Triple probe interrogation of spokes in a HiPIMS discharge

Using a triple probe situated above the racetrack and inside the magnetic trap of a magnetron, rotating spoke-like structures have been clearly identified in a single HiPIMS pulse as periodic modulations of 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 × B direction with a velocity of ~8.8 km s −1 . Defining the spoke shape from the footprint of the 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 × 10 19 m −3 , T e ~ 2.1 eV) and a relatively hotter but more rarefied trailing edge ( n e ~ 1 × 10 19 m −3 , T e ~ 3.9 eV). Measurements of V p show a potential hump towards the rear of the spoke, separated from regions of the highest density, with plasma potentials up to 8 V more positive than the inter-spoke regions. Azimuthal electric fields of ~1 kV m −1 associated with these structures are calculated. Transforming the triple probe time-traces to functions of the azimuthal angle θ and assuming a Gaussian radial profile for the plasma parameters, 2D spatial maps of n e , T e and V p have been constructed as well as the target ion current density J p from the embedded probes. The phase relationship between T e , V p and n e can be clearly seen using this representation with n e leading T e and V p with a phase shift between them of ~50°. Regions of maximum ion current to the target, delivered by individual spokes, coincide with the overlap of regions of high n e and T e measured above the target at a height of 15 mm. Ions created at elevated positions above the target in the observed dense region will take several micro-seconds to reach that surface, so contributing to the target ion current in the following spokes.