1 - Nuclear Sciences and Applications - IAEA

Shine through
IAEA-CN-50/A-VH-ll 463
(a) Conventional Limiter
(b) Impurity Control Limiter
Toroidal dispersal
Wall
Redeposition
0
FIG. 1. Schematic diagram oflimiler geometries: (a) conventional limiter directing impurities into the
plasma; (b) impurity control limiter (ICL) directing impurities towards the wall.
3. LIGHT IMPURITY PRODUCTION AT THE WALLS
Spectroscopic measurements of impurity fluxes from the
stainless steel walls have been made in DITE in both deuterium
and helium discharges over a wide range of densities. Typical
values are presented in Table I. These fluxes have been derived
from chordal emissivities of visible spectral lines of these
species and theoretical photon efficiencies [5]. In the
deuterium discharges the effective yields of impurities are 0.3
oxygen atoms per deuteron and 0.9 carbon atoms per deuteron.
These values are much higher than the physical sputter yields and
indicate either a desorption process or a chemical reaction [7].
Photodesorption can probably be ruled out because the yields are
too low [7]. The absolute flux of oxygen is a factor of A higher
in deuterium than in helium and the flux of carbon is a factor of
7 higher in deuterium. Residual gas analysis also shows an order
of magnitude increase in the impurity release rates (CH^ and CO)
evolved from the vacuum vessel following hydrogen discharges
compared to helium [7]. The impurity gas evolution increases
only slowly after the change in working gas, taking 10 to 20
discharges to reach equilibrium.
Wall