Power Dependence of the RF Surface Resistance of MgB>inf<2>/inf<Superconductor

Abstract

Magnesium diboride (MgB 2 ) is a superconducting material that has a transition temperature (T c ) of ~40 K, which is ~30 K higher than niobium (Nb) that has been used for most superconducting RF cavities in the past decades. Last year, it was demonstrated that the RF surface resistance of MgB 2 can be lower than Nb at 4 K.One of the problems with other high-T c materials such as YBCO was its rapid increase in RF surface resistance with higher surface magnetic fields. Recently, we have shown that MgB 2 shows little increase in the surface resistance up to ~120 Oe, equivalent of an accelerating field of ~3 MV/m. The highest field tested was limited by available power. This result is encouraging and has made us consider fabrication of a cavity coated with MgB 2 and test it. Also, there is a potential that this material has a higher critical magnetic field that enables the cavity to run at a higher gradient than Nb cavities in addition to the possibility of operation at higher temperatures.

Microwave Response of Coaxial Cavities Made of Bulk Magnesium Diboride

“…Due to the lower R s value of the MgB 2 material, the maximum value of the peak magnetic field achieved at the maximum power level is about 100 Oe, and the slight variation of R s starts at H mw ≈ 12 Oe (corresponding to P in = 20 dBm). The maximum peak magnetic field we achieved with the homogeneous cavity is smaller but of the same order of magnitude than those achieved in previous investigations in MgB 2 bulk and films [39], [40]; even in our MgB 2 material, the mw surface resistance weakly depends on the mw field, as already highlighted in [39] and [40].…”

${\hbox{MgB}}_{2}$ for Application to RF Cavities for Accelerators

“…In [13], we proposed to use PLD to coat a cavity. To check the feasibility of this technique, we coated some substrates made of Nb and .…”

“…The surface finish does not look as good as that coated on sapphire because the Nb substrates had a very rough surface of 400 nm in rms roughness. This could have contributed to the higher surface resistance compared to the samples coated on smooth sapphire substrates [12], [13]. It was found that, although the surface resistance at low field is similar for both films with PLD and reactive evaporation techniques, the power dependence is very different.…”

“…An independent measurement of low-field of the film coated with reactive evaporation method on r-plane sapphire having a surface roughness showed an slightly lower than Nb at 4 K [12], [13], whereas the films deposited on a rough surface of have shown about one order of magnitude higher than Nb as shown in Fig. 10.…”

Microwave surface resistance of thick MgB2films onc-plane sapphire: a study on the depth profile of the surface resistance

“…It has been understood that such extremely low R S , as observed from high-quality MgB 2 films at very low temperatures, is attributed to the s-wave nature of the order parameter in the π band [11,12]. The s-wave nature allows an exponential temperature dependence of the R S [13,14], with their R S values being lower than those of Nb films at temperatures above 4 K [15,16] 5 . Grain boundaries of MgB 2 films affect their R S only a little compared to those of YBa 2 Cu 3 O 7 (YBCO) films, with the R S of polycrystalline MgB 2 films being as low as 10 µ at 8.5 GHz at 7 K after being irradiated with 7 MeV protons [17].…”