Optical characterization of a novel miniature microwave inductively coupled plasma source in nitrogen flow

A miniature microwave (MMW) inductively coupled plasma (ICP) source is characterized by optical emission spectroscopy and by optical imaging of nitrogen plasma. The MWW source operates in two different modes (H-mode and hybrid-E/H mode) with different plasma parameters and different emission morphologies, depending on the absorbed microwave power (Pabs). The measured spectra of the second positive system (N2(C-B)) and of the first negative system (N2+(B-X)) of nitrogen reveal an electron density ne = (6.4 1.7)×1018 m-3 and a gas temperature of Tg = (650 20) K for Pabs = 12 W at a pressure of 1000 Pa. By increasing the absorbed power to Pabs = 78 W the parameters increase to ne = (3.5 1.7)×1019 m−3 and Tg = (1600 100) K. The discharge morphology in hybrid and H-mode is different. While in the H-mode the plasma resembles a 'donuts' shape, the hybrid mode has a very narrow shape close to the walls and to the gap capacitor of the resonator. For our discharge conditions the power absorption is limited to 158 W, above which the discharge spontaneously switches from H-mode to hybrid mode.