IPP Annual Report 2007 - Max-Planck-Institut für Plasmaphysik ...

It measures back-scattered radiation using the other ECRH
antenna and transmission line located in the same port. This
information will be used to infer the 1-D velocity distribution
of the confined fast ions.
The figure 17 above shows the first scattered radiation results,
an important milestone for the diagnostic. The centre most
channels are plotted as a function of time during a plasma
discharge where the receiver antenna is swept twice across
the gyrotron beam. The vertical lines are the time points
where the receiver antenna position is expected to have
maximum overlap. The commissioning is in its final stages
and physics exploitation is expected in the 2008 campaign.
6.7 Radial acceleration of solid hydrogen pellets
Fuelling pellets, injected into the plasma from the magnetic
high field side (HFS), undergo a strong radial acceleration
towards the magnetic low field side (LFS), as observed by a
fast framing camera system. The camera images allow for the
study of the pellet velocity through the whole pellet trajectory
inside the separatrix, revealing a monotonic increase of the
acceleration as the pellet penetrates deeper into the plasma.
The cause of this radial acceleration is thought to originate
from a rocket-effect by the asymmetric ablation of the pellet
surface. The ablation asymmetry is the result of the grad B
drift of the pellet cloud, which is then shifted relative to the
pellet. As the pellet cloud provides shelter for the pellet from
hot plasma electrons, the shift of the cloud towards the LFS
will result in a higher heat flux (i.e. a higher ablation rate) at
the HFS of the pellet, thus there will be a net reaction force on
the pellet caused by the impulse of the leaving particles.
Simulations, based on the simple but yet most powerful pellet
ablation model, the NGS (Neutral Gas Shielding), were performed
to estimate the ablation asymmetry. It was assumed
that the total ablation rate is given by the NGS model, but a
small fraction (ε) of it is asymmetrically produced on the
pellet’s HFS. The corresponding net radial force on the pellet
and hence acceleration is taken into account in the simulation,
resulting in curved pellet trajectories similar to the measurements.
Simulations were performed with pellet and injection
geometry settings identical to those in the measurements.
The asymmetry parameter (ε) was tuned until the simulated
pellet trajectory fitted the measured one separately for each
pellet event, and the corresponding value of ε was considered
as the output of the simulation. Up to now 16 pellet events have
been studied and asymmetry values fell in the range of 3-9 %.
6.8 New stochastic model for the sawtooth crash
The sawtooth oscillation is one of the fundamental instabilities
observed in tokamaks. Still no definitive explanation for
the crash process exists. Incomplete sawtooth reconnection
is often observed. It is associated with an internal m/n=1/1
kink mode which does not vanish after the crash phase (as
would be the case for complete reconnection).
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It was shown that these sawteeth cannot be fully described
by a pure m/n=1/1 mode and that higher harmonics play an
important role during the sawtooth crash phase. We demonstrated
that stochastization appears due to the excitation of
low-order resonances which are present in the corresponding
q-profiles inside the q=1 surface which reflects the key
role of the central q(0)-value. Depending on this value two
completely different situations are possible for one and the
same mode perturbations: (i) the resonant surfaces are present
in the q-profile leading to stochasticity and sawtooth crash
(q(0)=0.7±0.1); (ii) the resonant surfaces are not present,
which means no stochasticity in the system and no crash
event (q(0)=0.9±0.05). Accordingly the central safety factor
value is always less than unity in the case of a non-complete
sawtooth reconnection. Our investigations show that the stochastic
model agrees well with the experimental observations
and can be proposed as a promising candidate for an
explanation of the sawtooth reconnection.
6.9 Extrapolation of AUG H-mode discharges to ITER
Scaling laws predict ITER’s confinement and fusion performance
in H-mode. Besides the well established IPB98(y,2)
law, others have been proposed with a weaker β dependence,
such as Cordey’s (2005) and the GyroBohm scaling laws
(see figure 18).
Q/(Q+5)
1
0.1
0.05
IPB98(y,2) Cordey05
0.1 1
3.23 -1.23 -3.10
35.5 H β q
98 N 95
ES GB
ITER ITER
ITER
0.1 1 0.1 1
1.82 0.18 -2.20
6.07 H Cor β N q 95
2.22 -0.22 -2.71
22.8 H EGB β N q95
Figure 18: 0D figure of merit of Q/(Q+5) assuming a constant Greenwald
fraction for the different scaling laws, versus Q/(Q+5) predicted with the 1D
ASTRA simulations. IPB98(y,2) (red), Cordey 05 (green), GyroBohm (blue)
Theory based dimensionless models also predict the ITER
performance. However, even those based on the same instabilities
yield different core predictions. Moreover they are
strongly sensitive to the pedestal density and temperatures,
which are predicted with large uncertainties. In this work
we extrapolate AUG discharges to ITER, using the information
contained in the scaling laws with additional input
from present experiments, in particular the profile shapes,
the H factors, the thermal normalised pressure β N,th and q 95 .
A profile database of 92 well diagnosed H-mode discharges
has been selected, including only stationary time intervals.
The scaled profiles have then been used for ASTRA simulations
with ITER geometry, in order to predict the P fus and P rad ,
as well as P aux needed to obtain the target β N,th at the given
H-factor. The effect of varying the tungsten concentrations
is also investigated here. Dimensionless figures of merit are