Have you ever heard of (inverted) plasma fireballs?

 Plasma fireballs come in two different varieties: classical fireballs (or anode spots, as they were called in former times) and inverted fireballs. They emergy when a positivle biased electrode is inserted into an existing background plasma. If the potential on the elcetrode is high enough, the initially 2-D sheath will evolve into a 3-D fireball as shown here:

This expansion happens because the electrons, which are attracted from the surrounding plasma gain enough kinetic energy in the sheath and can, thus, induce ionisation processes within the sheath. Then the sheath has to expand into a 3-D plasma fireball in order to compensate for these additional ions. If this happens inside a highly transparent gridded electrode, we speak of an inverted fireball.

Now, why are those plasma phenomena interesting for ongoing research?

There are different answers for this question, depending on which line of research you are in. If you are working in fundamental physics, you might be interested in the basic properties of such fireballs, sinco not so much is understood about them. The exhibit a variety of interesting non-linearities and instabilities, they may become self-pulsating and their basic physics may also change due to their spherical symmetry (in contrast to the mostly used Cartesian symmetry). If you are working in applied physics or plasma technology then you might be interested in the facts that (inverted) fireballs are producing high energy ions (up to several tens of eV). They also have been demonstrated to work excellent for surface treatment, such as etching or deposition. Especially inverted fireballs are advantageous for these purposes because they consist of a very homogenious plasma with an increased plasma density compared to the background. Several papers have been published in the last few years highlighting those interesting possibilities. If you want to get an overview of research into plasma fireballs, check out the review paper 'Sheaths and Double Layers with Instabilities', which has been published in the Journal of Technological and Space Plasmas (www.jtsp.eu) in 2021.

On a sidenote: The Journal of Technological and Space Plasmas is full open access and has also published other fireball related papers and welcomes submission on different topics of plasma physics. For the rest of the year 2021 there will not be any open access publication fees for authors outside of the G7 countries. So, check out the papers and the journal and read about inverted and non-inverted plasma fireballs and leave some comments below!