Emissive Probe Diagnostics in Low Temperature Plasma - Effect of Space Charge and Variations of Electron Saturation Current (original) (raw)
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Electric probes for plasmas: The link between theory and instrument
Review of Scientific Instruments, 2002
Electric probe methods for diagnostics of plasmas are reviewed with emphasis on the link between the appropriate probe theories and the instrumental design. The starting point is an elementary discussion of the working principles and a discussion of the physical quantities that can be measured by the probe method. This is followed by a systematic classification of the various regimes of probe operation and a summary of theories and methods for measurements of charged particle distributions. Application of a single probe and probe clusters for measurements of fluid observables is discussed. Probe clusters permit both instantaneous and time-averaged measurements without sweeping the probe voltage. Two classes of applications are presented as illustrations of the methods reviewed. These are measurements of cross sections and collision frequencies (plasma electron spectroscopy), and measurements of fluctuations and anomalous transport in magnetized plasma.
Czechoslovak Journal of Physics, 2006
We report on experimental investigations of the change of the electron saturation current of a dc-heated emissive probe with the probe heating current. According to the simple theory of the emissive probe, the electron saturation current should not be affected by emission. However, in many experiments a variation of the electron saturation current with the emission current was observed. We consider two possible reasons for such variations: (a) the influence of the space charge around the probe shaft, (b) the change of the work function of the probe surface material due to heating. We tried to find sufficient experimental evidence for supporting one or the other (or both) of these two explanations. We used two different types of plasma to validate the results: a cylindrical magnetron plasma and the non-magnetized plasma of a DP machine. From our experiments follows that the electron saturation current of the emissive probe is affected by the space charge effect as well as it depends on the probe wire material.
Measurements with the emissive probe in the cylindrical magnetron
Czechoslovak Journal of Physics, 2006
This contribution describes the results of the measurements that were made with emissive probe in the argon dc discharge in the cylindrical magnetron. Main part of this emissive probe — the tungsten wire loop — was heated by the dc current to raise its temperature up to that allowing to reach the requested emission current. With such a probe we measured the dependence of floating potential on the heating current or voltage, I–V characteristics of the probe and radial distribution of potential between anode and cathode. Main goal of the measurements was the evaluation of plasma potential inside the magnetron and finding evidence, whether these methods bring us advantage for plasma potential estimation with respect to commonly used Langmuir probe method. For that purpose we compared several ways used for obtaining the plasma potential: (i) from the inflection point of the cold probe characteristic (ii) from the floating potential of sufficiently heated emissive probe (iii) from the abscissa of the crossing point of the cold and emissive probe characteristic (iv) from the ionization potential of the discharge gas (argon).
Use of emissive probes in high pressure plasma
Review of Scientific Instruments, 1996
The characteristics of emissive probes in unmagnetized high pressure ͑р1 Torr͒ argon and helium plasmas, produced by inductively coupled 13.56 MHz rf power, are studied. A procedure is given for interpreting emissive probe current-voltage ͑I-V͒ characteristics. The I-V curves indicate the amplitude of the rf fluctuation of the plasma potential. They also show ionization near the emissive probe when the potential drop between the emissive probe and the plasma potential is more than the ionization potential. Experiments show that when the temperature of the emissive probe wire and/or the neutral pressure is increased, ionization becomes significant. An increase in the local ion density due to the additional ionization was demonstrated by the I-V curves of an emissive probe and a nearby Langmuir probe. A simple procedure is presented for interpreting these results.
A comparison of emissive probe techniques for electric potential measurements in a complex plasma
Physics of Plasmas, 2011
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/ebelowtheplasmapotentialduetoavirtualcathodeformingaroundtheprobe.Thetheoreticalpredictionswerecheckedwithexperimentsperformedina2kWannularHallthrusterplasma(ne1.5T e /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 (n e 1.5Te/ebelowtheplasmapotentialduetoavirtualcathodeformingaroundtheprobe.Thetheoreticalpredictionswerecheckedwithexperimentsperformedina2kWannularHallthrusterplasma(ne 10 9 À10 10 cm À3 and T e $ 10À50 eV). The authors find that the floating point method gives a value around 2T e /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.
Surface temperature and thermal balance of probes immersed in high density plasma
Plasma Sources Science and Technology, 1998
The surface temperatures of thermal probes immersed in a low pressure inductively coupled argon discharge have been measured over a wide range of gas pressure. At a fixed discharge power, the measured temperature increases with gas pressure and decreases with increasing probe diameter. At a discharge power of 100 W, the surface temperature of a 0.4 mm diameter probe in the centre of the discharge ranges from 272 • C at 0.3 mTorr to 590 • C at 1 Torr. This temperature is considerably higher than the gas temperature. An analysis of the energy balance on a probe surface shows that plasma particle bombardment is the dominant heating process while radiation is the dominant cooling process. Probe temperatures found from an energy balance are in reasonable agreement with those measured in experiment.
Potential measurements by an emissive probe in a magnetized plasma
Physics Letters A, 1980
We fmd that the probe potential determined from the intersection between the characteristics of a usual cold and an emlssive probe is equal to the plasma potential. In a magnetized plasma the emissive floating potential method is still useful.
Test function for the determination of plasma parameters by electric probes
Review of Scientific Instruments, 1999
A ''test function'' sensitive to the electron distribution function is used to determine the plasma parameters by Langmuir probes. It was successfully applied to determine the parameters of the hot-electron group in a multipolar, magnetically confined Ar plasma, even when the ratio of bulk to hot-electron densities is greater than 50. The method can also be used to measure the positive-ion density from a probe potential slightly below the plasma potential, and to estimate the electron and negative-ion density and temperature in an Ar/SF 6 plasma. A characteristic plasma potential is also introduced and is correlated with the plasma parameters.