1. magnetic confinement - ENEA - Fusione

36
1. MAGNETIC CONFINEMENT
1.2 FTU Facilities
turbulence characteristics
were measured in
discharges with toroidal
magnetic fields of 5.2-8 T,
average densities of 0.35-
3.5x10 20 m -3 and plasma
currents 0.4-1.2 MA.
The heterodyne reflectometry
system is similar to
that of T-10 [1.45] and
operates in the 53–78 GHz
FTU
frequency band. There are two antenna arrays (fig. 1.30): the top array consists of
four antennas adjusted to operate in the O-mode and the bottom array has two
antennas to launch an extraordinary wave. It is, therefore, possible to probe the
plasma simultaneously with the O-mode (n e between 0.35 and 0.75×10 20 m -3 ) and
the extraordinary low-frequency (Xl) mode (n e between 1.4 and 2.9×10 20 m -3 ). One
of the antennas in the top array is used to launch the probing signal and two are used
to receive the incoming signal. The two receiving antennas are separated by 2.5° to
allow poloidal correlation measurements with the O-mode in the full frequency
band. As Q-band waveguides are used for the transmission lines, it is also possible
to work with the top antenna array in the Xl-mode from 60 to 78 GHz, therefore
extending the density range of poloidal correlation measurements to the maximum
critical density of 2.9×10 20 m -3 . A special processor controls the required frequency
variation during the discharge. The minimal sweep time for the whole band is 80 ms.
The amplitude (A) and the phase (ϕ) fluctuations of the reflected electric field vector
are decomposed by a quadrature detector into imaginary (U 1 =A×sinϕ) and real
(U 2 =A×cosϕ) parts. Thus, for each reflectometry channel two signals are recorded
with a sampling rate of up to 2 MHz during the whole discharge. For poloidal
correlation measurements, the signals of two poloidally separated antennas are
synchronously sampled using four ADCs. All signal processing is performed in
complex form. For each time interval of interest, the signals are divided into
subintervals and in each subinterval a complex fast Fourier transform is applied, to
provide the amplitude and the phase spectra for both signals. The absolute values of
the amplitude of the subspectra are then averaged to obtain two final amplitude
spectra (one for each channel). A signal equal to the normalised cross product of the
two complex spectra is also built; the signal is then averaged with the same
procedure as used for the original signals. The results are the cross-phase and
coherency spectra. Auto and cross-correlation functions of the two channels can also
be calculated over each subinterval by averaging over the whole time interval.
17.5°
10.8°
5°
4"O"
mode top
antennas
array
2"X" mode
bottom
antennas
array
Fig. 1.30 - FTU
reflectometer antennas.
The bottom pair are in
X–mode; the top four, in
O-mode.
[1.45] V.A. Vershkov, V.V.
Dreval, S.V. Soldatov,
Rev. Sci. Instr. 70, 1700
(1999)
[1.46] V.A. Vershkov, et
al., presented at the 28 th
EPS Conf. on Controlled
Fusion and Plasma Physics
(Madeira 2001), Vol.
25A, p. 1276
[1.47] G.D. Conway, et al.,
Phys. Rev. Lett. 84, 1463
(2000)
Figure 1.31 shows typical results of poloidal correlation measurements of a)
amplitude, b) cross-phase, c) coherency spectra, d) auto-correlation and e) crosscorrelation
functions. The results are similar to those of T-10 [1.46] and JET [1.47]. The
quasi-coherent fluctuations always rotate in the electron diamagnetic drift direction
with velocities of about 2×10 3 m s -1 . Their angular velocity is equal to or slightly less
than that of the m=2, n=1 mode at half radius and decreases significantly towards the
plasma centre. As per expectations, turbulence frequencies are rather high in some
regimes. The broadband fluctuations show frequencies of up to 1 MHz; the quasicoherent,
250 kHz. The estimated poloidal m numbers of the quasi-coherent
fluctuations can be as large as 100. However, in many discharges lower frequencies
and m numbers are also observed, suggesting a more complex dependence of the
wavelengths on the discharge parameters, rather than a simple scaling with the
toroidal magnetic field. The cross-phase slope and the cross-correlation function
show that the low-frequency component also rotates in the electron diamagnetic drift