Single Langmuir Probe Research Articles

Multichannel boxcar-averaged measurements of plasma parameters made using a digital storage scope

We present a technique for rapidly acquiring time-resolved, ensemble-averaged Langmuir probe characteristics. Fifty probe characteristics are acquired using a digital storage oscilloscope in the time it would take to acquire a one-probe characteristic using a single-channel boxcar averager. A single Langmuir probe is used, and the probe bias is swept quite slowly, so that the probe is always in equilibrium with the plasma. A method for the automatic extraction of electron temperature, electron density, and the plasma potential from the acquired probe characteristics is described. This technique for acquisition and analysis is applied to the study of plasma decay and the effects of rf excitation in a pulsed, strongly magnetized plasma.

Novel Langmuir probe technique for the real-time measurement of the electron temperature

A new method for the evaluation of the electron temperature from the voltage–current characteristic of a single Langmuir probe under the application of an ac signal was developed and tested. This method exploits harmonics in the current spectrum which arise from the nonlinearity of the characteristic; from the amplitude of these harmonics the electron temperature can be deduced. Langmuir probe measurements on the TEXTOR edge plasma show a good agreement of the new method with the conventional one. The advantages of this method over other methods are discussed. Proposals for further improvements are given.

The use of Langmuir probes and optical emission spectroscopy to measure electron energy distribution functions in RF-generated argon plasmas

A single Langmuir probe technique has been used to measure the electron energy distribution function (EEDF) in an RF (13.56 MHz)-generated argon plasma in a reactive ion etcher. This was achieved by applying a driving RF signal to the probe to compensate for the effects of RF fluctuations in the plasma potential and then using the second differential of the probe characteristics to obtain the EEDF. It is observed that the EEDF is not well represented by a Maxwellian, Optical emission spectra (4600-4900 AA) were also recorded. In argon plasmas, generated at a constant power level, the average electron energy deduced from the Langmuir probe increases as the pressure is reduced below 20 mTorr. At 5 mTorr the average energy is 8.5 eV whereas above 20 mTorr the average energy lies in the range 3.5-5.3 eV. This is found to correlate with the variation in the intensity of an emission line (4s'(1/2)10 from 5p(1/2) at 4702.32 AA) in an excited argon atom, which can be accounted for by the presence of an increasing fraction of higher-energy electrons at lower gas pressures.

Errors in measuring electron temperatures using a single Langmuir probe in a magnetic field

Single Langmuir probes used in the JET Tokamak environment have shown spuriously high values of electron temperatures when the voltage scan is allowed to exceed the floating potential. This note presents a theoretical explanation for this anomaly in terms of a suppression of electron current in the strong magnetic field. The theory is supported by experimental evidence.

Lower hybrid effects on the FT scrape-off plasma

The authors observe, by means of two single Langmuir probes that the scrape-off layer of FT (Frascati Tokamak) is modified during the lower hybrid (2.45 GHz) heating experiments. The modifications are completely different according to whether the plasma line average density is under or above the threshold density nth (Alladio et al., 1982, 1983) for RF-electrons interaction. Some interesting variations with the plasma current Ip and the toroidal field BT are found. The change in the transport properties in the scrape-off due to RF injection is discussed, together with the possibility of taking advantage of the observed phenomena.

Characterization of laboratory plasmas with probes

This paper presents a broad overview of various types of probes employed to study plasmas produced in laboratory environments. Methods reviewed include single, double, and triple Langmuir probes, biased calorimeters, E×B, RFA, momentum discriminating, and sample exposure probes.

The scrape-off layer characteristics as a function of the plasma current and density in the FT Tokamak discharges

The temperature and the density of the plasma at the limiter shadow of FT (Frascati Tokamak) during ohmic operation and their radial variation have been measured by means of single Langmuir probes. No clear dependence on the radius for the temperature has been found, while the density decays towards the wall exponentially with an e-folding length lambda n approximately 0.5 cm, in agreement with a simple model. The influence of the plasma current Ip and line averaged density n on the scrape-off conditions has been studied. The scrape-off density ne increases with n at a fixed value Ip, but is substantially reduced when Ip is raised, at fixed n. Neither Ip nor n seem to alter the scrape-off temperature Te appreciably, at least in the region accessible to the probes.

Studies on the Measurement of Plasma Parameters by Means of the RF Probe Method

The RF impedance of a probe in collisionless plasmas has been studied. Numerical results of the RF impedance vs. the probe bias potential for various ωp/ω are given, where ωp and ω are the angular frequencies of the plasma and the applied RF signal, respectively. Collision effect on these RF impedance characteristics is also analyzed and discussed. A new method based on this study in order to determine the electron temperature and the electron density of the plasma is proposed. This proposed method has been also assured in a different manner by measuring the electron temperature and the electron density employing a usual Langmuir single probe method simultaneously. Experimental results agreed with the theory for the gas pressures up to 3 Torr where the electron mean free path is smaller than the probe radius by the factor of about 1/5.

The Single Langmuir Probe under Orbital-Limited Conditions in a Low-Density Collisional Plasma

The cylindrical Langmuir probe under orbital-limited conditions was used to determine the charge density in a low-density collisional plasma. The Langmuir's theory was applied to both electron and ion saturation currents in their respective accelerating regions. Present study indicates that the length of the probe significantly affects the probe characteristics. A probe of suitable length under orbital-limited conditions may be useful under the experimental conditions where the radius of the probe is much smaller than the Debye lengt.

Behavior of a single Langmuir probe in a magnetic field

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The application of Langmuir probes to the study of flowing afterglow plasmas

The application of the single Langmuir probe techniques to the measurement of electron number densities and temperatures in helium flowing afterglow plasmas is described. Data are presented relating to electron loss by both ambipolar diffusion and recombination, which illustrates the value and quality of the diagnostic technique. Results are also presented which illustrate how metastable atom densities can be readily determined by exploiting the Penning effect, and also probe determinations of electron temperature which demonstrate the role of metastable atoms in elevating the plasma electron temperature. In conclusion, the potential value of the probe-flowing afterglow combination is discussed with reference to the determination of positive and negative ion densities and ionic mass, of ion-molecule reaction rates and electron temperature relaxation rates.

Observations of electron temperature relaxation rates in rare gas afterglow plasmas

The single Langmuir probe technique has been used to determine the relaxation rates of the electron temperature, Te, in repetitively produced, time-resolved afterglow plasmas. Comprehensive data is presented for krypton plasmas together with preliminary data for neon and argon plasmas. The electron temperature cooling curves for a given gas pressure, p0, have been interpreted in terms of a characteristic time, tau en, for electron cooling via electron-neutral collisions. Spatial measurements of Ie indicated that no significant Ie gradients exist in the plasmas during the periods over which cooling curves were obtained; it is also argued that there was no significant elevation of the gas temperature above the wall temperature during the afterglow. In all three gases good agreement is obtained with previously published data.

An appraisal of the single Langmuir probe technique in the study of afterglow plasmas

A detailed appraisal of the collisionless sheath single cylindrical Langmuir probe technique applied to the study of electron densities and temperatures in afterglow plasmas is described. It is shown that for positive probe potentials (V) no significant disturbance (`pseudodepletion') of the plasma occurs until the electron current to the probe (ie) approaches the natural diffusive positive ion current flowing to the reference electrodes. This is indicated by departures from linearity of ie2-V plots obtained in the orbital-limited current régime. Departures from linearity have also been observed due to probe sheath expansion resulting in departures from cylindrical geometry above a critical value of V. It is shown that, for values of V below those critical values signifying the onset of pseudodepletion and sheath expansion, meaningful probe characteristics can be obtained in afterglows from which accurate electron densities can be obtained. Also described are the effects which arise from temporal changes of electron temperature, and the resulting changes in floating potential Vf. The latter has been measured as a function of the electron temperature (obtained from plots of the electron retarding region) and is shown to substantiate the original Langmuir theoretical expression for Vf.

Diffusion cooling in neon, argon, and krypton afterglow plasmas

Measurements are reported of ambipolar diffusion coefficients as determined from ion density loss rates, and of electron temperatures determined using the single Langmuir probe technique, in afterglow plasmas of spectroscopically pure neon, argon and krypton. In each gas there was a critical pressure,p 0 c , above which the product of the ambipolar diffusion coefficient,D a , and the reduced gas pressure,p 0, was pressure independent and above which the electron temperature was found to cool asymptotically to the gas temperature. For pressures belowp 0 c ,D a p 0 decreased with decreasing pressure, and the electron temperature cooled asymptotically to a steady equilibrium value,T eq, below the gas temperature, this equilibrium value itself decreasing with decreasing gas pressure. These results clearly show that diffusion cooling of electrons was taking place at these low gas pressures. A detailed study of the krypton afterglow showed that the values ofD a p 0 andT eq for a given gas pressure were related in a manner predicted by simple diffusion theory. During the course of these measurements values for the zero field mobilities of Ne+, Ar+ and Kr+ ions in their parent gases were obtained and are also reported.

Asymmetrical double-probe systems

This note investigates the conditions which must be satisfied in order that the energy distribution function of electrons in a collisionless plasma may be obtained by the Druyvesteyn method using an asymmetrical double-probe system instead of a single Langmuir probe. If the error in the determination is to be less than 3% it is shown, in the case of nitrogen ions, that n2A2/n1A1 must exceed 6 × 103, where A1,A2 are the areas of the main probe and reference probe respectively and n1, n2 are the electron densities in the vicinity of the two probes.

Investigation of the helium afterglow II. Langmuir probe observations

Measurements are described of slow electron density decay rates in pure helium afterglow plasmas, using a very small single Langmuir probe, a technique not hitherto applied to the study of ambipolar diffusion. Values are obtained for the rate of conversion of He+ ions to He2+ ions and for the respective mobilities of these ions at a wall temperature of 295 °K, which are in reasonable agreement with values obtained by other workers using different techniques. This agreement is considered to indicate that depletion problems, which are expected to be most severe in slowly decaying plasmas, have been overcome by the utilization of very small probes. Evidence is also presented for the existence of elevated electron temperatures at times as late as 10 ms in the afterglow, and these results, together with those above, are critically discussed and compared in detail with the complementary mass spectrometric observations described in I.

A comparison of the methods of determining electron densities in afterglow plasmas from langmuir probe characteristics

A diversity of opinion exists at the present time concerning the interpretation of single Langmuir probe characteristics, in particular with respect to the location of space potential and the determination of charged particle number densities from the characteristics. This work describes measurements made with very small Langmuir probes in gaseous afterglow plasmas at room temperature where conditions for a study of this kind are expected to offer particular advantages. A comparison is made of the values of electron densities obtained from the characteristics using six methods which are commonly used. Good agreement is found to exist between those deduced from the orbital limited characteristics in the accelerating region for electrons and those calculated from the probe current at the inflexion point of the characteristics. That these are also in tolerable agreement with those obtained using the intersecting tangents method is thought to be fortuitous.

Use of Langmuir Probes for Low-Density Plasma Diagnostics

A review is given for the theory of operation of the single and double Langmuir probe. The method of using the double Langmuir probe for the purpose of measuring the temperature and density of electrons in a plasma is described. In the remaining part of this paper, a laboratory experiment using Langmuir probes is suggested and sample calculations pertaining to such an experiment have been given.

極超音速プラズマ流におけるラングミュア・プ口ーブ測定

Langmuir probe measurements have been performed in order to evaluate the properties of the flow in the working section of a plasma jet windtunnel. A single plane Langmuir probe has been used for conditions when the probe-collecting surface is parallel to the flow and the diameter of it is of the same order as the meam free path of a neutral atom. Electron temperature and Plasma density are calculated from Langmuir probe chharacteristics. A typical electrorn temperature is 3×103°K and a typical plasma density is 1013 particles/cm3.

A Visualizing Method of Electron Temperature and Plasma Density

Electron temperatures as well as plasma densities of a transient plasma can be visualized directly on the pattern of a cathode-ray tube. The principle is based on Langmuir's single probe method. Electron temperature is obtained by differentiating a logarithm of the probe current with respect to the probe potential. Then, if the probe potential is changed linearly with time, electron temperature is given by the ratio of probe current to its time derivative. (L.T.W.)