2000 PROGRESS REPORT - ENEA - Fusione

1. Magnetic Confinement
terms occurs at the center, while the q profile appears to be hollow with q 0 ≈3 and q min ≈2. The
plasma is MHD-quiescent as long as the sawtooth activity does not develop. The T e profile at
the time of the maximum temperature is narrower than in the ramp-up case, fig. 1.20, possibly
because of the reduced width of the low shear (s≤ 0.5) region. Z eff is ~3 in this scenario, while
current ramp-up phase has much higher values, with Z eff =7÷10. The pulse demonstrates that
high T e values can also be obtained on the current flat-top phase, provided that the current profile
is far from the standard sawtooth regime. In this pulse comparison between the experimental
neutron yield and the estimation, made through the ion temperature, evaluated according to
Chang-Hinton ion thermal diffusivity, a degradation of the ion transport during the high electron
temperature phase is shown. Indication of density pump-out is also observed.
1.3.3 ECRH in the post pellet phase
Ohmic plasmas in the post-pellet phase are characterized in FTU by the suppression of the sawtooth
activity; peaked density profiles; reduction of the ion energy transport to the neoclassical value; and
improved global energy confinement. ECRH has been applied to this scenario to check whether
these features are maintained with additional electron heating. The cut-off density at 140 GHz
(2.4×1020 m-3) sets a strong constraint to the experiment, as the pellet injector system was designed
to inject deuterium pellets of a given
size (1-2×1020 atoms), thus allowing
the high-density limit of a high-field
tokamak to be explored. To fulfil this
constraint, a low density plasma target
( ≤ 1×1020 m-3) has been chosen
and off-axis ECRH has been applied.
The results obtained for a plasma pulse
at I p =0.6 MA, B T =5.6 T, (q a =4.8) are
shown in fig. 1.26. ECRH power (0.8
MW) is applied 50 ms after the pellet,
in a phase when the line averaged
density is very slowly decreasing,
while peak density is slightly
increasing, so that density profiles are
peaking. MHD activity is rather
quiescent as the sawtooth is
suppressed by the pellet. Plasma reheating
is helped by ECRH, electron
temperature and neutron yield
increase until a strong central m=1
mode starts. At that point, the neutron
yield increase is quenched, density
decreases and density peaking is
reduced. The global energy
confinement time, that has reached
transiently 1.5 times the value of the
ITER89-P scaling, goes back to the L-
mode value. The comparison between
the experimental neutron yield and the
estimation, produced by the solution
of the ion energy diffusion equation
using the Chang-Hinton ion thermal
diffusivity, shows that, before the m=1
mode, the neoclassical value is in
10 20 (m -3 )
10 6 (W)
10 -2 (s) 10 18 10 12 (keV)
3
1
2
0
2
1
3
1
6
2
6
2
a)
b)
c)
d)
e)
f)
I sx
P TOT
T e0
Φ DD
0.5 0.6 0.7
t (s)
Fig. 1.26 - Time traces for pulse #17839, pellet injected at t=0.55 s: a)
peak and line averaged density; b) Fig. total 1.26 and ECRH power; c) peak
electron and ion temperature; d) neutron yield: experimental (full),
neoclassical (dashed), 2 times neoclassical (dot-dashed); e) soft x
emission; f) global energy confinement time: experimental (full),
ITER89-P (dashed). temperature and ECRH power (on-axis heating)
ITER89P
n e0
P ECRH
T i0
τ E
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