New data on self-organisation in magnetised plasmas

By Georgia Longstaff on August 11, 2017

We asked Professor James Bradley to tell us about his recent research in magnetised plasmas. His latest article “Triple probe interrogation of spokes in a HiPIMS discharge”, published in JPhysD, is available now on IOPscience.

Understanding the fundamental processes that underpin magnetron sputtering has been the task of physicists and engineers over the last 40 years or so. Recently, it has been discovered that these magnetised plasmas, developed for thin films and coatings production, exhibit self-organisation, and so changing our view on their operation. A number of exciting recent publications from groups across the world have shown that in high power impulse magnetron sputtering (HiPIMS) in particular, regions of high ionisation (and anomalously positive space potential) can rotate at high speed around the discharge above the cathode target. These regions, often called spokes, are known to be responsible for boosting the energy of gas and metal ions in the plasma, and affect conditions at the growing film. Spokes seem to be a property of many, if not all, plasmas that have E x B drifting electrons in a closed loop configuration.

Despite many fascinating optical studies of spokes, until now no quantitative information on the density and temperature of the electrons inside spokes has been obtainable. One way to obtain such spatially resolved information is to use a Langmuir probe, however since the spokes rotate at high speeds (~10⁵ ms^-1), data collection becomes extremely difficult. To address this, we decided to use a triple probe, often used on tokamak “fusion” devices, which can under certain conditions (and assumptions about the plasma) provide the electron temperature (T_e), ion saturation current (I_isat), and floating potential (V_f) in real-time. After simple calculation involving I_isat, T_e and V_f, the electron density (n_e) and plasma potential (V_p) can be found as a function of time. The trick is to keep the triple probe at a fixed position above the target and let the spokes intersect the probe tips as they rotate.

Figures 1 a) Three contour lines for the electron density n_e and temperature T_e overlaid on a colour map of the target current distribution J_p and b) three contour lines for n_e and the plasma potential V_p overlaid over the same colour map. Image taken from F Lockwood Estrin et al 2017 J. Phys. D: Appl. Phys. 50 295201 © IOP Publishing, All Rights Reserved.

To confirm that we were indeed observing spokes with the triple probe, we placed the diagnostic directly above a set of flush mounted probes built into the magnetron target. This allowed us to measure simultaneously the azimuthal spatial distribution of spoke plasma parameters and the current they delivered to the target. Figures 1a and 1b show an example of the triple probe measurements obtained (displayed as a function of azimuthal angle rather than time) in which the spoke mode number is m = 3. Interestingly, regions of higher electron density and temperature within the spokes are off-set from each other as described in detail in the paper. The highest densities (n_e) are at the leading edge of the spoke, while the highest electron temperatures (T_e) are at the rear. Regions of positive plasma potential (V_p) follow the distribution of T_e. From the V_p measurements, local electric fields of 1000 Vm^-1 associated with the spokes have been determined.

More triple probe measurements in our HiPIMS system are planned in order to obtain finer detail and axially-resolved data, missing in these initial studies.

About the Authors:

Dr Francis Lockwood Estrin has now completed his PhD in the study of HiPIMS plasma with applications in the production of superconducting coatings at the University of Liverpool. His PhD studies were funded from the EPSRC Centre for Doctoral Training in the Science and Technology of Fusion Energy (grant EP/L01663X/1) and also partially supported from the ASTeC group at STFC Daresbury.

Dr Shantanu Karkari is an associate Professor at the Institute for Plasma Research, Gandhinagar, India. He was sponsored by the University of Liverpool India Fellowship Programme as a visiting fellow to undertake this triple probe project on the HiPIMS experiment.

Professor James Bradley has been chair in plasmas and complex systems at the University of Liverpool since 2004, and now leads a plasma group of seven permanent academic members. His research interests are in the development, diagnosis and applications of plasma discharges. He has published 166 peer-viewed journal papers in these areas. He is also a member of the Cockcroft Institute, Daresbury, UK.