45,395 research outputs found
An equivalent-effect phenomenon in eddy current non-destructive testing of thin structures
The inductance/impedance due to thin metallic structures in non-destructive
testing (NDT) is difficult to evaluate. In particular, in Finite Element Method
(FEM) eddy current simulation, an extremely fine mesh is required to accurately
simulate skin effects especially at high frequencies, and this could cause an
extremely large total mesh for the whole problem, i.e. including, for example,
other surrounding structures and excitation sources like coils. Consequently,
intensive computation requirements are needed. In this paper, an
equivalent-effect phenomenon is found, which has revealed that alternative
structures can produce the same effect on the sensor response, i.e. mutual
impedance/inductance of coupled coils if a relationship (reciprocal
relationship) between the electrical conductivity and the thickness of the
structure is observed. By using this relationship, the mutual
inductance/impedance can be calculated from the equivalent structures with much
fewer mesh elements, which can significantly save the computation time. In eddy
current NDT, coils inductance/impedance is normally used as a critical
parameter for various industrial applications, such as flaw detection, coating
and microstructure sensing. Theoretical derivation, measurements and
simulations have been presented to verify the feasibility of the proposed
phenomenon
Angular and Polarization Response of Multimode Sensors with Resistive-Grid Absorbers
High sensitivity receiver systems with near ideal polarization sensitivity
are highly desirable for development of millimeter and sub-millimeter radio
astronomy. Multimoded bolometers provide a unique solution to achieve such
sensitivity, for which hundreds of single-mode sensors would otherwise be
required. The primary concern in employing such multimoded sensors for
polarimetery is the control of the polarization systematics. In this paper, we
examine the angular- and polarization- dependent absorption pattern of a thin
resistive grid or membrane, which models an absorber used for a multimoded
bolometer. The result shows that a freestanding thin resistive absorber with a
surface resistivity of \eta/2, where \eta\ is the impedance of free space,
attains a beam pattern with equal E- and H-plane responses, leading to zero
cross polarization. For a resistive-grid absorber, the condition is met when a
pair of grids is positioned orthogonal to each other and both have a
resistivity of \eta/2. When a reflective backshort termination is employed to
improve absorption efficiency, the cross-polar level can be suppressed below
-30 dB if acceptance angle of the sensor is limited to <60degrees. The small
cross-polar systematics have even-parity patterns and do not contaminate the
measurements of odd-parity polarization patterns, for which many of recent
instruments for cosmic microwave background are designed. Underlying symmetry
that suppresses these cross-polar systematics is discussed in detail. The
estimates and formalism provided in this paper offer key tools in the design
consideration of the instruments using the multimoded polarimeters.Comment: 22 pages, 15 figure
Cluster: A fleet of four spacecraft to study plasma structures in three dimensions
The four Cluster spacecraft are spin stabilized spacecraft which are designed and built under stringent requirements as far as electromagnetic cleanliness is concerned. Conductive surfaces and low electromagnetic background noise are mandatory for accurate electric field and cold plasma measurements. The mission is implemented in collaboration between ESA and NASA. A Russian mission will be closely coordinated with Cluster
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