Application of Non-Linear Optical Methods to Plasma Diagnostics

Abstract. The application of non-linear optical methods nowadays allows the measurement of an increasing number of specific plasma quantities which were previously not amenable to diagnostics. Among those are state resolved measurements of molecular density distributions, sensitive measurement of light atomic radical densi-ties, electric field measurements, and determination of quenching rates. In this paper an overview of the avail-able techniques and trends is given. Some selected topics are discussed in more detail. Since the early days of plasma physics the emission of light from the discharge has always been a very valuable tool for the study of basic phenomena. This is still true'today where emission spectroscopy is finding wide application not only in research but also in process monitoring in industrial applications [1,2,3]. The sensitivity of the spectra to changes of the discharge conditions and the relatively modest ex-perimental requirements have contributed to this development. However, to obtain an unambiguous pic-ture of the physical conditions that lead to the measured spectra turns out to be often very difficult and one has to be very carehl when drawing conclusions. This is partly due to the complexity of the problem and partly to the lack of reliable atomic and molecular cross-sections [4]. Laser spectroscopy often allows a more direct interpretation of the measured data and in addition can be a more sensitive probe with, at the same time, better spatial and temporal resolution. Although fo