Studies On Tunable Radio-frequency Magnetic Probe

Magnetic probes (B-dot) have been widely used to measure the time varying magnetic flux density in the experiment because of the simple structure. However, the common mode interference signals come from the capacitive coupling always exist in the actual measurement of magnetic probe, and the magnetic induction signal decreases with the reduce of the effective probe coil area in the high spatial resolution measurements of the magnetic field. Thus, it is necessary to increase the output voltage of magnetic probes.A tunable center-tapped transformer is proposed to improve the output voltage of a radio-frequency magnetic probe. The tuning is implemented by using two variable capacitors: one is parallel with the primary winding of the center-tapped transformer and the other is in series with the secondary winding. In addition, a planar faraday shield consisting of two separate slotted-metal-plates is installed between the primary and secondary windings of the transformer to further suppress the electrostatic coupling. It is found that the electrostatic coupling is suppressed effectively by using the center-tapped and the faraday shield. Tuning the parallel capacitor or the series capacitor can result in a resonance in the output voltage of the rf magnetic probe. The parallel-connected variable capacitor (without the series capacitor) is better than the series-connected variable capacitor (without the parallel capacitor) in improving the tunable magnetic probe output. The largest output voltage, achieved with the tunable magnetic probe under the optimal condition, is higher than that with a conventional one by an order of magnitude. The resonance output voltage presents a non-monotone behavior with the length of the coaxial, namely it obtains minimums at the middle length. Influences of parameters such as the inductance of the primary and secondary windings on the output voltage, the mutual inductance between the primary and secondary windings are also presented.A circuit model of magnetic probe with the theory of mutual circuit and the transmission line effect, and the resonant condition is analytically. A maximum output voltage, the circuit is not at a simple parallel resonance state, but is determined by the system impedance including the impedance of transmission line, and the impact of the conditions of the resonance parameters are analytically. The agreements between the numerical results and measurements are qualitatively good. The voltage of transformer's primary coils and the output voltage reached the maximum with the parallel variable capacitor, but different with the series variable capacitor. Without the variable capacitor, the output voltage also exist a resonance phenomenon with the variation of the length of the coaxial, and shows a periodical change. When with the variable capacitor, the resonance output voltage presents a non-monotone behavior with the length of the coaxial, namely it obtains minimums at the middle length. However, the relative tuning capacitor is decrease with the increase of the length of the coaxial, and also shows a periodical change. The resonance output voltage of parallel-connected capacitor presents a non-monotone behavior with the mutual inductance, namely it obtains maximum at the middle stage, but the series-connected is increase with the mutual inductance. Influences of parameters such as the inductance of the primary and secondary windings on the resonance output voltage and the tuning capacitor are also presented.