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The recently developed manufacturing technologies use high pressure andvarious doping additions to prepare bulk MgB2-based materials with a highcritical current density measured by the magnetization methods. We use acontactless transformer method, which is based on studying the superconductorresponse to an induced transport current, to measure AC losses in bulk MgB2rings synthesized under high pressure. The obtained dependence of the losses onthe primary current (applied magnetic field) is fitted by the power law withthe exponent ~2.1 instead of the cubic dependence predicted by Bean's model andpower-law electric field-current density (E-J) characteristics with a largeexponent. The unusually strong dependence of AC losses on the frequency wasalso observed. It was shown that the E-J characteristic of bulk MgB2 is wellfitted by the dependence used in the extended critical state model based onaccount of the viscous vortex motion in the flux flow regime. Numericalsimulation using this E-J characteristic gives the current and frequency ACloss dependencies which well agree with the experimental results.

AC losses in high pressure synthesized MgB2 bulk rings measured by transformer method