1 - Nuclear Sciences and Applications - IAEA

334 TEXTOR TEAM
The hydrogen recycle and wall inventory problems are important
but so involved, multi-faceted and interrelated that time
will be required to elucidate them fully, even with all necessary
diagnostics available.
Understanding carbon erosion and redeposition is urgent and
crucial since graphite is contemplated as first wall material in
future devices. The C flux from limiters has been measured by
the same spectroscopic method used for H and 0 [16, 17]. Measurements
with a sniffer probe equipped with a heatable inner graphite
target [22] have demonstrated the importance of chemical
erosion by hydrogen from the plasma. The same conclusion is
drawn from the measurements of the C fluxes from the limiters
[17, 18]. Chemical erosion by hydrogen at the wall had also to
be assumed to explain the redeposition profiles in the SOL
[23, 24]. In addition, chemical erosion by oxygen occurs
[12, 18, 22] but is in principle avoidable if we can reduce the
oxygen contamination. A rapid redeposition of carbon (order of
lO 15 atoms/em's) occurs in the SOL. The atoms leave probably the
wall as CH4, are ionized, dissociated in the SOL and guided by
the magnetic field to obstacles situated in the limiter shadow.
Thus a screening effect occurs. The redeposition rate depends on
the distance along field lines between the collector probe and
the next obstacle. When this distance is very large, carbon diffusing
out of the plasma is also collected on the deeper-lying
probe.
Results relating to the oxygen fluxes have been mentioned
in § 2.1. The presence of metal ions is continuously surveyed by
their X-ray radiation from the plasma center. TEXTOR is recarbonized
whenever their concentrations reach regularly a value higher
than 0.02 %. Impurity collector probes confirm the fact
that the metal fluxes are very low.
3.PROPERTIES OF SOL AND OF EDGE PLASMA
The TEXTOR results concerning the plasma edge have been reviewed
recently by Samm [18], Differences arising in the presence
of toroidal and poloidal limiters, the influence of the plasma
species (H, D, He), the detached and attached plasma conditions,
the particle transport and the influence of impurities
are addressed (see also [25, 26]). Lack of space restricts the
present summary to two major points:
Earlier measurements of the ne profiles in the SOL had
shown L25J that the e-folding length \ varied significantly
with the distance (along field lines) fr'om a poloidal limiter.