The article of Chauhan et al. [“Plasma fireball: A unique tool to fabricate patterned nanodots,” Rev. Sci. Instrum. 88(6), 063507 (2017)] describes the very interesting idea of utilising the plasma phenomenon of fireballs for the creation of patterned nanodots on a GaSb substrate. For this purpose, the authors obtained a large plasma fireball in a magnetised background plasma and used it to accelerate ions in the sheath, which surrounds such a fireball. Chauhan et al. were able to demonstrate the production of large ion fluxes that can be extracted from the fireball and that the properties of these fluxes define the geometric structure of the nanodots on the substrate surface. Hence, the nanodot pattern can be easily controlled by the discharge parameters of the plasma fireball. This is clearly a novel method of fireball-induced surface modification. However, plasma fireballs themselves have been known for about hundred years, although as a very particular plasma phenomenon. Therefore, this letter aims at providing some additional background information and references on this topic for the interested reader.
I. A SHORT HISTORICAL OVERVIEW OF FIREBALL RESEARCH
Fireballs or anode spots, as they are referred sometimes to, were discovered at the beginning of the 20th century by Lehmann.2 Lehmann observed the appearance of bright spots on the surface of an auxiliary anode in an existing background plasma when the electrode bias exceeded the ionisation energy of the working gas. These regions of bright glowing, dense plasma are created by electrons, which are accelerated in the sheath surrounding the anode. If the potential drop in front of the anode exceeds the ionisation potential, high energetic electrons are able to induce impact ionisation processes that alter the space charge distribution in front of the electrode in a way that the two-dimensional Debye sheath expands into a three-dimensional plasma fireball. The main physical properties of fireballs, like the plasma potential, charge densities, and temperatures, were described byseveral authors in great detail.3–7 Especially Baalrud et al. studied the ion fluxes emanating from a fireball in a magnetised plasma,8 which bears a direct relevance to Ref. 1.
In more recent years, a new configuration of plasma fireballs, a so-called inverted fireball was discovered by Stenzel et al.9,10 This type of fireball is formed inside a highly transparent gridded anode, where oscillating fast electrons induce additional impact ionisation processes. Fig. 1 shows the historical “evolution” of the discovery of plasma sheath phenomena.
“Evolution” of the sheath: From a two-dimensional Debye sheath (left) over a classical fireball (centre) to an inverted fireball (right).
“Evolution” of the sheath: From a two-dimensional Debye sheath (left) over a classical fireball (centre) to an inverted fireball (right).
The basic plasma parameters of inverted fireballs were examined by Gruenwald et al.,11 who also applied them with different co-workers for surface modifications, such as deposition, sputtering, or bias enhanced nucleation.12–14 The last references in particular show that different fireball configurations have already been used to modify surfaces of different substrates, which is considered to be important supplementary information to the work of Chauhan et al.1
