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Dec 2007

Volume 78, Issue 12, Articles (12xxxx)

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Rev. Sci. Instrum. 78, 121301 (2007); http://dx.doi.org/10.1063/1.2821148 (15 pages)

A. Westphalen, M.-S. Lee, A. Remhof, and H. Zabel
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A differential double-coil inductive transducer for measuring electrical conductivity

Jozef Kusmierz

Rev. Sci. Instrum. 78, 124701 (2007); http://dx.doi.org/10.1063/1.2804166 (5 pages)

Online Publication Date: 4 December 2007

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A differential system of two double-coil inductive transducers for the contactless measurement of the electrical conductivity of conducting materials has been presented. The differential system can be employed in applications that require smaller measurement uncertainty than that provided by the single-transducer system. A mathematical model of the differential system is based on the model of a single double-coil inductive transducer; in this case, a so-called processing function is defined as a ratio of voltages at the measurement coil terminals with and without the test sample. The relative differential voltage of the differential system is derived as a difference of processing functions of two single transducers and depends on a relative difference between conductivities of the test and reference samples. The conductivity of the test sample is obtained either using precalculated graphs or by numerically processing the equation of the differential voltage. In order to verify the obtained theoretical results, experimental investigations have been carried out using a computer-controlled measurement system with the differential system of the transducers. The conductivity measurements have been carried out using samples made of aluminum rods. During the measurements, the temperature of the reference sample was equal to room temperature (20 °C), whereas the temperature of the test sample was changed in the range of 0–20 °C to obtain the conductivity variation. The obtained experimental results confirmed the accuracy of the theoretical model of the differential transducer.
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07.07.Mp Transducers
72.10.-d Theory of electronic transport; scattering mechanisms
84.37.+q Measurements in electric variables (including voltage, current, resistance, capacitance, inductance, impedance, and admittance, etc.)

Nanosecond electro-optical switching with a repetition rate above 20 MHz

Holger Müller, Sheng-wey Chiow, Sven Herrmann, and Steven Chu

Rev. Sci. Instrum. 78, 124702 (2007); http://dx.doi.org/10.1063/1.2822101 (4 pages) | Cited 5 times

Online Publication Date: 11 December 2007

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We describe an electro-optical switch based on a commercial electro-optic modulator (modified for high-speed operation) and a 340 V pulser having a rise time of 2.2 ns (at 250 V). It can produce arbitrary pulse patterns with an average repetition rate beyond 20 MHz. It uses a grounded-grid triode driven by transmitting power transistors. We discuss variations that enable analog operation, use the step-recovery effect in bipolar transistors, or offer other combinations of output voltage, size, and cost.
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42.79.Ta Optical computers, logic elements, interconnects, switches; neural networks
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Method for nonlinear characterization of radio frequency coils made of high temperature superconducting material in view of magnetic resonance imaging applications

Olivier Girard, Jean-Christophe Ginefri, Marie Poirier-Quinot, and Luc Darrasse

Rev. Sci. Instrum. 78, 124703 (2007); http://dx.doi.org/10.1063/1.2825241 (7 pages) | Cited 1 time

Online Publication Date: 28 December 2007

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A contactless method based on reflectometry to accurately characterize an inductive radio frequency (rf) resonator even in the occurrence of a strong electrical nonlinearity is presented. Nonlinear extraction of the unloaded quality factor and resonance frequency is possible by combining an initial low-level swept-frequency calibration with high-level single-frequency measurements. The extraction protocol relies on a simple intrinsic R, L, C model and does not involve a fitting procedure according to a particular nonlinearity model. It includes a correction for strong coupling conditions between the probe and the rf coil, which allows extending the analysis over a wide range of transmitted power. Electrical modeling based on the extracted intrinsic data allows predicting the coil behavior when loaded by any kind of matching network. The method will have implications in different domains such as Magnetic Resonance (MR) applications with superconducting probe heads or analysis of rf properties in nonlinear materials. The method is demonstrated here by characterizing a high temperature superconducting (HTS) coil dedicated to MR imaging at 64 MHz. The coil consists in a multiturn spiral design that is self-resonant close to the MR frequency of interest. The Q factor and the resonance frequency are determined as a function of the actual power dissipated in the HTS coil accounting for losses occurring in the measurement system. Further characteristics of the HTS coil are considered in the present paper. The relation between the transmitted power and the magnetic field generated by the coil, which is the most relevant characteristics for MR applications, is directly accessible. The equivalent impedance of the coil under test is also expressed as a function of the total current flowing in the windings. The method could be extended to assess the fundamental properties of the nonlinear material (e.g., the London penetration depth or the critical current density) by including any pertinent model.
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84.71.Ba Superconducting magnets; magnetic levitation devices
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