Ignition Dynamics of a Self-pulsing y-mode discharge in a wedge-shaped micro-scaled atmospheric pressure plasma jet ($\mu$-APPJ)*

Microplasma jets, operated at atmospheric pressure, are susceptible to instabilities. A prominent one is the ``$\alpha $-$\gamma $ transition'' instability, often resulting in a constricted discharge at high gas temperatures destroying the device. Thus a safe and stable application of these devices for treating heat-sensitive biological materials is limited. In order to analyze the responsible mechanisms for this mode transition, the capacitative coupled, rf-excited (f$=$13.56 MHz) micro-scaled plasma jet ($\mu$-APPJ) has been modified. A wedge-shaped electrode configuration has been developed, forming a 30 mm long discharge gap increasing linearly from 1 mm at the gas inlet to 3 mm at the nozzle. A self-pulsing behavior is observed characterized by a periodical ignition of a constricted y-mode discharge feature at the gas inlet, propagating with the gas flow through the device towards the nozzle. Spectral- and phase-resolved optical emission spectroscopy (PROES) is applied to investigate discharge ignition dynamics and cross-checked with synchronized current /voltage measurements.