Lectures Ruhr University Bochum: Prof. Dr. Achim von Keudell, Julian Held, Sascha Monje

We invite to lectures given by collegues from Ruhr University Bochum:

4.3.2020 14:00 Prof. Dr. Achim von Keudell "Nanosecond Plasmas in Liquids - ignition, cavitation, plasma propagation"

Plasmas inside and in contact with liquids have been a highly promising field of study for the decontamination of water, wound healing in the field of plasma medicine and
modification of metallic and organic surfaces which are in contact with the treated liquid. Especially the ignition of a plasma inside a liquid has generated special interest due
to the enhanced mass transfer of the reactive species inside the liquid. Those plasmas can be ignited by different techniques such as the insertion of gas bubbles where the plasma
discharge appears inside these bubbles or the discharge directly at a high voltage electrode which is immersed inside the liquid. 

 The understanding of the physics of the plasma ignition and sustainment is, however, still in its infancy due to the complexity of the phenomenon. Plasma generation depends on the
characteristics of the plasma electrode, the powering of the electrode, and the properties of the liquid. For example, if the rise time of the voltage is of the order of many
microseconds, the Ohmic heating of the liquid by the dc current will locally cause water vapour bubbles to be formed until the ignition criterion inside that bubble is met. A
plasma is generated in the void and the plasma pressure continues to increase the bubble. If, however, the rise time of the voltage is of the order of ns and the voltage several
kV, electrostriction is very large so that rupture of the liquid occurs at a negative pressure of at least 24 MPa. This causes the formation of voids in the liquid, which are
filled by the water saturation pressure. If these cavitation voids are large enough, the Paschen criterion may be fulfilled and ignition occurs. As an alternative, microbubbles
form dissolved gas in the liquid may be present at ambient pressure, which may then serve as ignition sites for plasma generation. This ignition converts the liquid spontaneously
in the plasma state, which then expands this initial void to form a bubble. This creates a pressure pulse of the order of 10s of GPa depending on the HV voltage and the emission of
acoustic waves that can be observed via shadowgraphy. At the same time, the plasma pressure expands the initial bubble until a specific size is reached before the counteracting
force due to surface tension causes this bubble to collapse. This sequence is monitored by shadowgraphy and compared to cavitation theory. The dissipated energy by the plasma
drives the adiabatic expansion of water vapor inside the bubble from its initial super critical state to a low pressure, low temperature state at maximum bubble expansion reaching
values of 103 Pa and 50 K, respectively. These predictions from cavitation theory are corroborated by optical emission spectroscopy (OES). In addition the ignition process itself
is analyzed with optical spectroscopy at 2 ns time resolution. This reveals a background consisting of black body radiation from the boiling tungsten tip and the Hydrogen Balmer
series which is strongly affected by self absorption.

4.3.2020 10:00 Julian Held, Sascha Monje "Transport in HiPIMS plasmas - phase resolved plasma diagnostics"

High power impulse magnetron sputtering (HiPIMS) plasmas use conventional magnetron targets, but applying the power as short pulses with power densities at the target of several
kWcm-2 and pulse lengths of 10 to 200 µs and duty cycles of a few percent only. HiPIMS plasmas are characterized by a high degree of ionization and a very energetic metal growth
flux leading to superior material properties.

Many studies focus on unraveling the dynamic of these HiPIMS plasmas. The intense sputter wind in a HiPIMS pulse causes gas rarefaction after a time span of 10...30µs after the
onset of the plasma pulse. At target power densities above 1 kW cm-2, localized ionization zones, so-called spokes, are observed which rotate along the plasma torus in ExB
direction (or in the opposite direction) with a typical velocity of 10 km s-1.  It is assumed that the localized ionization zones correspond to regions of high electrical
potential, and are, therefore, the source of an energetic group of ions of typically few tens of eV in the growth flux on the substrate. The spoke pattern depends on target
material, plasma gas, power density and pressure. By adding a reactive gas such as oxygen or nitrogen to a HiPIMS plasma specific oxides and nitrides can be deposited on the
substrate.

This phenomenon is analyzed using time resolved probe diagnostics and optical diagnostics to follow the plasma evolution together with a mapping of the ransport along the target
using a marker technique. It shown that structure formation in these plasmas are conssitent withe the occurence of drift waves leading to the appearance of rotating spokes along
the racetrack of the magnetrons. The underlying mechanisms are discussed to explain the good performance of HiPIMS plasmas for material synthesis.

in Dean's Office, lower meeting room. Invitation flyer pdf.