Plasma, impurity concentration

Figure 2. Maximum allowed impurity concentration for ignition of D-T plasma as a function of plasma temperature assuming radiative losses only. (Reproduced, with permission, from Ref. 22. Copyright 1978, American Nuclear Society.)...

Comparision of Techniques. A figure-of-merit for the impurity concentration that exists in a plasma is a parameter based on the classical resistivity of the plasma (24), the "effective Z" ... 

Pig. 8. Maximum tolerable impurity concentration of a D-T plasma (see text)... 

Fig. 14.5. Beryllium erosion rate depends both on impurity concentration in the incident plasma, as well as the sample temperature during the exposure. From [16]...

The measured erosion of a beryllium surface will therefore be dependent not only on the impurity concentration in the incident plasma, but also on the temperature of the sample surface during the plasma exposure. An example of this effect is shown in Fig. 14.5, where the deposition of a carbon impurity... 

Spectroscopic methods for the deterrnination of impurities in niobium include the older arc and spark emission procedures (53) along with newer inductively coupled plasma source optical emission methods (54). Some work has been done using inductively coupled mass spectroscopy to determine impurities in niobium (55,56). X-ray fluorescence analysis, a widely used method for niobium analysis, is used for routine work by niobium concentrates producers (57,58). Paying careful attention to matrix effects, precision and accuracy of x-ray fluorescence analyses are at least equal to those of the gravimetric and ion-exchange methods. 

The sodium hydroxide is titrated with HCl. In a thermometric titration (92), the sibcate solution is treated first with hydrochloric acid to measure Na20 and then with hydrofluoric acid to determine precipitated Si02. Lower sibca concentrations are measured with the sibcomolybdate colorimetric method or instmmental techniques. X-ray fluorescence, atomic absorption and plasma emission spectroscopies, ion-selective electrodes, and ion chromatography are utilized to detect principal components as weU as trace cationic and anionic impurities. Eourier transform infrared, ft-nmr, laser Raman, and x-ray... 

One of us (RES) suggested over two years ago[6] that the resolution to this question lies in the electric field inherent to the arc plasma. As argued then, neither thermal nor concentration gradients are close to the magnitudes required to influence tip annealing, and trace impurities such as hydrogen, which might keep the tip open, should have almost no chemisorption residence time at 3000°C. The fact that w ell-formed nanotubes are found only in the cathode deposit, where... 

There is, however, one point where the different experiments do not agree Gougousi et al. found that the plasma decayed faster when the H2 concentration was increased. They concluded this from a large set of data over a significant range of H2 densities and they found the same to be true when Dj ions were studied in the presence of D2. Smith and Spanel carried out tests over a smaller range of H2 concentrations. Their data show the opposite dependence on H2 concentration, but unfortunately the authors discontinued this set of measurements since they became concerned about an increase in the concentration of impurity ions. 

Precursor Impurities detected by plasma emission (ppm) Material grown and temperature (°C) Carrier concentration, 77K (cm-3)a Mobility, /i77[c (cm2 V-1 s f... 

Two methods were used to determine the purity of the final material. The ash content was obtained by calculation of a weight loss of 1 g size graphite sample, after its exposure to 900°C for 4 hours (Superior Graphite AIMS procedure FBQ0001). This test gives a good idea of the total amount of impurities. The nature of impurities and their specific concentrations as measured in ppm or ppb levels, was determined with an ICP inductively coupled plasma spectrometer (Model Jobin Yvon Ultima POXX/681,... 

At present, reproducibility and control are the primary limitations to the implementation of plasma processes clearly, the large number of interacting parameters accounts for such problems. In addition, deposition and etching processes are inordinately sensitive to small (part-per-billion) concentrations of impurities. These difficulties can be overcome only by careful investigation of the complex chemistry and physics of glow discharges. 

Another system based on an SEC pre-column was employed by Earley and Tini for the analysis of a nonpeptidic inhibitor of human leukocyte elastase as part of a drug discovery project (13). The analytical column used here was octadecylsilane, although the authors commented that, depending on the particular application, any stationary phase could be used as the analytical column. In their application, a fraction of the effluent from the SEC column was transferred to the analytical column by the use of a C18 collection column that served to pre-concentrate and focus the analytes. They noted that frequent solvent flushing was required to avoid build-up of impurities on the collection column. A chromatogram obtained from a 50 p.L plasma standard injection is shown in Figure 15.6. This separation provides an excellent... 

Plasma impurities may also have a beneficial effect by cooling the plasma edge in the vicinity of the first wall. But this is partially offset by the fact that the sputtering yields due to impurities are higher as compared with that of hydrogen. Consequently, a relatively small concentration of impurities can significantly increase the wall erosion and plasma contamination. 

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