Jernej Kovacic

Potential formation in front of an electron emitting electrode immersed in a plasma that contains a monoenergetic electron beam

A one-dimensional fluid model of the sheath formation in front of a large, planar electron emitting electrode (collector) immersed in a plasma that contains a monoenergetic electron beam is presented. Expressions for the Bohm criterion, the total electric current to the collector, and for the zero electric field at the collector are derived. When there is no electron emission, the model predicts, in some cases, up to three different solutions. The low and the high solutions correspond to the fact that the presheath potential drop can be determined either by thermal or by the beam electrons. The middle solution between them has no physical meaning. When the electron emission is space charge limited, the model may have up to five solutions because the low solution can sometimes split into three parts. The current-voltage characteristic of the collector that exhibits triple floating potential is calculated and it shows qualitative similarity with experimental ones [C.-H. Nam et al., J. Appl. Phys. 63, 5674 (...

Comparison of measured and simulated electron energy distribution functions in low-pressure helium plasmas

Knowledge of the electron energy distribution function (EEDF) is of great interest in different branches of plasma physics ranging from laboratory to fusion plasmas. In the frame of this work systematic measurements of the EEDF in low temperature helium plasmas (Te ≈ 2 eV) at different working gas pressures and discharge currents (Idis between 1 and 2 A) will be presented and compared with numerical particle-in-cell (PIC) code simulations. The experiments were conducted in the Innsbruck double plasma machine and in the Ljubljana linear magnetic plasma device with helium as the working gas. The EEDF was obtained by the second derivative of the characteristic of a Langmuir probe. The PIC code was used to simulate the EEDF by taking into account most of the physical parameters in the plasma vessel.

Robust Highly Emissive Probe For Plasma Potential Measurements In The Edge Region Of Toroidal Plasmas

B.S. Schneider∗a, S. Costeaa, C. Ionitaa, R. Schrittwiesera, V. Naulinb, J.J. Rasmussenb, R. Stärzac, N. Vianellod , J. Kovacice and T. Gyergyek f aInstitute for Ion Physics and Applied Physics, University of Innsbruck Technikerstr. 25, 6020 Innsbruck, Austria bDTU Physics, Technical University of Denmark Anker Engelunds Vej 1, 2800 Kgs. Lyngby, Denmark cMechatronic Department, Management Center Innsbruck Universitätsstraße 15, 6020 Innsbruck, Austria dCentre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne CH-1015 Lausanne, Switzerland eJozef Stefan Institute Jamova cesta 39, 1000 Ljubljana, Slovenia f Faculty of Electrical Engineering, University of Ljubljana Trzaska 25, 1000 Ljubljana Slovenia E-mail: bernd.s.schneider@uibk.ac.at, stefan.costea@uibk.ac.at, codrina.ionita@uibk.ac.at, roman.schrittwieser@uibk.ac.at, vona@fysik.dtu.dk, jjra@fysik.dtu.dk, Ronald.Staerz@mci.edu, nicola.vianello@epfl.ch, jernej.kovacic@ijs.si, tomaz.gyergyek@ijs.si

Study of boltzmann relation in collisionless magnetized plasma with finite length scale ratios

Summary form only given. The issue of use of Boltzmann relation in magnetized plasmas has been recently again brought to attention by [1, 2]. Boltzmann relation for electrons is a usual assumption in fluid plasma modeling used also in fusion device modeling and it is in general a very good approximation, even though it strictly holds only for massless electrons. But when oblique or wall parallel magnetic field is considered, cross-field diffusion is impeded and even finite Larmor radius effects can influence potential formation in front of the wall. Our task was to study the validity of the Boltzmann factor via a fully-kinetic particle-in-cell code. We did so for finite ratios of three different length scales based on three characteristic dimensions, namely the Debye length, the Larmor radius and the ionization length. We considered a collisionless case, since effects of magnetization can be studied isolated, without side effects of a competing mechanism of collisions. In the simulations particle trajectories are fully resolved and artificial heating is used to maintain the assigned temperature in the volume source region. We qualitatively estimated the deviation of the results obtained from the simulations from the theoretical value calculated via Boltzmann relation in the logarithmic scale and made a simple integral method for the comparison too.