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[en] Langmuir probes are relatively simple to construct and to use for establishing local and time-resolved measurements of charged particles in ionized gases. The analysis requires the adoption of assumptions and implies constraints on operation that can often be met in laboratory plasmas that have been primarily devised for the testing of probe theories. Probe modelling can also be tested against simulations. More generally, the analysis of data from probes operated in less ideal plasmas, such as those used in materials processing, is more challenging. In these circumstances alternative strategies are required that are tolerant of the processing environment. Techniques for immediate, local measurements include the use of structures that involve standing waves and travelling waves, typically at microwave frequencies. Non-invasive electrical probing can also conveniently be done from isolated planar sections of the bounding surfaces.
[en] In 1929 Tonks and Langmuir predicted of the existence of electron plasma waves in an infinite, uniform plasma. The more realistic laboratory environment of non-uniform and bounded plasmas frustrated early experiments. Meanwhile Landau predicted that electron plasma waves in a uniform collisionless plasma would appear to be damped. Subsequent experimental work verified this and revealed the curious phenomenon of plasma wave echoes. Electron plasma wave theory, extended to finite plasmas, has been confirmed by various experiments. Nonlinear phenomena, such as particle trapping, emerge at large amplitude. The use of electron plasma waves to determine electron density and electron temperature has not proved as convenient as other methods.