1. magnetic confinement - ENEA - Fusione

32
1. MAGNETIC CONFINEMENT
1.2 FTU Facilities
new control system using modules addressable via the Net was set up and tested. In
the next phase, the beam will be tested at full power before installation on FTU.
The MSE optics has been designed with the observed chord divided in two sections
to reduce the aperture of each section. The relay optics will include two mirrors on
each section and a system of lenses to image the plasma outside the port. All of the
glass components will be made out of a low Verdet-coefficient material. The first
periscope components will be placed close to the plasma, inside a temperaturecontrolled
tube to avoid the window freezing in the cold FTU environment. The
photoelastic modulator polarimeter was pre-assembled, and lock-in detection was
compared with a Fourier analysis deconvolution for the interesting signal frequency
acquired on fast analog-digital converters (ADCs). The Fourier analysis was
preferred as it is less sensitive to hardware noise.
Charge-exchange spectroscopy will operate on 12 vertical lines and f/2 collection
optics. A high-aperture high-resolution image spectrometer with an echelle grating
has been set up and is being equipped with a high-gain image intensifier.
Oblique ECE measurements
It is generally assumed that the bulk electron distribution function is well described
by a Maxwellian distribution function. While qualitative theoretical arguments
support this assumption, determination of the exact form of the bulk distribution
function, in the presence of additional heating and transport processes, is beyond
today’s computational capabilities. From the experimental point of view, the
hypothesis of a Maxwellian distribution is at the basis of the interpretation of
temperature measurements. In the case of Thomson scattering, this is so even if the
scattered spectrum, in principle, contains information about the form of the
(typically perpendicular) 1-D distribution function, which could be used to ascertain
the above hypothesis. In the case of electron cyclotron emission (ECE), the
temperature measurements give the perpendicular slope of the distribution function
averaged over a small region of phase space (for optically thick plasmas), whose
extension is determined by the temperature and density profiles and the specific
instrumental parameters of the diagnostic. Multiharmonic ECE spectra, measured by
Michelson interferometry perpendicularly to the magnetic field, can provide a coarse
scan at different perpendicular energies, for low parallel energy.
Oblique ECE spectra can provide a continuous scan of the distribution in parallel
energy by changing the observation angle, for roughly constant perpendicular
energy. These two types of ECE measurements are complementary, and only their
combination can give a 2-D scan of the electron distribution function in velocity
space [1.36].
Recently, both theoretical [1.37] and experimental [1.38] evidence has emerged that
points to the existence of a distortion of the bulk electron distribution function
during on-axis ECH on FTU. The analysis of this phenomenon motivates the present
study, in which the first measurements on FTU with oblique ECE are reported.
An oblique view of FTU plasmas is achieved through one of the transmission lines
of the ECH system [1.39]. The line, consisting partly of closed metallic oversized
waveguides and partly of quasi-optical sections, was opened and collimating optics
was installed to focus the radiation emitted by the plasma and transported through
the ECH waveguide up to the receiving antenna. The emitted radiation is analysed
by a 32-channel heterodyne radiometer (2 nd harmonic, X-mode). The radiometer,
originally designed for perpendicular measurements (φ=0˚) during ECH with high
spectral resolution (∆f~1 GHz), was moved temporarily to the ECH line for the
oblique measurements. The ECH launcher, which is used to collect the radiation
[1.36] V. Krivenski and V.
Tribaldos, Proc. 20 th EPS
Conf. on Contr. Fusion and
Plasma Physics (Lisboa
1993) Vol. 17C, 1045
[1.37] V. Krivenski, Proc.
11 th Joint Workshop on
ECE and ECRH, Fusion
Eng. &. Des. 53, 23 (2001)
[1.38] O. Tudisco et al.,
Proc. 26 th EPS Conf. on
Contr. Fusion and Plasma
Physics (Maastrict 1999)
Vol. 23J, p. 101
[1.39] C. Sozzi et al., Proc.
13 th Topical Conf.
Applications of RF power
to Plasmas (AIP Press,
1999) p. 462