Effect of Thermal Strain, Induced by Cryogenic Cooling, on a High Homogeneity Superconducting Magnet for MRI Applications

The heart of the modern whole body MRI scanners is a superconducting magnet producing the required magnetic field with high homogeneity. The superconducting magnet operates at 4.2 K (-268.95 oC) and thus is kept dipped in liquid helium to maintain its superconducting state. This cooldown of the multi-coil magnet from room temperature to the LHe temperature generates thermal strains in the magnet structure which deforms the magnet. These deformations are found to affect the final magnetic field homogeneity and introduce artifacts in the MR images. We report our results on studies on the stresses induced in the bobbin and the coils consequent upon cooling the magnet to 4.2 K. The maximum von Mises stress in the coils is calculated to be 31.3 MPa while in the bobbin it comes out to be 60 MPa. We find that the cooldown causes a relative movement of the coils which in turn degrades the field homogeneity from 5.5 ppm to 278 ppm. The centre field is found to increase by 60 G is caused by a reduction in the overall cross-section of the coils. The change in homogeneity is also analysed in terms of Legendre polynomials where we found that the relative displacements of the coils introduce odd order terms to the polynomial expansion terms which were not present in the original design

Chemical Mechanical Planarization for Ta-based Superconducting Quantum Devices

We report on the development of a chemical mechanical planarization (CMP) process for thick damascene Ta structures with pattern feature sizes down to 100 nm. This CMP process is the core of the fabrication sequence for scalable superconducting integrated circuits at 300 mm wafer scale. This work has established the elements of the various CMP-related design rules that can be followed by a designer for the layout of circuits that include Ta-based coplanar waveguide resonators, capacitors, and interconnects for tantalum-based qubits and single flux quantum (SFQ) circuits. The fabrication of these structures utilizes 193 nm optical lithography, along with 300 mm process tools for dielectric deposition, reactive ion etch, wet-clean, CMP and in-line metrology, all tools typical for a 300 mm wafer CMOS foundry. Process development was guided by measurements of physical and electrical characteristics of the planarized structures. Physical characterization such as atomic force microscopy across the 300 mm wafer surface showed local topography was less than 5 nm. Electrical characterization confirmed low leakage at room temperature, and less than 12% within wafer sheet resistance variation, for damascene Ta line-widths ranging from 100 nm to 3 {\mu}m. Run-to-run reproducibility was also evaluated. Effects of process integration choices including deposited thickness of Ta are discussed.Comment: 31 pages, 16 figure

Engineering of Niobium Surfaces Through Accelerated Neutral Atom Beam Technology For Quantum Applications

A major roadblock to scalable quantum computing is phase decoherence and energy relaxation caused by qubits interacting with defect-related two-level systems (TLS). Native oxides present on the surfaces of superconducting metals used in quantum devices are acknowledged to be a source of TLS that decrease qubit coherence times. Reducing microwave loss by surface engineering (i.e., replacing uncontrolled native oxide of superconducting metals with a thin, stable surface with predictable characteristics) can be a key enabler for pushing performance forward with devices of higher quality factor. In this work, we present a novel approach to replace the native oxide of niobium (typically formed in an uncontrolled fashion when its pristine surface is exposed to air) with an engineered oxide, using a room-temperature process that leverages Accelerated Neutral Atom Beam (ANAB) technology at 300 mm wafer scale. This ANAB beam is composed of a mixture of argon and oxygen, with tunable energy per atom, which is rastered across the wafer surface. The ANAB-engineered Nb-oxide thickness was found to vary from 2 nm to 6 nm depending on ANAB process parameters. Modeling of variable-energy XPS data confirm thickness and compositional control of the Nb surface oxide by the ANAB process. These results correlate well with those from transmission electron microscopy and X-ray reflectometry. Since ANAB is broadly applicable to material surfaces, the present study indicates its promise for modification of the surfaces of superconducting quantum circuits to achieve longer coherence times.Comment: 22 pages, 7 figures, will be submitted to Superconductor Science and Technology Special Focus Issue Journa

Enhanced UV photosensitivity from rapid thermal annealed vertically aligned ZnO nanowires

We report on the major improvement in UV photosensitivity and faster photoresponse from vertically aligned ZnO nanowires (NWs) by means of rapid thermal annealing (RTA). The ZnO NWs were grown by vapor-liquid-solid method and subsequently RTA treated at 700°C and 800°C for 120 s. The UV photosensitivity (photo-to-dark current ratio) is 4.5 × 103 for the as-grown NWs and after RTA treatment it is enhanced by a factor of five. The photocurrent (PC) spectra of the as-grown and RTA-treated NWs show a strong peak in the UV region and two other relatively weak peaks in the visible region. The photoresponse measurement shows a bi-exponential growth and bi-exponential decay of the PC from as-grown as well as RTA-treated ZnO NWs. The growth and decay time constants are reduced after the RTA treatment indicating a faster photoresponse. The dark current-voltage characteristics clearly show the presence of surface defects-related trap centers on the as-grown ZnO NWs and after RTA treatment it is significantly reduced. The RTA processing diminishes the surface defect-related trap centers and modifies the surface of the ZnO NWs, resulting in enhanced PC and faster photoresponse. These results demonstrated the effectiveness of RTA processing for achieving improved photosensitivity of ZnO NWs

Research Trends in Library and Information Science: Analysis of ALIS and DJLIT

The study analyses the research articles published in two prestigious Indian LIS journals, Annals of Library and Information Studies (ALIS) and DESIDOC Journal of Library and Information Technology (DJLIT) to identify the trend of research in the Library and Information Science from 2009 to 2013. Bibliographic information of published research articles has been collected from the journals directly, as the journals are open access (OA) in nature. Bibliometric tools and techniques have been employed to analyze the data. The study shows that, the selected two OA LIS journals have published 457 research articles together during the study period (2009-2013) of which DJLIT published 280 (61.27%) research items followed by ALIS (177). The study revealed that the contemporary LIS researchers are concentrating on 22 research domains and these are Scientometrics (71), User study (55), Knowledge Organization (45), Bibliometrics (44), Webometrics (22), ICT application in Library (21), Digital library (20), IPR (19) and Information Retrieval (17), etc. Worthy to mention that, amongst the subject areas, scientometrics is leading with 15.53% of the total articles. The authorship pattern and citation analysis are found to be the core research areas in scientometrics. Research on visualization methods and scientific mapping have been emphasized by the community and becoming the core research areas of scientometrics

Effect of Thermal Strain, Induced by Cryogenic Cooling, on a High Homogeneity Superconducting Magnet for MRI Applications

581-585The heart of the modern whole body MRI scanners is a superconducting magnet producing the required magnetic field with high homogeneity. The superconducting magnet operates at 4.2 K (-268.95 oC) and thus is kept dipped in liquid helium to maintain its superconducting state. This cool down of the multi-coil magnet from room temperature to the LHe temperature generates thermal strains in the magnet structure which deforms the magnet. These deformations are found to affect the final magnetic field homogeneity and introduce artifacts in the MR images. We report our results on studies on the stresses induced in the bobbin and the coils consequent upon cooling the magnet to 4.2 K. The maximum von Mises stress in the coils is calculated to be 31.3 MPa while in the bobbin it comes out to be 60 MPa. We find that the cool down causes a relative movement of the coils which in turn degrades the field homogeneity from 5.5 ppm to 278 ppm. The centre field is found to increase by 60 G is caused by a reduction in the overall cross-section of the coils. The change in homogeneity is also analysed in terms of Legendre polynomials where we found that the relative displacements of the coils introduce odd order terms to the polynomial expansion terms which were not present in the original design

Task-specific representation learning for web-scale entity disambiguation

by Rijula Kar, Susmija Reddy, Sourangshu Bhattacharya, Anirban Dasgupta and Soumen Chakrabart

Evaluation of different consolidation methods for nano-materials

343-346In the present investigation the effect of high voltage and low frequency current pulses on Ni powder compacts and on alloys like Ni₈₀Si₂₀ or Ni₉₅Si₅ are investigated. The pulse application on ball milled Ni powder compacts and on Ni₈₀Si₂₀ result in a further increase in density. This suggests that the SPS and EDC technique can be used as an alternative method for the sintering of nano-materials and for the densification of nano particles but with the simultaneous application of pressure