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Limitations of magnetic probe measurements in pulsed discharges Tam, Yun-Kwong Sebastian

Abstract

A "gradient probe" consisting of two search coils has been developed to measure the current density in a pulsed plasma. This probe measures both the magnitude and the gradient of the magentic field simultaneously enabling more accurate measurements than the conventional magnetic probe which has only one coil. It has been used to measure the current densities and the magnetic fields in z-pinch discharges in helium at pressures between 500μ and 4 mmHg. The collapse curves obtained agreed with the predictions of a modified snow-plow equation which allowed for the loss of particles from the collapsing current shell. The flow of current in the plasma is distorted by the presence of a probe. Such an effect spoils the spatial resolution so that the measured values of the current density Jp are averages of the true current density J₀ over a finite region. To investigate this, a correction formula which relates Jp to J₀ has been developed. Our error analyses showed that any scatter in Jp due to experimental errors was magnified twenty times in J₀. For a pulsed plasma, therefore, one should try to reduce the perturbation of the probe instead of relying on the correction procedure.

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