Near-field microwave magnetic nanoscopy of superconducting radio frequency cavity materials

Abstract

A localized measurement of the RF critical field on superconducting radio frequency (SRF) cavity materials is a key step to identify specific defects that produce quenches of SRF cavities. Two new measurements are performed to demonstrate these capabilities with a novel near-field scanning probe microwave microscope. The first is a third harmonic nonlinear measurement on a high ResidualResistance-Ratio bulk Nb sample showing strong localized nonlinear response for the first time, with surface RF magnetic field B surf ace ∼ 10 2 mT. The second is a raster scanned harmonic response image on a high quality MgB2 thin film demonstrating a quench defect-free surface over large areas.Materials issues limit the ability to mass-produce bulk Nb Superconducting Radio Frequency (SRF) cavities with consistent high accelerating gradient performance. Much debate regarding this inconsistent performance has concluded that certain types of defects on the Nb cavity surface behave as a source of quenching in high RF magnetic fields [1] [2]. Quench occurs when the superconductor returns to the normal state in the presence of strong fields, thus limiting the utility of the entire SRF cavity. In addition it is widely observed that the quality factor (Q) of SRF cavities falls dramatically with increasing surface RF magnetic field, particularly as the quench field is approached [3]. A detailed microscopic understanding of this Q drop is lacking but is most likely related to the inevitable defects on the inner surface of bulk Nb cavities [1] [2]. These defects may be either nonsuperconducting or have lower critical temperature (T c ), making them sources of dissipation which interrupt the superconducting current flow [4]. Unfortunately, it is difficult to totally remove all of these defects even after sophisticated physical and chemical treatments. Because different defects each have their own quench limit, not all of the defects behave as sources of quenching under given cavity operation conditions. Therefore it is necessary to quantitatively understand the relation between physical defects and their individual electromagnetic properties such as local critical field.Currently, in order to isolate specific defects in SRF cavities, most measurements utilize a complete accelerator cavity [5] and image the quenching hot spots at high accelerating gradient via a thermometer array attached to the outside wall of the cavity [2]. Such measurements reveal the locations of quench sites only to within a few millimeters, far larger than the defect itself, making their unique identification difficult. Microscopic techniques have not been used in the past because traditionally it is difficult to create strong and localized RF magnetic fields at the cavity operating frequency (several GHz) [6][7]. Our idea is to look at the active surface of SRF cavities with a very intense and localized magnetic field (on the order of the thermodynamic critical field of bulk Nb), and directly measure the local electromagnetic response in the cavity operating regime a...

Time-dependent Ginzburg-Landau treatment of rf magnetic vortices in superconductors: Vortex semiloops in a spatially nonuniform magnetic field

“…Eqs. (24) and (25) were enforced on the spherical superconductor-vacuum boundary (r = 5λ 0 ) in both cases. When the two-domain method was used, the TDGL equations were solved in the inner sphere (r < 5λ 0 ) and only Maxwell's equations were solved in the vacuum domain (5λ 0 < r < 20λ 0 ).…”

“…For large parallel surface rf magnetic fields and a point-like surface defect, a vortex first enters the superconductor as a vortex semiloop. To study the dynamics of these vortex-semiloops a novel near-field magnetic microwave microscope was successfully built using a magnetic writer from a conventional magnetic recording hard-disk drive [20][21][22][23][24][25][26]. A magnetic write head can produce B RF ≈ 600mT rf magnetic field localized to a ≈ 100nm length scale [76].…”

“…In the experiment, a Seagate perpendicular magnetic writer head is attached to a cryogenic XYZ positioner and used in a scanning probe fashion. Probe characterization results and other details can be found in [22][23][24][25][26]. The probe produces an rf magnetic field perpendicular to the sample surface.…”

“…This phenomenon cannot be simulated unless the effects of the screening currents and fields on the superconducting order parameter are included self-consistently. This work is motivated by results for third-harmonic generation from a near-field microwave microscope utilized on Nb surfaces [20][21][22][23][24][25][26]. In this experiment, a magnetic writer probe from a conventional magnetic recording hard-disk drive is used to create a high-intensity, localized, and inhomogeneous rf magnetic field on the surface of a Nb superconducting sample.…”

Direct mapping of the electric permittivity of heterogeneous non-planar thin films at gigahertz frequencies by scanning microwave microscopy

“…13,14 In addition, SMM has also been applied to the study of complex oxides, 15 graphene, 16,17 carbon nanotubes, 18 doped semiconductors, 19 and superconductors. 20 Furthermore, SMM will contribute to the emerging field of high frequency nanoelectronic devices, where there is the demand of on-wafer measurement systems sensitive to the microwave electromagnetic properties of dielectric materials. 21 In spite of the large number of successful applications of the SMM, a main challenge still remains, namely, the difficulty in mapping the electric permittivity of heterogeneous samples exhibiting large height variations.…”