1 - Nuclear Sciences and Applications - IAEA

IAEA-CN-50/A-V-4 313
the laser-blowoff technique [10]. The impurity line brightnesses as a
function of time are simulated with the one-dimensional radial impurity
transport code, MIST, which solves the impurity continuity equations:
3/3t nq = - 1/r d/dr (r r ) + Iq_j n^ - (Iq + Rq )nq + Rq+1 nq+1
, + Sn
where n is the impurity density in ionization state q, F is the impurity flux
density, and I and R are ionization and recombination rates, respectively.
S_ is the impurity source which is assumed to be an instantaneous burst of
monoenergetic impurity atoms at 3 eV. x is the confinement time for parallel
flow to the limiter in the scrape-off region and is parameterized as X /D,
where X is a scale length of the scrape-off region, and D is the impurity
diffusivity. The definition of F is the same as used in the gas puff
experiments, with the additional assumption that V/D is equal to
cv3ln(ne)/dr, where cy is a peaking parameter which relates the impurity V/D
to the electron density profile scale lengths. In equilibrium, the total impurity
density profile would have the shape of the electron density to the power of
c . For these simulations D is assumed to be constant across the minor
radius, and both D and V are assumed to be independent of q.
This discussion will focus upon experiments where the brightness
and time evolution of lines emitted by edge (Ge , and Ge ), and central
(Ge ) states were measured in a series of ohmically heated plasmas. A
detailed study [11] of the sensitivity of the modeling to the ionization and
recombination rates and transport shows that the time evolution of the lines
emitted by central states is determined primarily by transport effects, while
those of the edge states are determined by ionization, recombination, and
transport. As a result, it is not possible to uniquely deduce the spatial
dependence of D and V from these data alone, and D and V are
parameterized as described above.
Impurity transport was studied in ohmically heated deuterium and
helium plasmas with L = 0.8-2.5 MA and ne = 0.6-6.0 x 10 19 m' 3 .
Impurity diffusion coefficients are found to be in the range of 0.5-1.5 m /s,
(2)