Speaker
S. Kuhn
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.4014.pdf
On universal properties of the plasma–sheath transition and large-size
sheath structures
L. Kos1 , N. Jelić1 , S. Kuhn1 , D. D. Tskhakaya (sr.)2,3 , T. Gyergyek4,5
1 Faculty of Mechanical Engineering, University of Ljubljana, 1000 Ljubljana, Slovenia,
2 Plasma and Energy Physics Group, Institute for Theoretical Physics, University of Innsbruck,
A-6020 Innsbruck, Austria,
3 Also at Institute of Physics, Georgian Academy of Sciences, 0177 Tbilisi, Georgia,
4 University of Ljubljana, Faculty of Electrical Engineering, Ljubljana, Slovenia, 5 Jozef Stefan Institute,
Ljubljana, Slovenia,
Recently a unified Bohm criterion has been formulated [1] in the form of the ion directional
energy expressed as a function of fluid, kinetic and electrostatic-pressure contributions. While
the famous purely kinetic criterion of Harrison and Thompson [2], is satisfied exclusively at the
plasma edge and for vanishing Debye lengths, the new unified Bohm criterion holds at any point
of the discharge even if the quasineutrality is not well satisfied (i.e., in vicinity of the sheath en-
trance) irrespectively on the ion temperatures and Debye length, provided that the latter is small
enough for the plasma criterion to be applicable. The plasma edge (PE) and the sheath edge
(SE) have been identified as the points of inflection and the maximum of the charge density
derivative with respect to the potential, respectively. Plasma quasineutrality is well satisfied up
to the PE, with a non-negligible electrostatic pressure taking place only between the SE and the
electrode/wall. The region between the PE and the SE, identified as the plasma–sheath transition
(PST), turned out to be characterised by a universal value (about one third of the electron tem-
perature) which is quite insensitive to ion temperature and the Debye length, while an increase
in Debye length from zero to finite values causes the location of the sonic point/potential (lying
inside the PST) to shift from the PE (for vanishing Debye length) towards the SE. Outside the
PST, the electrostatic-pressure term and its derivatives turn out to be nearly identical with each
other (independently of the particular values of the ion temperature and Debye length).
In the present investigation we investigate further the features described, however under the
condition that the ionisation within the sheath, and thus the kinetic contribution to the unified
Bohm criterion cannot be completely neglected. This is achieved by decreasing the plasma
density, so that the sheath thickness is comparable with the plasma length, and with employing
a constant ion production profile (independent of position, i.e., potential). The results obtained
by means of the theoretical method from Ref. [1] are compared with the results obtained here
after performing a series of kinetic numerical simulations. Special attention is paid to possible
universal sheath solutions of large size, under the condition of a non-negligible ion production
rate inside.
References
[1] L. Kos et al., Introduction to the theory and application of a unified Bohm criterion for arbitrary-ion-temperature
collision-free plasmas with finite Debye lengths to appear in Phys. Plasmas (2018);
[2] E. R. Harrison and W. B. Thompson Proc. Phys. Soc. 74, 145 (1959);
