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Optimization of magnetic probe measurements in transient plasmas Pachner, Jaroslav

Abstract

A significant improvement in magnetic probe measurements has been achieved by developing a 3-coil magnetic probe which partially corrects for the boundary error, i.e. for the error caused by an exclusion of the plasma current from the space occupied by the probe. The spatial resolution C of the three coil probe is roughly one-half of the probe radius, a. For current distributions which vary slowly with distance (scale length, ƛ ) it is shown that the fractional error in the magnetic field is 0.2 (C/ ƛ)². For a conventional probe the error is at least four times as large. Also it has been shown that spurious signals arising from poor probe geometry (and which often obscure the signals produced by the measured magnetic fields themselves) can be eliminated by making use of the symmetry of the discharge fields. Measurements have been made on a Z-pinch discharge which confirm the claimed 3-coil probe performance by revealing a "fine" structure of the current distribution in helium in the filling pressure range 0.5 to 4 Torr. The measurements are presented in the form of a catalogue of the spatial distribution of magnetic field and the current density for a Z-pinch discharge in He and Ar. Using the probe measurements a qualitative model of the collapse of the current sheet for 4 Torr in He is developed which differs from previous models.

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