Evidence for fireballs in bipolar HiPIMS plasmas

Using laser Thomson scattering (LTS) and 2D optical imaging, a fireball-like discharge is detected during the positive pulse period of bipolar voltage waveforms, for a circular planar unbalanced magnetron with W target operating in argon. These reverse discharges, excited for positive pulse voltages V (pp) from 200 to 300 V, sit primarily on the discharge centreline. Their establishment is delayed relative to the initiation of the positive pulse, with the delay time shortening with increased V (pp): they are clearly attached to the target (anode in this phase) and have lifetimes extending to the end of the positive pulse. LTS measurements of the electron temperature T (e) show dramatic electron heating (T (e) rises from 1 up to 3 eV) both on the discharge centre line and above the racetrack during the fireball event. This is consistent with greater light intensities from the broad band optical imaging of the discharge. In the fireball phase, the LTS measurements also show greatly reduced election densities (by a factor of 5) in the magnetic trap (directly above the racetrack) compared to the unipolar pulse case. The existence of such anodic fireballs is quite possibly an unwanted effect in bipolar HiPIMS.