Prime pagine RA2010FUS:Copia di Layout 1 - ENEA - Fusione

magnetic confinement (cont’d.)
progress report
2010
039
ENEA Frascati scientists have participated in the analysis of the data gathered during the last campaigns and
finalized journal papers and contribution to international conferences, such as EPS, SOFT, IAEA and others.
On the basis of data taken during the last experimental campaigns, significant progress was made in terms of
analysis, interpretation and diagnostic commissioning. Highlights regarding the main areas of interest are
reported below.
Pellet studies
The analysis of pellet ablation and particle deposition experiments has been completed and presented at the
2010 IAEA Conference [1.90]. Data show a clear evidence of ∇B drift effects in agreement with code
simulations (fig. 1.47). These simulations were performed within the JINTRAC code suite by using JETTO as
a core transport code and HPI2 for pellet ablation and ∇B drift effect calculations. In L–mode or moderately
heated H–mode this effect seems to be weak. At high heating power instead, injection from the vertical high
field side (VHFS) has proven to be much more effective than from low field side (LFS) in depositing particles
beyond the pedestal. In the latter case,
ablated particles are quickly lost due to a
combination of ∇B drift and edge
instabilities. The fuelling performance,
although investigated only in a limited
range of plasma and pellet parameters,
seems to be promising for the capability of
pellets to raise the density without
increasing the neutral pressure in the main
chamber. This latter feature is important
for ITER, since it confirms that pellets are
less demanding in terms of pumping
capabilities.
High beta operation
Stability of high–beta advanced scenarios
has been analyzed for a wide range of
q–profiles and normalized beta values up
to β N
=4. Global n=1 instabilities limit the
achievable β N
or degrade confinement.
Most recent results were presented at the
2010 IAEA Conference [1.91]. Stability
boundaries in terms of q min
and pressure
peaking have been determined. For
relatively broad pressure profiles the limit
decreases from β N
=4 at q min
=1 to β N
=2
at q min
=3, while at fixed q min
it decreases
with increasing pressure peaking. Bursting
and continuous n=1 instabilities have also
been analyzed. A new form of instability
that grows on typical resistive time-scales
but has kink internal structure has been
identified. This instability systematically
occurs above the no–wall ideal stability
limit as calculated for n=1 modes by the
MISHKA code. The measured mode
structure is in good agreement with
eigenfunctions given by the code. An
overview of the mode analysis results is
given in figure 1.48 on a β N
vs q min
plane.
Δn/Δn peak
1.0
0.6
0.2
JPN 77863 – LFS hybrid
-0.2
0 0.4
r/a
a) b)
JPN 77864 – VHFS hybrid
0.8 0 0.4 0.8
r/a
Figure 1.47 – Particle deposition code simulations (blue) show a clear
effect of ∇B drift on pellets injected from a) the LFS, and b) VHFS into
high power (∼21 MW) hybrid discharges. Particles injected from LFS
are rapidly displaced outwards as compared to ablation (red) and
leave the main plasma, while in the VHFS case they are shifted inwards,
thus producing a significant deposition (black), as measured by the
HRTS
βN
4
3
2
1
0.5 1.5 2.5
q min
no n=1
Broad b.
Chirping
Disrupt.
Cont.
Figure 1.48 – Distribution of stability characteristics in the β N –q min
diagram for MHD modes with toroidal number n=1. Circles and
squares are taken at maximum β N in discharges without any n=1 mode
and with weak broadband n=1 activity respectively. Triangles
represent discharges with bursts of short–lived (10 ms) intense
chirping modes, in particular downward pointing triangles represent
disruptive cases. Stars represent the onset of long–lived n=1 modes