1. magnetic confinement - ENEA - Fusione

38
1. MAGNETIC CONFINEMENT
1.2 FTU Facilities
components may rise simultaneously
if the non-locality of the measurement
is taken into account, since it
integrates the low frequency at some
outer radial zone. It is also important
that the low-frequency rotation
remain unchanged. It is worth noting
that the quasi-coherent rotation is
sensitive to gradients, whereas the
low-frequency is closer to that of the
bulk plasma. Figure 1.33 shows the
evolution of some turbulence
parameters in an experiment with the
injection of four deuterium pellets
[1.50]. The discharge has I p =1.2 MA
and B t =7.9 T. The probing Xl mode
was used with a cut-off density of
2.76×10 20 m -3 . The trace of the
reflected wave amplitude (fig. 1.33b)
shows that reflection appears just
transiently after the first pellet at
t=800 ms and becomes permanent
after injection of the second. This is
eV/cm 104 s-1 Part in signal cm a.u. 1019 m-3
ω rotation
pellets
Electron density
Reflection radius
Broad band
Low frequency
the first time that poloidal correlation measurements in the Xl mode have been
carried out in tokamaks, with the antennas located at the low-field side and the
poloidal angle 17.5° above the equatorial plane. Such geometry results in a
significant deviation of the incident wave from the perpendicular direction, due to
the dependence of the refractive index on the magnetic field and on the Shafranov
shift of the plasma column. This makes it impossible to measure reflections close to
the centre of a plasma with a flat density profile. Thus, reliable results can be
obtained only when the amplitude of the reflected wave is larger than some
minimum value, as shown in figure 1.33b. The most distinctive feature is the drop of
the quasi-coherent rotation velocity after the second pellet. An explanation for this
could be that the reflection layer gradually moves towards the central region where
the gradients are smaller due to density decay. At the same time, the low-frequency
component rotates more slowly and does not show any strong variation in time. This
may indicate again that this component is related to plasma rotation, whereas the
quasi-coherent one depends on gradients. The simultaneous suppression of both
components in the central region, as observed in these experiments, suggests a
physical relation between these turbulence features and also the presence of a good
confinement zone near the centre.
The fully non-inductive discharges with LHCD [1.51] make it possible to reveal the
fine structure of quasi-coherent turbulence. In figure 1.34 the usually smooth spectral
maxima of these fluctuations split into an envelope of five peaks with a 3.1-kHz
spacing in frequency. The depth of amplitude modulation shows that only the quasicoherent
fluctuations are 100% modulated whereas the broadband are not. This
supports the idea that the two components have different underlying physical
mechanisms. Poloidal correlation measurements during the same time slice give the
rotation velocity and show that the mean m number is equal to 48. Thus, the 3.1-kHz
frequency step corresponds to an m number increment of three instead of one. In
order to solve this problem, consider the reflection from a plasma region with a flat
current profile around the q=3 surface but with a rather steep density gradient. The
modes with m/n ≠3 will be far away from the reflection layer and will not be “seen”
by the reflectometer. Thus, only modes with an m increment of three will be
observed, in accordance with experimental data. Analysis of the bursting of the
4
2
20
10
0
20
10
0
0.5
0.0
0.2
0.0
1.0
0.5
0.0
m=2 HF
Signal amplitude
Reliability level
LF
100
0 dT/dr
700 800 900 1000 1100 1200
Time (ms)
Fig. 1.33 - Evolution of
turbulence behaviour
during a high-confinement
phase of a pellet-fuelled
discharge.
[1.50] V. Pericoli Ridolfini
et al. , Phys. Rev. Lett. ,
82, 93 (1999)
[1.51] S. Cirant et al.,
Mode coupling trigger of
tering mides in ECV
heated discharges in FTU,
presented at the 18 th
IAEA Conference,
(Sorrento 2000), paper
IAEA-CN-77/EX3/3