Different mechanisms of electron energization in spokes observed in ExB plasmas

Magnetron discharges operated at low pressures produce plasmas used for

thin film deposition. Similar ExB plasmas are employed for many other purposes, for example,

in Hall-effect thrusters utilized for space propulsion. Under certain conditions, such plasmas exhibit the development

of large-scale self-organized structures called spokes, which are related to zones of enhanced ionization.

With the plasma potential undergoing a sharp jump at the spoke front and a large population

of highly energetic electrons close to the front, this phenomenon can be explained by the electrons

gaining energy via crossing the equipotential surfaces toward the potential hump. Since the

ExB drift is aligned with such surfaces, this can occur only due to other types of motion arising

in a nonuniform magnetic field (grad-B drift [1]) or in a nonuniform electric field. The latter also takes

place in ExB plasmas with a uniform magnetic field, where spokes are also observed [2].

In this talk, we focus on different spoke electron energization mechanisms (adiabatic and

nonadiabatic) linked to the corresponding electron motion in nonuniform electric and magnetic fields.

[1] J.-P. Boeuf and M. Takahashi, Phys. Rev. Letters 124, 185005 (2020)

[2] L. Xu, D. Eremin, and R. Brinkmann Plasma Sources Science and Technology 30, 075013 (2021)