Spatially resolved experimental investigation of a surface wave sustained discharge in nitrogen

Abstract

A spatially resolved experimental investigation of the electron energy distribution function (EEDF) and wave propagation characteristics in a high frequency surface wave (SW) discharge in nitrogen has been performed. The measurements reveal the specific changes of the EEDF and its integrals which occur as a result of a different coupling between the electrons and the inhomogeneous SW electric field as a function of gas pressure. At a pressure of 0.5 Torr it was found that the EEDF and corresponding integrals depend on the spatial position, which means that local plasma response occurs. As a result, a strong radial inhomogeneity of the discharge is observed which relates to the radial variation of the wave field intensity. On the contrary, at 0.05 Torr nonlocal plasma response is observed. In a molecular plasma, electron energy relaxation occurs faster than in inert gases, this being why the transition from the local to the nonlocal regime takes place here at a lower gas pressure. A turning back of the axial wave number due to the simultaneous effects of collisions and the radial inhomogeneity is also observed. A notable feature of the experiments is that the minimum electron density for the wave to propagate may be smaller than the minimum density for propagation in the collisionless approximation. A strong correlation between the behavior of the plasma parameters and the wave electrodynamics is pointed out.