Commentary: Rotating structures in low temperature magnetized plasmas - insight from particle simulations

In a recent paper [1], Boeuf has addressed the question of electron transport in low temperature E × B discharge devices using PIC MCC simulations. Using a general approach, it has been demonstrated that particle simulations can be an extremely powerful tool to unravel some of the mysteries of electron transport in these devices. Even through the aim of the paper was not to provide a complete review of instabilities and anomalous transport in low-temperature E × B discharge devices but rather to address these questions on a few specific examples, one of the examples—electron vortices at low pressure—deserves some comments. This is because of a lack of connection in [1] between PIC MCC simulation results and a large volume of experimental data on the subject. In particular, discussion on the results of Abolmasov et al. [2] (cited as Ref. [18] in Boeuf 's paper) that describe different type of instability/oscillations as shown below is misleading. Kaganskii et al. [3] were perhaps the first who noticed that Penning discharges in argon at low pressures can exhibit two types of oscillations. In the majority of cases, the oscillations are incoherent, radio-frequency, with a broad spectrum of frequencies present at the same time. The frequency of these oscillations is gas pressure independent and scales with the electric and magnetic field in the same manner as the frequency of diocotron instabilities (ν ∝ E/B) [4]. It appears that electron vortices revealed by 2D PIC MCC simulations in low pressure cylindrical magnetrons [1] have very similar features. The vortex dynamics is usually chaotic (see Figure 4 in Boeuf [1]), which explains irregularity in the observed current curves (Figure 5 in Boeuf [1]). However, there are certain conditions under which the Penning discharge in argon acts as a source of intense, coherent, almost sinusoidal oscillations in the range of 1–100 kHz [3]. Similar coherent, low-frequency oscillations have been observed in inverted magnetrons [5] and in an anode layer ion source (ALIS) [2]. In contrast to the rf oscillations, the frequency of these oscillations is gas pressure dependent and scales with pressure and magnetic field in the same manner as the classical cross-field electron mobility (µ e⊥ ∝ p/B 2) [4]. Despite these differences, both types of oscillations result in the production of excess-energy electrons that move along the magnetic field to the electrodes at the cathode potential. It is interesting therefore to pose the question …