A simple miniature magnetic probe with inherent electrostatic rejection

Inductive magnetic probes are often used to measure oscillating magnetic fields in plasma.r3 The sensitivity of a magnetic probe to electrostatic potentials (“capacitive pickup”) is usually an important consideration. Centertapped construction of such a probe, shown in Fig. 1 (b), helps to minimize capacitive pickup with respect to a simply wound probe [Fig. 1 (a)]. This relies on the fact that inductive pickup changes sign when a measurement coil is rotated 180” while capacitive pickup does not. The two signals from such a probe are then subtracted, giving twice the inductive signal and greatly reducing any capacitive signal. The subtraction usually takes place at some remote distance from the probe tip. We present a modification of this basic design as shown in Fig. 1 fc). Winding A is grounded at one end Fig. 1 (d)] and provides the signal to a 50-Q system at its other end. Winding 3, wound bifilar with winding A, is grounded at the opposite end to winding A. The other end of winding B is connected to ground through a small 50-a resistor so that this winding has the same load impedance as winding A. This provides a system where a centertapped coil and a subtraction transformer are effectively one. The design remains effectively “center-tapped,” with one of the windings shown in Fig. 1 (b) moved so as to totally overlap the other.4 Only one of the windings, however, contributes inductive signal [winding A in Fig. 1 (c)l. Capacitive pickup creates currents in opposite directions in windings A and 3. The induced magnetic field from the capacitive currents in winding 3 therefore opposes that in winding A. The close coupling between the two windings, with the mutual inductance equaling the self-inductance, thus acts to subtract capacitive signals from winding A, reducing capacitive pickup in comparison with a single winding. If necessary an extra turn can be wound on winding B to compensate for less than ideal coupling. As with all inductive magnetic probes, w <R/L, where R ( = 50 ti) is the impedance of the system, L is the inductance of the probe windings, and o is the desired frequency response. This design eliminates both the need to propagate two signals from the probe tip and the need for a subtracting transformer or differential amplifier. Construction is simplified by the single signal design, allowing the use of a