Verlag des Forschungszentrums Jülich
JUEL-3036
Unterberg, Bernhard
Strahlungskühlung durch Verunreinigungen in der Plasmarandschicht des Tokamaks TEXTOR - vergleichende Untersuchungen zur Injektion von Neon und Silizium
120 S., 1995
Abstract
The concept of a cold radiating plasma mantle might help to solve the problem of
energy exhaust in a fusion reactor. Line radiation of impurities localized at the boundary of
the bulk plasma could distribute the heating power onto the whole vessel wall and avoid
dangerous heat loads at the plasma facing components as limiters or divertor plates.
Important requirements have to be fulfilled within such a concept: on the one hand a
stationary impurity concentration at the edge on a high level, sufficient to radiate a
substantial fraction of the heating power and to reduce the plasma temperature at the
boundary and the convective heat flow to the limiters or divertor plates, but on the other
hand a limitation of the central impurity concentration to values compatible with ignition in
a fusion reactor at the same time.
First experiments at the tokamak TEXTOR with neon as the radiating impurity gave
evidence for the feasibility of this concept. Within this work further experiments with neon
and a comparison with silicon as the radiating impurity have been performed. The results are
discussed with the help of model calculations to judge these two elements with respect to
their suitability for radiation cooling by characterizing the mechanisms of their plasma wall
interaction and the properties of radiation and transport.
Whereas up to 95 % of a heating power of 1.6 "!:If.\V can be radiated by neon under
quasi- stationary conditions using a feed- back system to adjust the neon-level, the maximum
fraction of radiated power is limited to 75% with silicon as radiating impurity for the same
heating power. L'1 contrast to the highly recycling neon silicon has a high probability to stick
at the limiters and wall. Therefore the wall and limiters of the tokamak TEXTOR have been
coated by a silicon containing film (siliconization). In that case silicon is released by physical
sputtering, the effective sputtering yield is depending on the plasma edge temperature which
is decreased by radiation from the sputtered particles, Because of this competition between
sputtering and cooling the silicon flux into the plasma is restricted. Additional puffing of
silane (S~) or disilane (Siz!4) increases the radiated power and the plasma density at the
same time. Altogether the silicon flux depends on the density and heating power of the
plasma. It cannot be adjusted freely and is limited for given conditions, leading to the
observed limitation of the radiated power.
The radiated powerfrom neon and silicon is dominated by line radiation and restricted
to the plasma boundary, accordingly a selective cooling of the edge can be observed while
no temperature reduction occurs in 1.1).e central plasma. The local heat flux at the limiters
decreases to t.~e same extent as the radiated power increases. Model calculations of the
density and radiation density of different ionization stages of the impurity species used show
that the radiating plasma mantle is extended up to the radial position where the lithium- like
ionization stage exists. There the local electron temperature amounts to about half the
corresponding ionization energy of the lithium- like ion.
The ion densities of noon and silicon measured at the plasma center show that the
transport of these species is dominated by particle diffusion for the conditions investigated.
Detailed modelling confirms that the effect of neoclassical drifts towards the center on the
shape of the impurity density profile is small compared to the effect of anomalous diffusion.
The resulting profiles are essentially flat in contrast to the peaked electron density profiles.
This leads to hollow impurity concentration profiles and conditions very favorable for
radiation cooling: high impurity concentration at the edge where high radiation losses are
necessary, but moderate concentration at the center (in the case of neon max. 0.6% for 90%
radiation) where temperature reduction and dilution have to be avoided. Additionally it turns
out that due to the dominant influence of diffusive particle transport the processes of plasma
wall interaction and ionization of neutrals at the edge, both together determining the
penetration depth of impurity neutrals, are of major importance for the resulting central
impurity density.
Although the averaged velocity of silicon atoms released by physically sputtering is
similar to that of neon atoms - composed of desorbed and reflected particles - the penetration
depth of silicon atoms is a. factor of 5 smaller than the penetration depth. of neon atoms under
comparable conditions due to the smaller ionization energy of silicon neutrals. This fact leads
to lower ion densities in the plasma bulk for a given flux of neutrals. Therefore the resulting
radiation potential - the radiated energy of an impurity particle during its dwell time in the
plasma - is surprisingly about the 5a111e for neon and silicon in spite of the difference in the
charge number Z. The radiation potential rises non- linearly as the plasma edge temperature
drops and the penetration depth accordingly increases. As a result the largest radiated power
for a given fuel dilution and bremsstrahlung in the center is observed at high plasma
densities. Silicon radiates for a given fuel dilution in the center of the tokamak TEXTOR
about 2.5-3.5 times more than neon. This factor reduces if additional losses by
bremsstrahlung in the center are taken into account. With respect to the limitations by fuel
dilution and bremsstrahlung silicon is a more powerful radiator as neon, the experimental
difficulties to inject silicon on a level sufficient for a radiated power close to the heating
power nevertheless restrict this advantage.
The experiments with radiative edge cooling by neon and silicon show that it is
possible to control of the impurity flux into the plasma, and to establish a high radiation
power at the edge and a moderate impurity concentration at the center at the same time. The
application of the concept in a fusion reactor will strongly depend on the possibility to
remove the helium ash from the plasma sufficiently fast because otherwise the play to inject
additional impurities will be restricted too strongly. The use of a sticking impurity as silicon
may cause problems because of excess wall deposition. This ill fact requires an integrated
concept for the choice of the radiating impurity and the material of the first wall.
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