1 - Nuclear Sciences and Applications - IAEA

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IAEA-CN-50/A-IH-l 155
t = 1 545 s
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r (cm)
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Experiment
Theory
Cu (%o)
« 25602
1.1 1.2 1.3 1.4 1.5 1.6
Time (s)
FIG. 7. Development of the central concentrations for C and Cu during ctr-NI (D° — D + , 0.9 MW).
Experimental values (solid lines) are derived from bolomelry for Cu and CXR spectroscopy for C and
compared with numerical results (dotted lines) from transport code calculations. The insert shows
measured and calculated radiation profiles at the end of the accumulation phase.
mined in this way are shown in Fig. 7. The final concentration obtained during the
radiation collapse phase (t > 1.5 s) is ~0.7%. These results are consistent with
infrared bremsstrahlung measurements. There an increase of Zeff{0) from 2.1 (for
t < 1.3 s) to 3.3 at t = 1.45 s is observed, corresponding to an increment of
Ancu(0)/ne(0) = 0.4%. In contrast to this marked increase of Zeff in the plasma
centre, no changes are observed in the outer region (Zeff (r/a = 5/8) = 2.3).
In addition, direct measurements of the C-density at four different radii (r = 5,
15, 25, 35 cm; a = 40 cm) have been performed by means of CX spectroscopy. The
temporal variations of the C-concentrations nc/n,. are found to be surprisingly small
at all radii. The trace for the innermost position, r = 5 cm, is also shown in Fig. 7.
Obviously there is no indication of accumulation of C from these measurements. It
should be noted that the absolute value of the plotted C-concentration has been determined
from Zeff{0) measurements at early times (t ~ 1.2 s) assuming an oxygen/
carbon ratio of 2/1 as measured in the boundary region. The absolute values obtained
from CX measurements are about two times higher. They depend, however, sensitively
on the proper recombination cross-sections and ion f-mixing collisions.
In order to assess whether our experimental results can be understood in terms
of neoclassical transport they are compared with corresponding code predictions. Our
basic concept is the assumption that under normal low confinement conditions the
stationary state is characterized by a balance between neoclassical inward fluxes and
anomalous diffusive outward fluxes. Turbulence enhances the anomalous diffusion
coefficient under low confinement conditions to values of about Dan ~ 0.6 m 2 /s, so
that the relatively large neoclassical inward convective drift of the order of 1.3 m/s
at r = a/2 is compensated. In cases of improved confinement, Dan is decreasing,