On the two modes of operation of planar-coil-driven inductive discharges in hydrogen

Abstract

Langmuir probe and laser photodetachment technique diagnostics as well as optical emission spectroscopy and phase-resolved optical emission spectroscopy are used to study a planar-coil-driven inductive discharge in hydrogen sustained in the first chamber of a two-chamber plasma source. The discharge is operated in the power range 50–400 W at 27 MHz. The gas-pressure range studied is p = 20–60 mTorr. The results obtained over the first half of the discharge length, that is starting from the position of the coil, outline discharge maintenance in the two modes of the inductive discharge. The inductive mode sustained at a high rf power appears with rf power deposition by ring-shaped rf electric field intensity close to the coil and high electron density there, followed by its strong drop in the remote plasma region. The capacitive mode sustaining low-density plasmas also appears with two regions specified by different mechanisms of rf power deposition: plasma heating by an electron beam acceleration in the wall sheath during its expansion and Joule heating in the plasma bulk. The similarity—at high and low rf power—in the axial structure of the discharge over the second half of its length, that is touching the transition between the two chambers of the source, and the appearance of a second maximum of the dc potential there are related to the configuration of the source. The obtained axial variation of the negative ion density obeys that of the potential of the dc electric field in the discharge.