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The discharge pulses in a high power impulse magnetron sputtering (HIPIMS) system generate large currents in the plasma between the sputtering cathode and the anode. Large Hall currents, circulating in the magnetic field trap close to the magnetron, also arise due to the presence of the magnetic field (E×B drift of electrons). Measurements by Bohlmark et al. (Plasma Sources Sci. Technol. 13, 654 (2004)) show that the ratio between the Hall current, jH and the discharge current, jD for a HIPIMS discharge is unexpectedly low, with jH/jD~2 compared to around 35 for classical magnetron sputtering. This indicates a more efficient electron transport across the magnetic field lines in the HIPIMS case, which requires an anomalous charge transport mechanism. It was concluded that classical theory of diffusion and Bohm diffusion do not meet the requirements to be the only transport phenomena for current flow. It has previously been reported on various instabilities giving rise to an anomalous electron transport in a plasma gun experiment (Hurtig et al, Phys. Plasmas 12, 012308 (2005)). This mechanism for anomalous transport in the HIPIMS discharge has been explored in this work. Electric field oscillations in the megahertz range were measured by electric field probe arrays, indicating an instability in the plasma. The key feature is that the ions and electrons react differently to this instability, due to the scaling ωgi << ωmeasured << ωge, where ωgi is the ion cyclotron frequency and ωge the electron cyclotron frequency. The ions are too heavy to follow the oscillating motion, but not the electrons, causing a charge perturbation and a net transport of electrons across the magnetic field lines. The perturbation is described in literature as a modified two-stream instability, which is known to generate oscillations in the megahertz range (lower hybrid range). Measurements of the oscillations confirm trends, specifically frequency dependence on ion mass, expected from the theory of lower hybrid waves. |