1. magnetic confinement - ENEA - Fusione

1. MAGNETIC CONFINEMENT
37
[1.48] F. Alladio et al.,
Pressure Anisotropy in
Ohmic FTU Discharges,
presented at the 18 th
European Conf. on Control.
Fusion and Plasma Physics,
(Berlin 1991)
[1.49] V.A. Vershkov, et
al., Nucl. Fusion, Iokohama
special issue 2, IAEA, 39,
1775 (1999)
Fig. 1.31 - Turbulence
behaviour of a typical
FTU discharge: a)
spectrum, b) cross phase,
c) coherency, d) cross
correlation, e) autocorrelation.
The spectrum
consists of three distinct
parts: low frequency,
quasi-coherent and broad
band. The crosscorrelation
function
shows a fast and a slow
rotating structure.
Fig. 1.32 - Evolution of
turbulence behaviour
during a spontaneous
density peaking event.
Coherency Cross-phase Amplitude
π
a.u.
Amplitude
kHz ω, 104 s-1 a.u. Part in signal cm 1019 m-3
1.5
1.0
0.5
0.0
1
0.6
0.4
0.2
0.0
-300 -200 -100 0 100 200
0.4
0.2
0.0
1.0
0.5
0.0
-100
6
4
2
0
10
0
0.5
0.0
0.4
0.0
0.4
0.0 2
1
0
100
0
0
-1
a)
b)
c)
d)
e)
LF
g)
d)
d)
e)
-12,84 µs
(LF)
QC
-50
300 400
Time (ms)
Frequency kHz
0 50
Time lag, µs
i
Line averaged density
Reflection radius
Broad band
Low Frequency
Quasi-coherent
Angular velocity
Quasi-coherent
frequency
f)
-3.84µs (QC)
500
a)
b)
Cross
correlation
c) c)
Auto
correlation
300
100
1.2 FTU Facilities
direction but at a
much lower speed
(about 0.5×10 3 m s -1 ).
This differs from the
T-10 results, where
this component does
not move in the
plasma core.
Distinctive
turbulence behaviour
during a spontaneous
density peaking
“event” [1.48] is
shown in figure 1.32.
A strong decrease in
the quasi-coherent
and broadband
angular velocities
and a simultaneous
increase in the quasicoherent
amplitude
are observed at t=260
ms when the density
starts to rise.
Suddenly, at t=340
ms, this process
reverses and the
rotation of the high
frequencies starts to
increase, together
with a transient
disappearance of the
quasi-coherent component, which
appears again after a few ms at a
much higher frequency. The growth
of the low-frequency amplitude
begins about 10 ms earlier, while its
velocity remains constant. Such
complex turbulence behaviour is very
similar to that observed in T-10
during SOC to IOC transition [1.49]
and may be explained by the
formation at the periphery of a
transient velocity shear zone, which
travels towards the centre. It initially
flattens the gradients in the plasma
core and the quasi-coherent velocity
therefore decreases. The shear wave
arrives at the reflecting radius at
t=340 ms, as shown by a steep
velocity rise and by a transient
suppression of the quasi-coherent
component. Note that, in reality, the
low-frequency and quasi-coherent