Langmuir probes

Electrodes or Langmuir probes may be inserted into plasmas that are large enough (>1 cm) and relatively cool (<10 K). The net current to the probe is measured as a function of the appHed voltage. Electron temperatures, electron and ion densities, and space and wall potentials may be derived from the probe signals. Interaction of plasmas with soHd probes tends to perturb plasma conditions. 

In order to relate material properties with plasma properties, several plasma diagnostic techniques are used. The main techniques for the characterization of silane-hydrogen deposition plasmas are optical spectroscopy, electrostatic probes, mass spectrometry, and ellipsometry [117, 286]. Optical emission spectroscopy (OES) is a noninvasive technique and has been developed for identification of Si, SiH, Si+, and species in the plasma. Active spectroscopy, such as laser induced fluorescence (LIF), also allows for the detection of radicals in the plasma. Mass spectrometry enables the study of ion and radical chemistry in the discharge, either ex situ or in situ. The Langmuir probe technique is simple and very suitable for measuring plasma characteristics in nonreactive plasmas. In case of silane plasma it can be used, but it is difficult. Ellipsometry is used to follow the deposition process in situ. 

Figure 7. Schematic diagram of a flowing-afterglow electron-ion experiment. The diameter of flow tubes is typically 5 to 10 cm and the length is 1 to 2 meters. The carrier gas (helium) enters through the discharge and flows with a velocity of 50 to 100 m/s towards the downstream end of the tube where it exits into a fast pump. Recombination occurs mainly in the region 10 to 20 cm downstream from the movable reagent inlet, at which the ions under study are produced by ion-molecule reactions. The Langmuir probe measures the variation of the electron density in that region. A differentially pumped mass spectrometer is used to determine which ion species are present in the plasma.

Dean et al. (1974) use a Langmuir probe technique in a rare gas repetitive afterglow plasma. The electron temperature is extracted from the semi-log plot... 

Niu H, Houk RS (1994) Langmuir probe measurements of the ion extraction process in inductively coupled plasma mass spectrometry-I. Spatially resolved determination of electron density and electron temperature. Spectrochim Acta 498 1283-1303... 

Such measurements have been performed by Koons and Fio-cco 2791 in a low-density reflex discharge in helium, using an argon laser at X = 4880 A as light source. The results (N = 4.8 x lO cm", Te = 1.3 eV) are in good agreement with those obtained with a Langmuir probe. 

Doering and Mahan119 also photoionized NO using the Kr 1236-A line. They measured the photostationary concentration of ions by use of a Langmuir probe and found limits of 2.4 x 1014 and 1.2 x 1015 M-1 sec-1 for the dissociative recombination... 

Parallel-plate reactor Dissociation of CH4 for amorphous carbon Well mixed Mean electron density energy distribution measured with Langmuir probe 82... 

A decade ago, while considerable data had been compiled on the kinetic measurement of dissociative recombination (DR) reactions of small polyatomic ions, laboratory information on the product distributions of such reactions was restricted to the results of a few merged-beam and stationary-afterglow studies on DR of C02 and of H( [157,158], and the first explorations of combined flow tube/Langmuir probe/spectroscopic detector techniques, independently pursued by Rowe and co-workers (at Rennes) [159,160] and by Adams and co-work-ers (at Birmingham, and subsequently Atlanta) [161, 162]. Considerable advances have since been made, both in measurement of recombination coefficients (particularly for larger ions) and in the elucidation of product distributions for a still small but growing sample of important IS ions. 

Measurements of the poloidal distribution of the ELM energy deposition at the divertor target, such as those in Fig. 3.18, are usually taken at only at one toroidal location. Few experiments have been carried out with measurements of the divertor ELM energy and particle fluxes (with infra-red cameras and Langmuir probes) at various toroidal positions. These experiments show that the divertor ELM energy flux is, approximately, toroidally... 

Fig. 5,2. Fulcher lines observed near the divertor plates in an attached divertor plasma in JT-60U [7], Calculated Q-branch line intensities for the v = 0 — 0,1 — 1,2 — 2 transitions are also shown as vertical lines. The electron temperature and density measured with a Langmuir probe near the separatrix strike point were 20 eV and 0.7 x 1019 m-3, respectively...

Fig. 5.8. Profiles of (a) electron temperature, (b) density, and (c) ion flux measured with Langmuir probes at the divertor plates in an attached and a detached divertor plasma. The distance from the strike point (dl) is measured along the outer divertor plates as shown in the right-hand figure...

Langmuir probes. Electrostatic probe measurements give access in principle to ne, (Fp), Te, and to the electron energy distribution. If the implementation is easy (collection of the current using a biased conductor), it is much more difficult to obtain reliable measurements because the method is very intrusive. Most of the probes have a cylindrical geometry, but some probes are planar or spherical. The following conditions have to be fulfilled preferably. Ideally the probe dimension has to be smaller than XD, to limit perturbation of the plasma, also the sheath thickness around the probe has to stay smaller than XUn or Xehx in order to limit... 

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