поскольку время теплообмена с термостатом значительно превышает длительность импульса. Вследствие этого величина захваченного поля в зависимости от амплитуды имеет узкий максимум [15]. В области температур T = 78 K захваченные поля при импульсном намагничивании примерно такие же, как в случае изотермического намагничивания.…”