1. magnetic confinement - ENEA - Fusione

34
1. MAGNETIC CONFINEMENT
1.2 FTU Facilities
temperature profile is obtained by
assuming a Maxwellian distribution,
and then this temperature profile is
used to compute the expected
emission spectra for different
observation angles, these spectra
overestimate the actual radiation
temperature, if the distribution
function is not Maxwellian.
The temperature profiles measured
during an ECH pulse by the Michelson
interferometer (for φ=0°) and by the
radiometer (for φ=10°) are compared in
figure 1.27. The radiometer was
calibrated using the Michelson
temperature in the Ohmic phase of the
discharge, when the temperature
profile is rather flat and the
distribution function nearly
Maxwellian. The peak of the
temperature profile measured with the
radiometer shows the characteristic
frequency upshift due to the Doppler
effect. The radiometer peak is also
thinner than the Michelson peak, due
to better instrumental resolution. For a
more direct interpretation of the
experiment, the perpendicular
emission should also be measured
with the radiometer. Unfortunately,
these spectra are not available because
stray radiation from the gyrotron
cannot be effectively filtered for φ=0°.
Therefore, the perpendicular spectra of
the radiometer were simulated, first
deriving the temperature profile over
this frequency range from the
Michelson spectra (assuming a
Maxwellian distribution) and then
computing the corresponding spectra
of the radiometer for φ=0° and 10°.
(This is the same procedure as that
followed to obtain the results of figure
1.26, but now using the experimental
Michelson spectrum.) In the
calculations, the nominal instrumental
resolution of the two diagnostics was
0
240 260 280 300 320
Frequency (GHz)
used. Figure 1.28 summarises the result. The perpendicular spectrum of the
radiometer is both higher and thinner than the corresponding spectrum of the
interferometer, due to the better instrumental resolution of the former. The computed
oblique spectrum (assuming a Maxwellian distribution) is higher than the measured
spectrum, in qualitative agreement with the discrepancy expected when the
distribution function has a non-Maxwellian bulk (fig. 1.25). Further experimental
investigation is needed to confirm this conclusion and to test the quantitative
agreement with the theory.
Tradiation (keV)
Tradiation (keV)
Tradiation (keV)
12
10
12
10
8
6
4
2
8
6
4
2
Maxw.
FP
Fig. 1.26
Michelson
# 19462
0° 10°
20°
Radiometer
Radiom.
0.134 s
0.124 s
0.1 s
0
220 240 260 280 300 320 340
Frequency (GHz)
14
12
10
8
6
4
2
Maxw.
Radiom.
Maxw.
Michelson
0°
10°
Exp.
Michelson
0
240 260 280 300 320
Frequency (GHz)
Exp.
Radiom.
Fig. 1.26 - Angular
dependence of emission
spectra computed for a
non-Maxwellian bulk (FP)
and for the temperature
profile which, assuming a
Maxwellian distribution,
gives an identical spectrum
over this frequency
range for φ=0°. (Instrumental
parameters of the
radiometer).
Fig. 1.27 - Measured ECE
spectra from perpendicular
(φ=0°, Michelson)
and oblique ECE (φ=10°,
radiometer) during
central ECH.
Fig. 1.28 - Measured
spectra at τ=0.134 s and
simulated spectra for the
radiometer instrumental
parameters (φ=0°, 10°),
computed with the temperature
profile obtained
over this frequency range
from the interferometer
and assuming a Maxwellian
distribution. Compare
with the angular dependence
in fig. 1.26.