Capacitively coupled micro atmospheric pressure plasma jets (µAPPJs)
are important tools for the generation of radicals at room temperature for various
applications. Voltage Waveform Tailoring (VWT), which is based on the simultaneous
use of a set of excitation frequencies, has been demonstrated to provide an efficient
control of the Electron Energy Probability Function (EEPF) in such plasmas and,
thus, allows optimizing the electron impact driven excitation and dissociation processes
as compared to the classical single-frequency operation mode. In this work, the
effects of changing the driving frequencies on the spatio-temporally resolved electron
power absorption dynamics, the generation of helium metastables and the dissociation
of nitrogen molecules are investigated in He/N2 plasmas based on experiments and
simulations. We find that under a single-frequency excitation, the plasma and helium
metastable densities are enhanced as a function of the driving frequency at a fixed
voltage. When using valleys-type driving voltage waveforms synthesized based on
consecutive harmonics of the fundamental driving frequency, the spatial symmetry of
the electron power absorption dynamics and of the metastable density profile is broken.
Increasing the fundamental frequency at a constant voltage is found to drastically
enhance the plasma and metastable densities, which is a consequence of the change
of the EEPF. Finally, we compare the energy efficiency of the formation of radicals
under single-frequency and VWT operation at different driving frequencies. For a
given power dissipated in the plasma, VWT yields a higher helium metastable as well
as electron density and a higher dissociation rate of N2.