Using a triple probe situated above the racetrack and inside the magnetic trap of a magnetron, rotating spokes-like structures have been clearly identified in a single HiPIMS pulse as periodic modulations in the electron temperature T e , electron density n e , ion saturation current I isat , floating potential V f and plasma potential V p. The spokes rotate in the E x B direction with a velocity of ~ 8.8 km/s. Defining the spoke shape from the footprint of ion current they deliver to flush mounted probes embedded in the target, each spoke can be characterised by a dense but cool leading edge (n e ~ 2.0 x 10 19 m-3 , T e ~ 2.1 eV) and relatively hotter but more rarefied trailing edge (n e ~ 1 x 10 19 m-3 , T e ~ 3.9 eV). Measurements of V p show a potential hump towards the rear of spoke, separated from regions of highest density, with plasma potentials up to 8 V more positive than the inter-spoke regions. Azimuthal electric fields of ~1kV/m associated with these structures are calculated.
The temporal evolution of the electron temperature T
e and density n
e has been measured at two positions on the centre-line of an asymmetrically pulsed bi-polar HiPIMS plasma using incoherent laser Thomson scattering (LTS). The magnetron was operated with a tungsten target in argon atmospheres. The results show that in the plasma afterglow when positive voltage pulses are applied (above a threshold of at least 200 V) significant heating of the electrons can occur in which T
e can rise to values comparable to the those measured in HiPIMS on-time. The on-set of the rises in T
e are significantly delayed relative to the start of the positive pulse, with the delay time decreasing with the magnitude of the positive voltage. The delay is only weakly dependent on the operating pressure. The presence of large positive pulses can also affect the local electron density with n
e seen to decay significantly more quickly in the afterglow than for the corresponding unipolar pulsing case, in which no positive pulse is applied. The LTS measurements were complemented by a time-resolved study of the plasma optical emission (neutral argon and tungsten lines). With increasing positive potentials applied in the afterglow the Ar(I) line intensities grow consistent with increasing T
e. Interestingly, W(I) line intensities are detected in the afterglow with positive voltages >200 V despite the termination of all target sputtering, suggesting that tungsten is being re-sputtered from the vessel walls. With the aid of emissive probe measurements of the spatial and temporal evolution of plasma potential profile along the centre-line we discuss the phenomena of plasma electron heating and wall sputtering in the positive pulse. This is done in terms of the existence of a non-sustained reverse discharge, in which the vessel walls become an effective cathode.
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