Verlag des Forschungszentrums Jülich
JUEL-3985
Kalupin, Denis
Influence of an external gas puff on the RI-mode confinement properties in TEXTOR
91 S., 2002
An actual subject of experimental and theoretical studies in present day fusion
research is the development of an operational scenario corobining simultaneously high
confinement, with at least H-mode quality, and high densities, around or above the
eropirical Greenwald limit. Recently, this subject was studied in TEXTOR Radiative
Improved (RI) mode discharges, in which the seeding of a small amount of iropurities
is helpful in a transition to the iroproved confinement stage. It was found that by the
careful tailoring of extemal fuelling and optimisation of the wall conditions it is
possible to maintain the H-mode or even higher quality confinement at densities much
above Greenwald density limit. However, more intense fuelling, aimed to extend
maximal achievable densities, led to the progressive confinement deterioration.
The theory explains the transition to the RI-mode as a bifurcation into the stage where
the transport govemed by the Ion Temperature Gradient (ITG) instability is
significantly reduced due to a high density gradient and high value of the effective
charge. The numerical studies of an influence of the gas puff intensity on confinement
properties of plasma, done with the help of the 1-D transport code RITM, show that
the same theory can be used for an explanation of the confinement rollover triggered
by a strong gas puff.
The code was modified in order to simulate the effect of the gas puff on the
confinement properties. The anomalous transport coefficients in the plasma core
include contributions froro the ITG and Dissipative Trapped Electron (DTE)
instabilities. The transport at the plasma edge under RI-mode conditions might be
described by the electrostatic turbulence caused by electric currents in the scrape-off
layer of the limiter. The present computations show that this assuroption for the edge
transport does not allow the modeling of an effect of the gas puff intensity on the
profiles evolution in agreement with experimental observations.
The level of the edge transport must be increased significantly in order to reproduce
the evolution of the plasma density and the effective ion charge profiles during
confinement degradation caused by a strong gas puff. An increase of this order is in
agreement with reflectometer measurements and could tentatively be explained by the
effect of neutrals on Drift Resistive Ballooning instability.
Although, an increase of the edge transport is a necessary condition for the core
confinement deterioration, the latter is produced by the resuroption of ITG instability
in the plasma core. The present studies show, that the reduction of iropurity content of
plasma and high positive time derivative of the density provided by a strong gas puff
are crucial conditions for the restart of the ITG mode.
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