Plasmadiagnostik mit Emittierenden Sonden Durch Ausnutzung der Örtlichen Störung des Ionisationsgleichgewichts

The Use of Emissive Probes in Laboratory and Tokamak Plasmas

“…This we also see in Nevertheless, this is a surprising result since, as briefly mentioned in section 2, the general opinion was up to now that for T e /T em > 10 the floating potential of a strongly emissive probe would stay below the plasma potential due to a space charge built up by the emitted low temperature electrons around the probe [5,10,11,12,13,14]. This effect was probably first presumed in [17] as the possible cause for this deviation. Recent theoretical [13] and experimental [15] investigations, supported by computer simulations, came to the conclusion that it depends critically on the value T e /T em whether or not V fl,em agrees with Φ pl .…”

“…1a). This effect was described already by Klagge [17] (see also [18]), who also discusses its usefulness on the determination of the plasma potential and for gaining information on the ionization conditions and rates of the gas.…”

Emissive probes

“…The first is to simply average the low and high plasma potentials (points D and E, respectively), just as can be done in low neutral pressure systems. Additionally, the average plasma potential can be determined by taking the average of the potentials at which probe induced ionization begins (points B and G) [38,83,84]. The two methods agree with each other and both provide a good measure of the average plasma potential.…”

“…The current in region AB is due to two effects. One is the temperature-limited emission current and the other is ionization near the probe from emission, which occurs during the part of the RF cycle when the difference between the bias voltage and the plasma potential is larger than the ionization potential [84]. In region BC there is no ionization current, so only the temperature-limited emission current affects this region.…”

Accurate space potential measurements using an emissive probe in high pressure plasma and neutral gas

“…[20,[23][24][25] With the realization of an automatic emissive probe apparatus which can automatically execute the conventionally cumbersome procedure of the improved inflection point method, the accuracy of the method in plasma potential measurements has been greatly improved to be 0.1 V, demonstrated by the measurement of vacuum space potential distribution between two parallel plates. [20,24,25] Although it has been proved that the emissive probe can provide an accurate measurement of the plasma potential at low pressure (≤1.33 Pa), there are few reports about the plasma potential measurement at a pressure more than 1.33 Pa. [26][27][28][29] As the pressure increased, the reliable measurement of the plasma potential is more challenging because that the additional ionization of the neutral particles caused by the increase of the collision rate between the emitted electrons and the neutral particles can inevitably perturb the local plasma. [28,30] Different from the emissive probe I-V characteristics obtained at low pressure, it has been validated that there are two inflection points in the I-V traces obtained at high pressure due to the extra ionization caused by the emitted electrons.…”