Mode transition in a planar-coil inductively driven discharge caused by an external magnetic field

A hydrogen discharge inductively driven by a planar coil is studied by employing the phase resolved optical emission spectroscopy method, which permits observations not only on the stationary discharge structure but also of its time evolution over the cycle of the rf signal producing the discharge. Since the discharge is considered as a single element of a matrix source of negative hydrogen ions, it is equipped with an extraction device forming an additional grounded metal wall on the side opposite to that where the coil is positioned. Regarding use of a magnetic filter (a localized external magnetic field), the modifications in the discharge caused by the magnetic field have been studied. The results show: (i) transition of the discharge from a capacitive mode to an inductive one with the shift of the magnetic filter from the extraction device towards the coil, (ii) asymmetry both of the stationary and time-varying discharge structure of the inductive mode caused, respectively, by a diamagnetic drift and an ( E-tilde ×B)-drift in the rf field, (iii) formation in the capacitive mode of the discharge of two electron beams, starting from the position of the magnetic filter, in addition to the beams well known as electron acceleration at the wall sheath expansion and (iv) asymmetry in the structure of the capacitive mode due to (E×B)-drifts in the dc and rf fields. (paper)