Spectroscopic study of microwave induced plasmas : exploration of active and passive methods

Abstract

Microwave induced plasmas (MIPs) are used for a number of high-tech applications like material processing, light generation, gas cleaning and spectrochemical analysis. Especially the feature that MIPs can be operated remotely and that the propagation of the microwaves can be manipulated with slits, chokes and dielectra others numerous technological opportunities. This thesis concentrates on low-pressure MIPs as used for the production of optical fibers by the company Draka Communications in Eindhoven. To improve this fabrication process, more insight in the plasma is needed. The modeling of these low pressure plasmas is difficult, since they are far from local thermodynamical equilibrium (LTE). Therefore, an accurate description of transport, chemistry and electromagnetic power coupling is required. Experimental data are needed to validate the results of the plasma models. This study describes the application of several spectroscopic techniques to low-pressure MIPs in pure argon and in mixtures of argon/hydrogen and argon/oxygen. These techniques can be divided in passive methods such as emission spectroscopy and active methods like Thomson scattering, absorption spectroscopy and laser induced fluorescence. Due to the non-LTE character of the plasma, the processing of experimental data obtained from the passive methods is not straightforward: plasma models are required to extract important plasma properties such as the electron density and electron temperature from the data. Therefore, the results have to be compared with the results of active spectroscopy. After validation, the passive methods can be applied to plasmas for which it is not possible to apply the experimentally difficult active techniques, such as in industrial situations. Active spectroscopy can not be applied to the MIPs used at Draka Communications. These plasmas are not easily accessible with passive spectroscopy either since they are shielded from their environment. Therefore, at the TU/e, a microwave setup has been built which resembles the microwave plasmas used at Draka. A surfatron is used to launch electromagnetic surface waves into a small quartz tube filled with argon or oxygen gas. This surfatron setup is small and flexible and is therefore easy to investigate with both passive and laser aided diagnostics. The surfatron induced argon plasma was studied using a number of passive diagnostic methods like absolute line intensity measurements, continuum measurements and H¯-broadening. These results were compared with the results of one active spectroscopic method, laser Thomson scattering. A good agreement between the results was obtained for atomic plasmas. For molecular plasmas, a number of discrepancies were found. These require further investigations. Another aspect of this work is the application of global plasma models. This type of models relates external control parameters such as the microwave power, plasma length, tube radius and gas pressure to the electron density, electron temperature and gas temperature. Also, these models are used to explain the trends observed in experimental results. For atomic argon plasmas, a good agreement between the experimental and global model results was found. For molecular plasmas, it was found that molecule assisted recombination is the main loss process for the electron density instead of ambipolar diffusion, as is the case for atomic plasmas. This implies that an accurate description of the plasma chemistry and kinetic reactions is required.