Diagnostics of low-temperature plasmas suitable for plasma processing applications using light scattering techniques is a research field of growing importance. The three scattering diagnostic techniques discussed in this paper may be applied to a variety of industrially used thermal and nonthermal plasma processing techniques. These methods are nonintrusive with high temporal and spatial resolution and could help to analyse the absolute composition of a plasma, where as many species as possible in different excitation and ionization states should be included. They furthermore deliver information about the energy of particles under investigation, their temperature, density and fluxes. The scattering theory in random media is summarized very briefly and this approach is applied to Rayleigh and Thomson scattering. The difference between incoherent and coherent scattering is figured out. A short overview over the process of coherent anti-Stokes Raman scattering (CARS) and an introduction into experimental techniques is given, which are required to detect Rayleigh, Thomson and CARS signals from a plasma. Finally, applications of these three diagnostic techniques to miscellaneous plasma experiments are shown.

Atomic (H) and molecular (H 2 ) hydrogen densities and temperatures have been determined in a magnetized hollow cathode arc plasma burning at low pressure (p = 4-40 Pa). Rayleigh scattering measurements are used to derive the sum of atomic and molecular densities, each weighted with its scattering cross section. Coherent anti-Stokes Raman scattering (CARS) has been used to determine the population density differences of rovibrational molecular H 2 states n H 2 (v, J ) − n H 2 (v + 1, J ). The CARS intensity of many rotational states (J 9) of H 2 can be detected and these levels are found to be populated according to a Boltzmann distribution. In the low-pressure plasma only the fundamental vibrational band of H 2 can be found experimentally owing to the low particle densities. In order to evaluate the H 2 density properly from the measured CARS data, the H 2 vibrational population for v > 0 is calculated from a spatially one-dimensional diffusion reaction model. Within the plasma centre the dissociation degree d = n H /(n H + 2n H 2 ) ≈ 0.4 and about one third of the molecular hydrogen is found in vibrationally excited states. Here, the vibrational temperature is about T vib ≈ 5000 K, which far exceeds the gas temperature of T gas ≈ 1000-3000 K. The dissociation degree and the vibrational distribution are mainly determined by electron-impact processes in the inner plasma region and recycling processes at the vessel walls, whereas the influence of inelastic neutral-neutral collisions is rather marginal.

A spatially resolving incoherent Thomson scattering technique has been used for time-averaged and time-resolved investigations of a magnetized hollow cathode arc burning in hydrogen and helium. In these plasmas strong self-excited oscillations with frequencies between typically 10 and 60 kHz are found generating modulated plasma emission intensities and discharge voltage signals. In the case of periodically oscillating plasmas. space-and time-resolved measurements of electron density and temperamre are performed over the x c cross section by proper triggering of the pulsed laser system with the periodically oscillating voltage signal from the plasma The evaluation of the electron density and temperature contour maps reveals that the region of m i m u m electron density is shifted eccentrically and rowing azimuthally around the arc axis. The region of maximum electron temperature shows B n e d y annular shape, following the dimensions of the hollow caihode. When increasing the external magnetic field and neutral gas pressure, the arc motion and time behaviour of individual plasma parameters become irregular and chaotic.

In this paper a method to determine spatially-resolved profiles of the electron temperature T e and density n e in an electron-cyclotron-resonance (ECR) discharge is presented. This technique is based on the observation of line emission from a neutral Li atom beam, which is injected into the plasma and excited by electron collisions. A collisional-radiative model valid for the injected Li atoms is used to predict the emission intensities as function of n e and T e for several lines theoretically. In contrast to the electron temperature regime representative for the edge of tokamak discharges (T e > 5 eV), the ECR discharge offers a T e range where selected line intensity ratios strongly depend on the electron temperature. Therefore, a comparison of the measured ratios with the calculated ones yields T e profiles for the first time. The n e measurement is performed as in tokamaks by observing the attenuation of the beam due to ionization in the plasma. We present radial profiles of T e and n e for discharges in argon and xenon under different operating conditions. These results are compared with results obtained by Thomson scattering. Our measurements give evidence for a satisfying agreement between the two methods.

A two-dimensional self-consistent model of a large-volume ECR discharge is presented. The set-up of the plasma source is introduced briefly. The model treats electrons as a fluid assuming a Maxwellian velocity distribution and determines electron density and temperature from a particle and energy balance. Wave propagation in the plasma is calculated from a wave equation derived from Maxwell's equations in matter. The self-consistent coupling of the equations is achieved by determining the material properties of the plasma from the dispersion relation of right-hand circularly polarized waves propagating parallel to the external magnetic field. We do not adopt the cold plasma approximation so that thermal effects like Doppler broadening of the resonance zone and collisionless electron cyclotron heating of the plasma are included. The neutral temperature is estimated from a global balance of energy. Results of the simulation for different operating conditions are discussed and compared with theoretical predictions and experimental data of other authors.