Investigation of Water-Vapor Plasma Excited by Microwaves as Ultraviolet Light Source

The potential of using water-vapor plasmas excited by microwaves as a ultraviolet (UV) light source has been investigated by using various pressures and input powers. The UV irradiation power increased and saturated at a pressure range dependent on the input power. On the other hand, other visible and infrared emissions corresponding to four atomic lines, i.e., the Balmer series of hydrogen at 486.1 nm ( Hbeta) and 656.3 nm (Halpha) and oxygen atoms at 777.2 and 844.6 nm, were clearly decreased with an increase in the total gas pressure. It was found that pressures (1.4-2.0 kPa) near the saturated water-vapor pressure were found to give the most intense UV irradiation. With a vapor pressure of 1.6 kPa and a total microwave power of 300 W, the power density of UV (Gamma_uv) was measured to be 10.5 muWmiddotcm^-2 at a distance of 30 cm from the center of the discharge tube as measured through an optical viewing port on the cavity discharge applicator. This value for (Gamma_uv) is comparable to that for a mercury lamp. However, the (eta_uv) efficiency was estimated to be considerably lower than that of a mercury lamp.