Experimental and numerical study on the optimization of pulse-modulated radio-frequency discharges

Abstract

In this paper we present an experimental and computational study on the optimization of pulse-modulated radio-frequency (rf) discharges at atmospheric pressure. Based on the measured and simulated data, to improve the electron density and electron temperature in a pulse-modulated rf discharge, a lower modulation frequency, smaller than 50 kHz, with a higher duty cycle, usually larger than 30%, should be used. But to reduce the power consumption and lower the gas heating, the duty cycle should be smaller than 60%. On the other hand, to achieve a quasi-continuous rf plasma in a whole modulation cycle, a modulation frequency larger than 100 kHz is preferred. Thus, by choosing an appropriate modulation frequency and duty cycle, the pulsed rf discharge can be effectively optimized to produce a high-density and non-equilibrium atmospheric plasma without the risk of gas heating but with a reduced power consumption.

Numerical Simulation on the Production of Reactive Oxygen Species in Atmospheric Pulse-Modulated RF Discharges with He/O2 Mixtures

“…Moreover, the previous experimental and computational investigation have shown that when the duty cycle is small than 60% the gas heating effects can be effectively eliminated. [1,12] According to the present computational data and the investigation in refs., [14,15] the production of ground state atomic oxygen is mainly through…”

“…In a pure helium, RF plasma under pulse modulation, the experimental and computational results have indicated that the current density always increases with the duty cycle (with different increasing rates), and the maximum current density under pulse modulation is impossible to be larger than the amplitude of current density in a continuous RF discharge. [12] Figure 1 presents the RMS current density versus the duty cycle for various modulation frequencies at a given oxygen admixture of 0.6%. In a continuous RF discharge, the amplitude of current density under the given discharge conditions is a constant value of about 42.68 mA cm À2 , indicated as a black dash line in Figure 1.…”

“…However, in a pure helium plasma, the electron density generated under pulse modulation could not be larger than that in a continuous RF discharge. [12] Two factors should be considered to be responsible for the variation of peak electron density, one side as the duty cycle is increased, more RF cycles are operated during the pulseon phase with more power coupled into the plasmas, thus an enhancement in electron density and current density can be expected; on the other side, under pulse modulation electrons and oxygen molecules may not fully interact during the pulse-on phase, leading to a less consumption of electrons. Therefore, at some duty cycles the electron density is higher than that in a continuous RF case, as well as the current density.…”

“…[10,11] In addition, if the RF source is modulated by pulse, the gas temperature could be significantly reduced together with a reduced power consumption. [1,9,12] However, due to the complexity of plasma chemistry, the production and destruction of ROS and RNS in atmospheric RF discharges is still under investigation. Therefore, the insights into fundamental plasma chemistry and physics are vital for the development and optimization of ROS and RNS, and many efforts have been applied for further understanding the production of ROS in atmospheric He/O 2 plasmas.…”

“…[14,15,20] In our previous report, we have presented a fluid model to tailor and optimize the pure helium plasmas generated in a pulse-modulated RF discharge. [12] In this study, the fluid model incorporated with a simplified He/O 2 reaction set is explored to examine the generation of ROS in a pulsemodulated RF discharge, we focus on the influences of duty cycle and modulation frequency on the generation of ROS, which would provide interesting and useful information for many applications. The rest of this paper is arranged as follows: Section 2 gives a brief description of the model, and Section 3 discusses the generation of ROS depending on the duty cycle and oxygen admixture in a pulse-modulated RF discharges, a summary is given in Section 4.…”