Optical and electrical characterization of pulse-modulated argon atmospheric-pressure inductively coupled microplasma jets

The critical parameters determining the generation of the pulse-modulated argon atmospheric-pressure inductively coupled plasma (AP-ICP) microjet were studied by varying the power, P , pulse-modulation frequency, f , and duty ratio, DR. The temporal changes in the net output power, P net , monitored between the very high frequency power supply and matching network by an rf sampler, and ArI 4 s ′ [ 1 / 2 ] 1 O - 4 p ′ [ 1 / 2 ] 0 emission from the antenna were measured to elucidate the behavior of this plasma. The AP-ICP microjet, which produces high-density ( 0.9 - 1.1 × 10 15 cm − 3 ) nonequilibrium plasma, consists of an alumina discharge tube with the inner diameter of 0.8 mm. The generation diagram of the pulse-modulated plasma was created by having f as the horizontal axis and DR as the vertical axis while varying P up to 50 W. At f ≤ 10 kHz , the plasma was generated at above the linear lines of f and DR, which indicated the existence of the critical power-off period of approximately 80 μ s . At f > 10 kHz , the pulse-modulated plasma was produced above constant DR and almost independent of f . The time-averaged power, P ¯ , which is the product of P and DR, had to be more than 8-10 W to sustain the pulse-modulated plasma. From the measurement of the temporal changes in the net power and ArI emission, the dynamic behavior of the pulse-modulated plasma was revealed as follows. The prebreakdown period was present for ∼ 5 μ s after the power was turned on. Once the plasma was generated, the impedance was changed and the reflected power gradually decreased. A strong emission peak was observed immediately after the breakdown, followed by the gradual increase up to the steady state. Finally, the intense afterpeak was observed at 0.8 μ s after the power was turned off.