Speaker
J. Ledig
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1016.pdf
Measurement of a tilting cylindrical probe in a RF magnetized plasma
discharge
J. Ledig1
, E. Faudot1
, N. Lemoine1
, S. Heuraux1
, M. Usoltceva2
, J. Moritz1
1
Institut Jean Lamour, Nancy, France
2
Max-Planck-Institut für Plasmaphysik, Garching, Germany
I-V curve coming from Langmuir probe measurements is assumed to provide many
physical plasma parameters as density, temperature, plasma potential, and floating potential.
In the presence of magnetic field, probe understanding becomes more difficult because the
magnetic field breaks down isotropy. Some works had already been done in this field for
electron collection on a wall and parallel to magnetic field [1] and for a semi-infinite
cylindrical probe aligned with magnetic field [2]. However, probe measurements in
magnetized plasma is still a challenge especially in the case of a tilted probe with respect to
the field (which occurs in Tokamaks). For the present work measurements were done in a
linear plasma reactor, ALINE, a chamber of 1 m length and radius of 35 cm. The 1.2 Pa
helium plasma was generated by an RF antenna at frequency 25 MHz. The injected power
went from 30 W to 200 W. The 1 cm long cylindrical probe of radius 75 µm compensates
RF fluctuations thanks to a compensation electrode and small chokes. Magnetic field was
set between 0 and 100 mT and few tilting angles were used for Ψ from 0° (probe aligned
with B) to 90° (probe perpendicular to B). Comparaisons of IV curves showed that
increasing Ψ tend to increase electron saturations curents, but ion saturations currents
remains approximatly the same. For low Ψ a current bump between exponential part of I-V
characteristics and electron saturation current was noticed. A previous theory of electron
collection for Ψ=0 on cylindrical probe for different values of magnetic field was used here
using a simple projection to calculate plasma parameters in every tilting angle.
[1] J. Moritz et al., Plasma sheath properties in a magnetic field parallel to the wall,
Physics of Plasma 23, 062509 (2016)
[2] J.G. Laframboise et al., Theory of a cylindrical probe in a collisionless
magnetoplasma, Physics of Fluids 19, (1976)
