Superconducting Microwave Cavity Made of Bulk MgB2

We report the successful manufacture and characterization of a microwave resonant cylindrical cavity made of bulk MgB2 superconductor (Tc ≈ 38.5 K), which has been produced by the Reactive Liquid Mg Infiltration technique. The quality factor of the cavity for the TE011 mode, resonating at 9.79 GHz, has been measured as a function of the temperature. At T = 4.2 K, the unloaded quality factor is ≈ 2.2×10; it remains of the order of ×10 up to T ∼ 30 K. We discuss the potential performance improvements of microwave cavities built from bulk MgB2 materials produced by reactive liquid Mg infiltration. PACS numbers: 74.25.Nf; 74.70.Ad Superconducting Microwave Cavity Made of Bulk MgB2 2 The very low surface resistance of superconducting materials makes them particularly suitable for designing high-performance microwave (mw) devices, with considerably reduced sizes. The advent of high-temperature superconductors (HTS) further improved the expectancy for such applications, offering a potential reduction of the cryogenic-refrigeration limit, with respect to low-temperature superconductors. A comprehensive review on the mw device applications of HTS was given by Lancaster [1]. Among the various devices, the superconducting resonant cavity is one of the most important applications in the systems requiring high selectivity in the signal frequency, such as filters for communication systems [2], particle accelerators [3, 4], equipments for material characterization at mw frequencies [1, 5]. Nowadays, one of the commercial applications of HTS electronic devices are planarmicrostrip filters for transmission line, based on YBa2Cu3O7 thin or thick films [1, 6], whose manufacturing process allows having an high degree of device miniaturization. Nevertheless, the need of high performance, in many cases, overcomes the drawback of the device sizes, as, e.g., in the satellite-transmission systems, radars, particle accelerators, and demands for cavities with the highest quality factors. Since the discovery of HTS, several attempts have been done to manufacture mw cavities made of these materials in bulk form [2, 7, 8]; however, limitations in the performance were encountered. Firstly, because of the small coherence length of HTS, grain boundaries in these materials are weakly coupled giving rise to reduction of the critical current and/or nonlinear effects [9], which worsen the device performance; furthermore, the process necessary to obtain bulk HTS in a performing textured form is very elaborated. For these reasons, in several applications, such as particle accelerators and equipments for mw characterization of materials, most of the superconducting cavities are still manufactured by Nb, requiring liquid helium as refrigerator. Since the discovery of superconductivity at 39 K in MgB2 [10], several authors have indicated this material as promising for technological applications [4, 11, 12]. Indeed, it has been shown that bulk MgB2, contrary to oxide HTS, can be used in the polycrystalline form without a significant degradation of its critical current [11, 12, 13]. This property has been ascribed to the large coherence length, which makes the material less susceptible to structural defects like grain boundaries. Actually, it has been shown that in MgB2 only a small amount of grain boundaries act as weak links [14, 15, 16, 17]. Furthermore, MgB2 can be processed very easily as high-density bulk material [18], showing very high mechanical strength. Due to these amazing properties, MgB2 has been recommended for manufacturing mw cavities [4, 19], and investigation is carried out to test the potential of different MgB2 materials for this purpose. However, papers discussing the realization and/or characterization of mw cavities made of MgB2 have not yet been reported. Recently, we have investigated the mw response of MgB2 samples prepared by different methods, in the linear and nonlinear regimes [17, 20]. Our results have shown that the residual surface resistance strongly depends on the preparation technique and the purity and/or morphology of the components used in the synthesis process. In Superconducting Microwave Cavity Made of Bulk MgB2 3 particular, the investigation of small plate-like samples of MgB2 prepared by Reactive Liquid Mg Infiltration (RLI) process, has highlighted a weak nonlinear response, as well as relatively small values of the residual surface resistance. Furthermore, bulk samples produced by RLI maintain the surface staining unchanged for years, without controlledatmosphere protection. This worthwhile property is most likely related to the high density, and consequently high grain connectivity, achieved with the RLI process, as well as to the small and controlled amount of impurity phases [21]. On the contrary, samples prepared by other techniques, though exhibiting lower values of the residual surface resistance, need to be kept in protected atmosphere to avoid their degradation. These interesting results have driven us to build a mw resonant cavity using MgB2 produced by the RLI process. In this work, we discuss the properties of the first mw resonant cavity made of bulk MgB2. As the first attempt to apply the MgB2 to the cavity-filter technology, we have manufactured a simple cylindrical cavity and have investigated its microwave response in a wide range of temperatures. All the parts of the cavity, cylinder and lids, are made of bulk MgB2 with Tc ≈ 38.5 K and density ≈ 2.33 g/cm. The MgB2 material has been produced by the RLI process [18, 22], which consists in the reaction of B powder and pure liquid Mg inside a sealed stainless steel container. In particular, the present cylindrical cavity (inner diameter 40 mm, outer diameter 48 mm, height 42.5 mm) was cut by electroerosion from a thicker bulk MgB2 cylinder prepared as described in Sec. 4.3 of Ref. [22], internally polished up to a surface roughness of about 300 nm. A photograph of the parts, cylinder and lids, composing the superconducting cavity is shown in Fig. 1. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15