Pulsed microwave plasmas in Argon: A surprising lack of effects due to fast power modulation

Summary form only given. Pulsed power operation of plasmas is usually considered as a tool to control deposition processes as well as a diagnostic method. In this contribution, we will investigate the pulsed operation of a low pressure microwave plasma source, the surfatron. Thomson scattering and emission spectroscopy are used to study the evolution of the plasma during the ignition as well as the afterglow phase. Special attention is paid to the time/space coupling along the plasma column during the ignition phase. The plasma is generated inside a quartz tube with an inner radius of 3 mm at a driving frequency of 2.45 GHz. The pressure is changed from 5 mbar till 90 mbar while the pulse frequency can be varied from 1 Hz to 1 MHz with a rise time of the power supply of about 100 ns. The effect of the pulse frequency and duty cycle is investigated as a function of power and pressure. The experimental results show that pulsing does not notably affect the (time) averaged plasma properties. This is quite in contradiction with the classical picture of pulsed power operation. It is found that the electron temperature does not overshoot its steady state value much during ignition despite the very fast rise time of the power supply. The electron density gradually rises to its steady state value without any overshoot. This is partly due to the time and space coupling of the power dissipation which adapts depending on the local plasma properties during ignition. The electromagnetic wave is only progressively absorbed by the plasma column while extending towards its steady state length. The plasma bulk ionization front velocity is found to be a strong function of pressure but also of applied peak power. The afterglow electron kinetics are found to be ruled by ambipolar diffusion at low pressure. Around 10 mbar, this changes to a volume recombination regime where dissociative recombination of molecular ions is the main sink of electrons. The afterglow kinetics of excited states such as 4p and 5p states are, on the other hand, ruled by three-body electron ion recombination.