The performance of superconducting radiofrequency (SRF) cavities used for particle accelerators depends on two characteristic material parameters: field of first flux entry Hentry and pinning strength. The former sets the limit for the maximum achievable accelerating gradient, while the latter determines how efficiently flux can be expelled related to the maximum achievable quality factor. In this paper, a method based on muon spin rotation (µSR) is developed to probe these parameters on samples. It combines measurements from two different spectrometers, one being specifically built for these studies and samples of different geometries. It is found that annealing at 1400 • C virtually eliminates all pinning. Such an annealed substrate is ideally suited to measure Hentry of layered superconductors, which might enable accelerating gradients beyond bulk niobium technology.Recently, to reach high quality factors, a treatment procedure has been established baking cavities at 800 • C and injecting nitrogen gas at the end of this treatment. arXiv:1705.05480v3 [cond-mat.supr-con]
Spiral array transducers with a sparse 2D aperture have demonstrated their potential in realizing 3D ultrasound imaging with reduced data rates. Nevertheless, their feasibility in high-volume-rate imaging based on unfocused transmissions has yet to be established. From a metrology standpoint, it is essential to characterize the acoustic field of unfocused transmissions from spiral arrays not only to assess their safety, but also to identify the root cause of imaging irregularities due to the array's sparse aperture. Here, we present a field profile analysis of unfocused transmissions from a density-tapered spiral array transducer (256 hexagonal elements; 220 μm element diameter; 1 cm aperture diameter) through both simulations and hydrophone measurements. We investigated plane-and diverging-wave transmissions (5-cycle, 7.5 MHz pulses) from 0° to 10° steering for their beam intensity characteristics and wavefront arrival time profiles. Unfocused firings were also tested for B-mode imaging performance (10 compounded angles, -5° to 5° span). The array was found to produce unfocused transmissions with a peak negative pressure of 93.9 kPa at 2 cm depth. All transmissions steered up to 5° were free of secondary lobes within 12 dB of the main beam peak intensity. All wavefront arrival time profiles were found to closely match the expected profiles with maximum rootmean-squared errors of 0.054 μs for plane waves and 0.124 μs for diverging waves. The B-mode images showed good spatial resolution with a penetration depth of 22 mm in plane-wave imaging. Overall, these results demonstrate that the densitytapered spiral array can facilitate unfocused transmissions below regulatory limits (mechanical index: 0.034; spatial-peak, pulseaverage intensity: 0.298 W/cm 2 ) and with suppressed secondary lobes while maintaining smooth wavefronts.
High-volume-rate ultrasound imaging enables the rendering of complex flow dynamics in 3-D; however, high data rates pose significant hurdles towards real-time implementation. A sparsely populated 256-element 2-D spiral array coupled with unfocused transmissions is a potential solution to these challenges. This work analyzes the impact of sparse element distribution on flow velocity estimation with spherical waves (virtual point source 20mm behind the probe) to extend the field of view beyond the array footprint. Field II was used to simulate straight tube flow (6mm-diameter; 45° tilt; 30cm/s plug-flow; Transmission: 7.5 MHz, 3-cycle) to analyze the velocity estimation bias without the influence of surrounding tissue. It was found that the bias at the inlet, mid-section, and outlet were 20.3%, 8.7%, and 4%, respectively; this variation in bias is due to the change in beam-flow angle from 57.8 deg to 34.9 deg. To evaluate the wide-angle and grating-lobe performance of this configuration, the median Doppler power was compared between the center flow region, flow at the extended regions, and the grating lobe region; the relative power to the center flow region was found to be -12.3dB and -19.5dB for extended and grating-lobe regions respectively. These results suggest that the flow estimation region can be extended beyond the array footprint area but extra considerations on the beam-flow angle must be taken, especially in the case of vector flow.
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