IPP Annual Report 2007 - Max-Planck-Institut für Plasmaphysik ...

Helmholtz Junior Research Group
“Computational Material Science”
Head: Dr. Ralf Schneider
The group studies effects on materials in contact with fusion
and low-temperature plasmas. The major objective is the
development and application of computational physics tools.
Development of a Multi-scale Model for the Interaction of
Hydrogen with Graphite
The multi-scale model was extended further by including the
formation and destruction of hydrogen molecules and the chemical
reactions between hydrocarbons and hydrogen (Küppers-
Hopf cycle). The model has been applied to study hydrogen
retention and release from deposits collected from the leading
edge of the neutralizer of Tore Supra. The simulations showed
that the macropores play the dominant role in the retention and
release behavior of hydrogen. The hydrogen released from
the micropores and mesopores is adsorbed on the surfaces of
the macropores. This internal deposition of hydrogen increases
the tritium retention problem for carbon in a fusion reactor.
Atomistic Description of the Plasma-wall Interaction using Ab
Initio Methods
Highly accurate global potential energy surfaces for C2H +
3 and
C2H +
5 were generated using cluster expansions. They agree
well with spectroscopic frequencies within several cm-1 .
Density Functional Theory methods were applied to study
the diffusion of atomic hydrogen trapped within crystalline
graphite. The ab initio molecular dynamics calculations at
10 K and 300 K show that the H atom forms a chemical bond
with one of the C atoms in graphite. At these temperatures
the H atom is not able to overcome the barrier and thus is
not able to diffuse. These results drastically differ from
those obtained by the use of the empirical Brenner potential
where the H atom is able to diffuse freely. The potential
energy curves calculated using the Brenner potential show a
flat potential in the central region between graphite planes
whereas for DFT calculations a higher potential energy
region between the two planes is observed due to the contribution
from the non-local pseudo-potential energy term.
Kinetic Modelling of Complex Plasmas
Kinetic modelling of plasmas with PIC (Particle-in-Cell)
methods was done in close collaborations with experiments
at the Ernst-Moritz-Arndt University in Greifswald within
the Transregio TR-24 project. Calculation of spatial-temporal
emission profiles in RF-discharges in oxygen including
the formation of negative ions and comparison with experiments
allowed for an estimate of the heavy ion collision
cross-section for dissociative excitation of atomic oxygen
+ by energetic O2 . The oxygen molecular ions get their maximum
energy at the electrode after crossing the sheath.
Theoretical Plasma Physics
91
Figure 4: Distribution of the plasma potential in an azimuthal segment for
quasi steady-state in SPT-100. The fluctuations are clearly visible.
PIC-modelling of electric propulsion Hall thrusters (SPT-100)
demonstrated the importance of secondary electron emission
for the operation of this system: electrons emitted from
the surface of the SPT-100 have lower energies than the
impinging ones. They leave the walls on a different spiral
trajectory which is displaced towards the anode. This creates
a high density low energy layer close to the walls which
increases the electron conductivity strongly in this region.
Another reason for electron cross field transport and axial
currents beyond classical estimates are azimuthal fluctuations,
which were visible in the simulations.
Figure 5: Time evolution of the azimuthal profile of the plasma potential in
the center of the discharge channel. The propagation of the fluctuations
can be seen.
Scientific Staff
K. Matyash, A. Rai, R. Schneider, A.R. Sharma, F. Taccogna.