Microwave Response of Coaxial Cavities Made of Bulk Magnesium Diboride

Abstract

Abstract-We report on the microwave properties of coaxial cavities built by using bulk MgB 2 superconductor prepared by reactive liquid Mg infiltration technology. We have assembled a homogeneous cavity by using an outer MgB 2 cylinder and an inner MgB 2 rod and a hybrid cavity by using an outer copper cylinder and the same MgB 2 rod as inner conductor. By the analysis of the resonance curves, in the different resonant modes, we have determined the microwave surface resistance R s of the MgB 2 materials as a function of the temperature and the frequency, in the absence of dc magnetic fields. At T = 4.2 K and f ≈ 2.5 GHz, by an mw pulsed technique, we have determined the quality factor of the homogeneous cavity as a function of the input power up to a maximum level of about 40 dBm (corresponding to a maximum peak magnetic field of about 100 Oe). Contrary to what occurs in many films, R s of the MgB 2 material used does not exhibit visible variations up to an input power level of about 10 dBm and varies less than a factor of 2 on further increasing the input power of 30 dB.

Frequency dependence of the microwave surface resistance of MgB2 by coaxial cavity resonator

“…In a previous work, we have investigated bulk MgB 2 produced by Mg-RLI technique [12]. Because of the shorter rod used in that work, the fits of the R s ðf Þ curves were done on only three points, so the results could be inaccurate; for this reason, we have investigated a longer MgB 2 rod.…”

“…The frequency dependence of R s of SC can be conveniently measured by coaxial cavity resonators, with the SC sample as inner conductor, provided that the SC rod is long enough to feed the cavity in different resonant modes [8][9][10][11]. Recently, we have done a feasibility study [12] to build coaxial cavities using bulk MgB 2 produced by reactive liquid Mg infiltration technique (Mg-RLI) [13]. The study has been carried out using a MgB 2 rod about 45 mm long; so, in the range of frequencies available with our experimental apparatus, it has been possible to investigate only three resonant modes.…”

High magnetic shielding properties of an MgB2 cup obtained by machining a spark-plasma-sintered bulk cylinder

“…Indeed, MgB 2 long coherence length is suitable for supporting the fabrication of large untextured polycrystalline samples, because the flow of high critical current density, J c , across clean grain-boundaries is not prevented by weak-link effects, even when two pellets are joined [19]. This has fostered the development of processing techniques able to produce and assemble dense MgB 2 bulk samples with almost isotropic and homogeneous J c , into complex geometries [17,[20][21][22][23]. The starting elements are cheap, non-toxic and do not include rare-earths, and the small weight density of this superconductor makes it attractive for portable applications such as space ones [24].…”