Spatially resolved measurements and diagnostics of digitally controlled rotating field pulsed plasma operated in helium at 20 kHz

Using optical emission spectrometry, fundamental properties are investigated of a stable, planar atmospheric pressure micro discharge, several dozen microliters in volume, driven by a digitally controlled 20 kHz rotating microsecond pulsed power. The discharge is generated by rectangular wave pulses using helium as the working gas. At a low cost, the digitally controlled plasma source produces a highly symmetrical, non-stationary helium discharge maintained in open air within 5 electrodes positioned in the plane toward the center. It has been shown that the geometrical shapes of the momentary discharges, which occur between the electrodes, are not arc-like shaped, but rather have a diffusive character and the resulting plasma can become doughnut-like in shape. Rotational and vibrational temperatures from OH and N₂ bands, excitation temperatures from He lines and ionization temperatures from Ca lines, as well as electron number densities from H_β Stark broadening have been estimated along the plasma diameter using axial viewing. The results demonstrated that T_exc (He) reaches stable value of 3800 K for selected plasma generation conditions (one anode and two cathodes commutation mode, cathode pulse width 8 microseconds, supplied power 200 W, helium gas flow 1 L·min⁻¹), while the T_rot (OH) is considerably lower (1700 K). The electron number density has been evaluated to be (1.7–3.3) × 10¹⁴ cm⁻³ and both T_ion (Ca) and T_vib (N₂) varied, throughout in the 4500–5100 K and 4000–4800 K ranges respectively, reaching its peak value near 2 mm off the plasma axis. Spatial measurements revealed symmetrical distribution of the plasma parameters, while the measurements of calcium and nitrogen ionic emission confirmed symmetrical doughnut shape of the discharge. Moreover, the processes running inside the discharge and their interaction with the surrounding atmosphere have been described in accordance to the recorded spectra. Spectroscopic observation has shown the existence of N₂⁺ molecular ions, although their emission profile differs significantly from neutral N₂. Namely, the population of ionic molecules forms a doughnut shape, while the N₂ population exhibits disk shape, indicating that plasma ionization zone is located off-axis. - Highlights: • Spatial profiles of 20 kHz digitally controlled rotating field helium plasma have been determined. • Three zones of the examined doughnut-shaped plasma have been distinguished and examined. • A doughnut plasma shape was established with minimum T_ion and electron density on-axis. • Atomic and ionic emission profiles of both N₂ and Ca are similar indicating plasma ionization zone.