Plasmas have been used in industry for decades with great success. However, in many cases their application is based on trial and error. A deeper understanding of the processes that take place in the excited gases, sometimes in a very short time and on tiny length scales, has been largely lacking until now. Researchers from Bochum and Ulm are venturing into this new territory together in the frame of the Collaborative Research Centre 1316 “Transient Atmospheric Pressure Plasmas: From Plasmas to Liquids to Solids”.

Their fields of work are complementary: Professor Thomas Mussenbrock, who holds the Chair of Applied Electrodynamics and Plasma Technology at RUB, focuses on the plasma as a whole, whereas Professor Timo Jacob, head of the Institute of Electrochemistry at Ulm University, specialises in atomic processes. “We calculate processes at different ends of the time and length scales and try to fill the gap in between,” as Mussenbrock recaps.

Complicated molecules

Timo Jacob is the expert for interfaces between plasmas and their environment. This interface can be between plasma and solid, but also between plasma and liquid. “If we want to use plasmas to coat a surface or to change its structure, we have to understand the interaction between the plasma species and the surface on an atomic scale,” he explains. This process seems simple and predictable. But his team goes into much more detail, because complicated molecules can also form in plasmas, which are present in specific excited states. They can then change both the properties and the structure of catalytic surfaces or even actively impact reactions at the interface directly. Until now, such details had been little understood. “We had to develop the tools for this first,” says Jacob.

He and his team established methods that can describe how the plasma changes the molecules during a reaction. The influence of the molecules, for example, can result in an effect on the interface that becomes meta-stable but of particular interest for catalytic reactions.

The effort usually increases cubically

The processes calculated in this manner take place on the Ångström or nanometre scale within a tiny fraction of a second. The fewer effects the researchers have to calculate simultaneously, the more accurate the simulation can become. “Our highly accurate calculations involve a maximum of 300 to 400 atoms; using semi-empirical dynamic simulations even several 100,000 atoms are possible,” explains the researcher. “We sometimes use specifically acquired computing clusters or mainframes at high-performance computing centres for this purpose.” The effort usually increases cubically: i.e. if twice as many atoms are to be included in the calculation, the computational effort increases eightfold.

The fundamental insights thus gained should help, for example, to make plasmas usable for new applications – most importantly, catalytic processes. The researchers concentrate on both the reduction of carbon dioxide (CO₂) and the decomposition of relatively simple hydrocarbons as a model, using n-butane as an indicator.

Data is extremely scarce

It is of course not enough to consider only a few hundred atoms. This is where the research of Thomas Mussenbrock’s team completes the overall picture. His length scale is a billion times larger than Jacob’s. Quantum mechanical effects that are important at the atomic level no longer need to be explicitly considered in this case. The idea is to map plasma dynamics and plasma chemistry. “We look at the plasma based on classical physics.” The methods for this are known and well understood, but the needed data is extremely scarce.

A major challenge is that industrial processes with plasmas should take place at atmospheric pressure, if possible, and preferably at air pressure. “At this relatively high pressure, many interactions come into play,” explains Mussenbrock. What reacts how strongly with which molecules and atoms? Based on the results of Timo Jacob’s calculations, his team scales up the simulation. The results can be matched with experiments – deviations must be understood and explained.

“Bringing the two ends of the time and length scales together is uncharted scientific territory,” points out Thomas Mussenbrock. Consequently, the teams are breaking new ground. Perhaps at some point, thanks to their findings, it will be possible to make large-scale industrial processes such as ammonia production more energy-efficient.

adapted from Meike Drießen (RUB)

Plasmas play a vital role in many industrial applications. The energetically excited gases can be used, for example, to deposit coatings on surfaces, such as scratch-resistant protective layers on plastic spectacle lenses, or high-precision optical filters on quartz glass. In the process, the plasma is used to bombard the coatings that grow on the substrate by vapour deposition with ions, thus effectively hammering them into place.

But such processes must meet many requirements. For example, they must take place at low temperatures to prevent damage to the coated surfaces. “In modern production processes, more and more emphasis is also being placed on precision,” stresses Professor Ralf Peter Brinkmann, who holds the Chair for Theoretical Electrical Engineering at RUB. All resulting products must be exactly the same, and the coating must be defect-free. To achieve this level of precision, it is necessary to constantly monitor the plasma. The electron density is particularly important in coating processes. If it were to fluctuate too much, this would negatively affect the quality of the finished coating. “Ideally, the electron density should be constantly measured and automatically readjusted if necessary, so that no human has to interfere in the process,” explains Brinkmann.

Small, reliable, maintenance-free

A measuring instrument that can do all this has to meet a variety of requirements: It should be as small as possible, reliable, maintenance-free, and it must neither interfere with the coating process nor be damaged in the plasma. Experts refer to this as “process-compatible plasma diagnostics”.

Researchers have been pursuing one idea for a long time: The electrons, which move freely in the plasma, can be made to oscillate by applying a small external voltage. If the right frequency is hit, a detectable resonance occurs. Since the resonance frequency depends on the electron density, this can then be theoretically calculated.

However, earlier attempts to put this idea into practice encountered some difficulties. A Japanese research group, for example, proposed a very simple construction: The team used a coaxial cable, similar to an analogue antenna cable, allowed the inner conductor to protrude a little, and inserted the end of the cable into the plasma. If a voltage was applied, resonances of the plasma could be measured. However, resonances of equal value occurred at several different frequencies – a veritable zoo. “The problem was: which one should be used for diagnostics,” explains Ralf Peter Brinkmann.

Analyses by the Bochum-based researchers provided an answer to the question of where the different resonances came from: As simple as the design of the measuring equipment was, different oscillations with different resonance frequencies arose at different sections of the apparatus. “Picture this like driving an old car,” as Ralf Peter Brinkmann illustrates the principle. “At a certain speed, the exhaust rattles, at another, it’s the wing mirror.”

The more symmetrical, the better

To remedy the situation, the team devised a concept that aimed for the simplest possible oscillations. Their aim was: the more symmetrical, the better. “The spherical shape is the simplest configuration imaginable,” says Brinkmann. “A floating marble would’ve been our first choice.” However, it was not quite that simple, of course. Electricity always needs a forward and a return conductor. Accordingly, two metallic hemispheres were chosen. The construction is rotationally and mirror-symmetrical, both electrodes are the same size. “Here, too, we measure resonances at different frequencies,” explains Ralf Peter Brinkmann. “But they can be clearly sorted. The strongest resonance is the dipole resonance; the other, weaker ones, represent the overtones to this keynote, so to speak.” The name “multipole resonance probe” (MRP) was derived from the mathematical method of “multipole analysis” used for this purpose.

The reason why researchers opt for the MRP is that the relationship between plasma density and resonance frequency is given by a simple mathematical formula. The plasma density is the only unknown in this equation. Once the equation is solved, it can be calculated from the measured values. “This means that it is not necessary to calibrate the measuring probe before use, i.e. to compare it with other measured values,” elaborates Ralf Peter Brinkmann.

Plasma was kept consistent through constant monitoring

So much for the underlying theory. For practical implementation, the researchers teamed up with three other chairs at the faculty. Professor Ilona Rolfes’ High Frequency Engineering Institute carried out a high-frequency optimisation of the entire system consisting of probe head and holder. For example, it was possible to make the holder practically invisible to the plasma. Professor Thomas Musch’s Institute of Electronic Circuits designed control electronics based on radar technology. And finally, Professor Peter Awakowicz’ Institute for Electrical Engineering and Plasma Technology tested the finished probe in a number of plasma processes.

The Federal Ministry of Education and Research funded the joint projects Pluto and Pluto plus to develop the MRP to the point where it could be used in practice. This also provided the opportunity to test the probe with industrial partners. And it turned out that if the electron density in the plasma was kept consistent through constant monitoring by means of MRP and automatic adjustment of the control, the fluctuations in the process results were significantly reduced.

Today, a spin-off is about to get off the ground.

adapted from Meike Drießen (RUB)

Plasma physics is the study of the behaviour of ionised gases. Statistical physics, fluid dynamics, electrodynamics as well as atomic and molecular physics come together to form a discrete discipline. Plasmas determine both stellar evolution on astronomical scales and etching of nanostructures in the semiconductor industry. Plasma-based engines are already powering satellites in space, and very hot magnetised plasmas may provide clean energy through controlled nuclear fusion in the future. Tiny cold plasmas at atmospheric pressure offer a wide range of applications, from CO₂ conversion to medicine and biology. Major advances in measuring the internal parameters of plasmas and in their simulation have recently contributed to a much better understanding of these complex systems. Wherever the journey into the future takes us, it will not be without plasmas.

by Uwe Czarnetzki (RUB)