AC losses in high pressure synthesized MgB₂bulk rings measured by a transformer method

Abstract

The recently developed manufacturing technologies use high pressure and various doping additions to prepare bulk MgB 2 -based materials with a high critical current density measured by the magnetization methods. We use a contactless transformer method, which is based on studying the superconductor response to an induced transport current, to measure AC losses in bulk MgB 2 rings synthesized under high pressure. The obtained dependence of the losses on the primary current (applied magnetic field) is fitted by the power law with the exponent ~2.1 instead of the cubic dependence predicted by Bean's model and power-law electric field-current density (E-J) characteristics with a large exponent. The unusually strong dependence of AC losses on the frequency was also observed. It was shown that the E-J characteristic of bulk MgB 2 is well fitted by the dependence used in the extended critical state model based on account of the viscous vortex motion in the flux flow regime. Numerical simulation using this E-J characteristic gives the current and frequency AC loss dependences which well agree with the experimental results.

MgB2-based superconductors for fault current limiters

“…A low ratio of the primary coil inductances in the current limitation regime and nominal one, 0.09/0.28≈3.2, is explained by peculiarities of the design of the magnetic system for the big ring: ratio of the height of the magnetic core to its diameter was about only 0.39. For more realistic SFLC model with other rings the inductance ration increases up to [5][6][7]. In liquid helium all tested models returned into an initial state for 1-2 s. A quenching current in rings, I q , changes in the range of 4,500 ÷ 24,000 A, corresponding to the quenching current density J q = 19,000÷63,200 A/cm 2 .…”

“…Basically, the device resembles a transformer with the short-circuited secondary coil which can be performed completely from a superconducting material in the form of a hollow cylinder or ring or as a coil containing a superconducting switching element ( Figure 1) [2,3,7]. The primary coil of this transformer is in series connected to the protected circuit.…”

“…The used contactless method suitable for samples in the form of a closed loop (hollow cylinder, ring, or short-circuited coil) is based on using the transformer configuration ( Figure 1) [7]. A superconducting closed loop forms the secondary coil of a transformer in which the primary coil is connected with an AC source.…”

“…Series: Materials Science and Engineering 171 (2017) 012144 doi:10.1088/1757-899X/171/1/012144 measuring contacts are required; iii) measurement results are not influenced by heating due to losses in contacts. More detailed description is presented in [7]. …”

“…Devices based on the superconductor magnesium diboride MgB 2 with critical temperature T c = 39 K can operate at temperatures of liquid helium, hydrogen or even neon. This compound can thus be envisaged for many applications, as fault current limiters, MRI magnets, cables, electromotors, magnetic bearings, etc [2,[5][6][7]. Note that hydrogen is already used for cooling conventional generators in electrical power stations.…”