ABSTRACT
IAEA-F4-TM-42581
IAEA Technical Meeting on Atomic, Molecular and Plasma Material Interaction Data for
Fusion Science and Technology
Relevance of molecules in ionizing and recombining plasmas in the
divertor and in negative ion sources for fusion
U. Fantz, D. Wünderlich
Max-Planck-Institut für Plasmaphysik, Garching, Germany
ursel.fantz@ipp.mpg.de
The plasma edge of fusion devices is characterised by cold temperatures with respect to the
hot core. Specifically in the divertor region the electron temperature is typically a few eV; in
the detached plasma regime where volume recombination dominates it is even below one eV.
Here, the survival length of hydrogen molecules produced at surfaces by recombination of
hydrogen particles is high. These molecules can undergo a variety of reactions, among them
dissociation, dissociative ionization and dissociative recombination. The relevance of these
reactions depends on the plasma regime, i.e. whether the plasma is ionizing or recombining or
in the transition between both regimes. Another important parameter is the vibrational and
rotational excitation in the ground state of the molecule and the ion, which in particular
depends on the isotope.
Molecules are also important in the negative hydrogen ion sources under development for the
neutral beam heating systems for ITER. In the low temperature plasma of these sources the
degree of dissociation is less than one and vibrationally excited hydrogen molecules play a
dominant role in the plasma chemistry. In particular they are important for the production of
negative hydrogen ions in the plasma volume. In these sources, the plasma is separated by a
magnetic filter field into an ionizing and a recombining part and dissociative recombination of
molecular ions becomes important in the transition region. Close to the extraction system
exists an ion-ion plasma (i.e. positive and negative ions are dominant whereas electrons play a
minor role) and mutual neutralization with molecular ions is a dominant process. Molecules
are also involved in the beam neutralisation and important for beam spectroscopy. Since these
sources operate in hydrogen and in deuterium isotope effects are of particular interest.
In addition, the presence of molecular species in the plasma may influence the interpretation
of diagnostic results. An example is the Balmer line radiation, which is sensitive on
dissociative excitation and dissociative recombination and has consequences on the
determination of the recycling flux. As a consequence, collisional radiative models need to be
used in which the different species are coupled to the excited states of the hydrogen atom:
molecules, molecular ions, the atom itself and its ion, and negative ions. The realization
however, relies strongly on the availability of the underlying cross sections, which should be
taken into account preferably vibrationally – and even more demanding rotationally –
resolved. In particular here, the low energy range from the threshold to a few tens of eV is
relevant. In contrast, the high energy range of several keV is of interest for beam
characterisation.
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