Energy exchange modulation for selective control of gas temperature and electron number density in cold atmospheric plasmas

Selective control of the key parameters of the cold atmospheric plasmas (CAPs) is crucial for diverse applications ranging from materials processing, clinical medicine to clean energy generation. In particular, the low gas temperature (T (g)) and high electron number density (n (e)) are both critical for obtaining high treatment efficiency of heat-sensitive materials, yet are challenging to achieve because of the very frequent species collision nature in CAPs. In this paper, selective control of T (g) and n (e) in a helium CAP driven by a radio-frequency power supply and operated in an open environment is achieved successfully for the first time numerically and experimentally with the quasi-independent variation windows from -33.7 degrees C to 49.5 degrees C (i.e. 239.3 to 322.5 K) for T (g) and from 2.7 x 10(16) to 6.3 x 10(16) m(-3) for n (e). This result has expanded the key CAP parameter windows significantly into a previously unachievable domain. The further theoretical analysis of the energy transfer and balance based on the 'energy tree' concept and numerical modeling reveals the unique non-equilibrium energy transfer channel allowing selective control of T (g) and n (e). This energy transfer channel is enabled by the two 'valves', one for controlling the energy deposition from the external circuit to the discharge cell (valve 1), and another one for controlling the energy exchange between the discharge cell and the environment (valve 2). Our conceptual approach and proof-of-principle demonstration open a new way for the active and selective control of the key CAP parameters, which will be quite important for designing CAP sources with specific requirements and for advancing or even creating new CAP applications in the future.