On double-layer and reverse discharge creation during long positive voltage pulses in a bipolar HiPIMS discharge

Time-resolved Langmuir probe diagnostics at the discharge centerline and at three distances from the target (35mm, 60mm, and 100mm) was carried out during long positive voltage pulses (a duration of 500$\mu$s and a preset positive voltage of 100V) in bipolar High-Power Impulse Magnetron Sputtering of a Ti target (a diameter of 100mm) using an unbalanced magnetron. Fast-camera spectroscopy imaging recorded light emission from Ar and Ti atoms and singly charged ions during positive voltage pulses. It was found that during the long positive voltage pulse, the floating and the plasma potentials suddenly decrease, which is accompanied by the presence of anode light located on the discharge centerline between the target center and the magnetic null of the magnetron's magnetic field. These light patterns are related to the ignition of a reverse discharge, which leads to the subsequent rise in the plasma and the floating potentials. The reversed discharge is burning up to the end of the positive voltage pulse, but the plasma and floating potentials have lower values than the values from the initial part of the positive voltage pulse. Secondary electron emission induced by the impinging Ar$^+$ ions to the grounded surfaces in the vicinity of the discharge plasma together with the mirror configuration of the magnetron magnetic field are identified as the probable causes of the charge double-layer structure formation in front of the target and the ignition of the reverse discharge.