Long and uniform plasma columns generated by linear field-applicators based on stripline technology

Abstract

Long plasma columns generated by high-frequency (HF) fields and extending over distances longer than the free-space wavelength of the applied electromagnetic (EM) field are of interest in various applications. A commonly used method to achieve such long plasma columns calls for the propagation of EM surface waves that use the plasma as their propagating medium. In such a case, the HF field applicator, called a wave launcher, is much shorter than the actual length of the plasma column. Long plasma columns can also be sustained by using field applicators that run along the full length of the discharge tube. Most such linear applicators rely on waveguide components. However, it is possible to use transverse electric magnetic planar-transmission-lines based on stripline technology to design efficient linear field applicators. Using such an approach, we have developed a new type of HF linear field applicator that operates on a relatively wide frequency range (typically, 200–2450 MHz). Comparison of the discharge that it generates with a surface-wave discharge (SWD) sustained under similar operating conditions shows that the discharge volume is larger than that obtained with a SWD at the same power level, hence a lower gas temperature and a plasma column more axially uniform, two valuable features for some applications. The contraction of these plasma columns is shown to occur at higher gas pressures than with SWDs. All these measurements are carried out in argon as the discharge gas.