%0 Journal Article %J Applied Physics Letters %D 2013 %T Drifting potential humps in ionization zones: The “propeller blades” of high power impulse magnetron sputtering %A André Anders %A Matjaž Panjan %A Robert Franz %A Joakim Andersson %A Pavel A. Ni %K Electric fields %K ionization %K plasma ionization %K sputter deposition %K sputtering %X

Ion energy distribution functions measured for high power impulse magnetron sputtering show features, such as a broad peak at several 10 eV with an extended tail, as well as asymmetry with respect to E × B, where E and B are the local electric and magnetic field vectors, respectively. Here it is proposed that those features are due to the formation of a potential hump of several 10 V in each of the traveling ionization zones. Potential hump formation is associated with a negative-positive-negative space charge that naturally forms in ionization zones driven by energetic drifting electrons.

%B Applied Physics Letters %V 103 %P 144103 %8 10/2013 %N 14 %! Appl. Phys. Lett. %R 10.1063/1.4823827 %0 Journal Article %J Applied Physics Letters %D 2013 %T Spectroscopic imaging of self-organization in high power impulse magnetron sputtering plasmas %A Joakim Andersson %A Pavel A Ni %A André Anders %K Aluminium %K ionization %K Ionizing radiation %K plasma ionization %K visible spectra %X

Excitation and ionization conditions in traveling ionization zones of high power impulse magnetron sputtering plasmas were investigated using fast camera imaging through interference filters. The images, taken in end-on and side-on views using light of selected gas and target atom and ion spectral lines, suggest that ionization zones are regions of enhanced densities of electrons, and excited atoms and ions. Excited atoms and ions of the target material (Al) are strongly concentrated near the target surface. Images from the highest excitation energies exhibit the most localized regions, suggesting localized Ohmic heating consistent with double layer formation.

%B Applied Physics Letters %V 103 %P 054104 %8 07/2013 %N 5 %! Appl. Phys. Lett. %R 10.1063/1.4817257 %0 Journal Article %J Journal of Applied Physics %D 2012 %T Drifting localization of ionization runaway: Unraveling the nature of anomalous transport in high power impulse magnetron sputtering %A André Anders %A Pavel A Ni %A Albert Rauch %K Cameras %K ionization %K Magnetic fields %K Plasma density %K plasma ionization %X

The plasma over a magnetron’s erosion “racetrack” is not azimuthally uniform but concentrated in distinct dense ionization zones which move in the E x B direction with about 10% of the electron E x B/B2 drift velocity. The ionization zones are investigated with a gated camera working in concert with a streak camera for Al, Nb, Cu, and W targets in Ar or Kr background gas. It is found that each ionization zone has a high plasma density edge, which is the origin of a plasma-generating electron jet leaving the target zone. Each region of strong azimuthal plasma density gradient generates an azimuthal electric field, which promotes the escape of magnetized electrons and the formation of electron jets and plasma flares. The phenomena are proposed to be caused by an ionization instability where each dense plasma zone exhibits a high stopping power for drifting high energy electrons, thereby enhancing itself.

%B Journal of Applied Physics %V 111 %P 053304 %8 03/2012 %N 5 %! J. Appl. Phys. %R 10.1063/1.3692978