A comparison of emissive probe techniques for electric potential measurements in a complex plasma

Abstract

The major emissive probe techniques are compared to better understand the floating potential of an electron emitting surface in a plasma. An overview of the separation point technique, floating point technique, and inflection point in the limit of zero emission technique is given, addressing how each method works as well as the theoretical basis and limitations of each. It is shown that while the floating point method is the most popular, it is expected to yield a value ∼1.5Te/e below the plasma potential due to a virtual cathode forming around the probe. The theoretical predictions were checked with experiments performed in a 2 kW annular Hall thruster plasma (ne ∼ 109−1010 cm−3and Te ∼ 10−50 eV). The authors find that the floating point method gives a value around 2Te/e below the inflection point method, which is shown to be a more accurate emissive probe technique than other techniques used in this work for measurements of the plasma potential.

Extraction of negative charges from an ion source: Transition from an electron repelling to an electron attracting plasma close to the extraction surface

“…However, it is known that the determination of Φ pl with a single probe can be erroneous. 24 In order to estimate the error bar of the determined plasma potential Φ pl , the voltage difference between the maximum and the minimum of d 2 I/dU 2 around the zero crossing is used, which is 2 V for all measurements. Thus, the error bar is ∆Φ pl = ±1 V. For the comparison of probe measurements with the source performance it should be kept in mind that the parameters determined with the Langmuir probe are measured locally whereas the electrically measured during the full pulse length for all pulses shown in this paper.…”

Plasma potential mapping of high power impulse magnetron sputtering discharges

“…[26][27][28][29][30][31][32][33][34][35] As the name suggests, an emissive probe is sufficiently hot to emit electrons via the thermionic emission mechanism, where the current density is approximately described by the Richardson-Dushman equation (cf. e.g.…”

Transient plasma potential in pulsed dual frequency inductively coupled plasmas and effect of substrate biasing

“…[25][26][27] The details of working principle and design of the emissive probe can be found elsewhere. 21,23,[25][26][27] The present study investigates the temporal evolution of plasma potential and the effect of substrate biasing on it, in the pulsed discharges produced by a dual-frequency/dual-antenna ICP source by using an emissive probe. The plasma potential has been measured using a Tektronix voltage probe TPP0201 (input impedance of 10 MΩ, oscilloscope impedance).…”

“…The magnitude of associated error can be determined by relative magnitude of plasma potential to electron temperature (eV p /T e ). 21 If the electron temperature is high enough and close to plasma potential, this offset in plasma potential can be significant. In the present experimental study, typical plasma potential, during pulse 'on-time', is ∼40 V and the electron temperature measured by Langmuir probe (not shown here) is ∼4 eV, thererefore the offset in plasma potential due to error associated with emissive probe technique in floating point method is not significant.…”

“…This technique can be used in three different ways, that is, separation point mode, inflection point mode, and the floating point (saturated floating potential) mode. 21 Among these modes, due to inherent simplicity in using and real time measurements of time resolved plasma potential, the floating point technique has been extensively used, particularly in pulsed plasmas. [25][26][27][28][29] The working principal of emissive probe in this mode of operation is as follows.…”