Dc and ac currents to a rf resonance probe were investigated experimentally in the frequency range from 1 to 100 Mc/s in a quiescent Cesium plasma. The measurements in the plasma are explained by calculations and measurements with a lumped-constant equivalent network (“plasma simulator”).
It is shown that the ac current to the probe is strongly influenced by the capacitance of the cable leading to the probe. A device to compensate this cable-capacitance is developed and used. Furthermore, it is shown for the first time that the double (or devided) main resonance maximum, occurring frequently with dc and ac currents to the resonance probe, is generally due to the influence of a second reference electrode.
A simple and direct (“focus”) method for very precise plasma density determination (from the intersection point of the rf-current-vs.-frequency curves with probe voltage as parameter) is developed and compared with ac transmission current and LANGMUIR probe measurements at plasma densities between 2 × 106 and 3 × 107 cm–3. Agreement is obtained between these three experimental methods within their margins of error. The comparison leads to a better understanding of the measured LANGMUIR characteristic and of the Cs-plasma of the “Alma I” device.
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