1. magnetic confinement - ENEA - Fusione

1. MAGNETIC CONFINEMENT
33
1.2 FTU Facilities
emitted by the plasma, can be steered at different toroidal angles (φ=0˚, ±10˚, ±20˚,
±30˚, with respect to the normal direction) and can also perform a continuous
poloidal scan. Due to the narrow frequency range of the radiometer (247-287 GHz),
observation of the plasma centre during on-axis ECH is possible only for φ=0° and
±10°.
In order to minimise the amount of O-mode radiation reaching the radiometer for
oblique viewing, a polariser was installed in front of the antenna. The polariser is a
quarter-wave plate, whose optical axis can be rotated to compensate for the change
in polarisation of the X-mode when the observation angle is changed. For these
preliminary measurements, the polariser is not a perfect quarter wave device and
therefore conversion of the X-mode elliptical polarisation into linear is not complete.
It was estimated that, for measurements at φ=±10°, the polariser reduces the
contribution of the O-mode emission from 15% to 4%.
[1.40] P. Buratti and M.
Zerbini, Rev. Sci. Instrum.
66, 4208 (1995)
[1.41] O. Tudisco et al.,
Rev. Sci. Instrum. 67,
3108 (1996)
[1.42] P. Buratti et al.,
Phys. Rev. Lett. 82, 560
(1999)
Two other ECE systems are routinely used on FTU: an absolutely calibrated
Michelson interferometer that measures the ECE spectrum over five harmonics with
moderate temporal resolution (5 ms) [1.40] and a 12-channel grating polychromator
with a 10-ms time resolution [1.41]. Both spectrometers measure the emission with
the line of sight in the mid-plane, normal to the magnetic field.
The radiometer has better spatial resolution (∆r≈±1 cm, ∆z≈±1.8 cm) than the
interferometer (∆r≈±2.5 cm, ∆z≈±2 cm) and polychromator. This difference is
appreciable in the presence of peaked temperature profiles, like those discussed here.
Oblique ECE measurements were performed on the current ramp phase, with central
ECH and at low or reversed magnetic shear and moderate density [1.42]. The time
evolution of the radiation temperature profile for this type of discharge, as measured
by the Michelson interferometer, is shown in figure 1.25. Although only one gyrotron
(360 kW) was available in this experiment, very peaked radiation temperature
profiles were obtained, with maximum values of the central temperature up to
11 keV.
Analysis of the Michelson spectra measured in these conditions shows that, at the
3 rd , 4 th and downshifted 2 nd harmonics, the level of emission (corresponding to a
scan of the distribution function in perpendicular energy if the energy of the
electrons responsible for emission at these harmonics is taken into account) is much
lower than expected from the high value of the central temperature. This anomaly
occurs because the bulk distribution function is distorted due to strong localisation
of ECH energy, which provokes an increase in the 2 nd harmonic emission. This
effect is in good quantitative agreement with simulations [1.37].
Fig. 1.25 - Time evolution
of the radiation temperature
profile as
measured by the
Michelson interferometer.
The ECH pulse is
applied at t=0.10 s.
Tradiation (keV)
12
# 19462
0.134 s
10
0.129 s
8
6
4
2
0.119 s
0.109 s
0.104 s
0.1 s
0
0.8 0.9 1 1.1 1.2
R(m)
A scan of the distribution function in
parallel energy by ECE measurements at
different observation angles should also
reveal the existence of a non-Maxwellian
bulk [1.37], which is the goal of the
present experiment.
Figure 1.26 shows the computed angular
dependence of the emission spectra for
conditions similar to the experimental
ones and compares the effects of
Maxwellian and non-Maxwellian bulks.
If the perpendicular emission spectrum
is selected in the frequency range near
the 2 nd harmonic and the corresponding