Prime pagine RA2010FUS:Copia di Layout 1 - ENEA - Fusione

magnetic confinement (cont’d.)
progress report
2010
025
scan in gain should also be applied, though this technique has to be
fully proven in laboratory.
256
Now the gas detector is at the Frascati laboratories in order to continue
the characterization and prepare irradiation tests under 2.5 MeV
neutron flux using the Frascati Neutron Generator (FNG) facility and
in environment with radiofrequency interference (the FTU’s hall).
Since this detector has three GEM foils for the electron amplification,
it could provide the advantage to be less sensitive to neutrons and
gammas as compared with the single stage one. These tests are
scheduled for February 2011.
2) Feasibility studies of polycapillary lenses. Polycapillary lenses [1.14]
have been studied in laboratory for preliminary characterization, with
the aim of using them in SXR diagnostics such as imaging and
tomography [1.15]. The first tests were performed to characterize the
polycapillary lenses (convergence, divergence, efficiency, spectral
dispersion, aberrations and so on) in the SXR range 5–25 keV and for
distances much larger than the optical focal length of the lenses, both
for the detector and the source. A silicon based C–MOS imager
(Medipix 2) has been used as 2D detector and the micro focus x–ray
tubes as “point–like” sources.
The full lens has been characterized by using an electronic tube with a
Mo anode powered at 25 kV, 150 μA (continuous spectra with the K α
line of Mo at 17.4 keV in addition), while the half lens by using an
x–ray tube with a Cu anode and a Ni filter (K α
line of Cu at 8.04 keV),
powered at 15 kV, 300 μA. The images have been acquired with the
Medipix 2 [1.16]. In particular, the output focus of the full lens has
been found by a scan of the distance detector–lens. The intensity of the
spot is found to be fairly Gaussian and the spot broadening is linear
with the distance (60 cm) much larger than the focal distance (4.4 cm).
The total geometry divergence results to be approximately equal to 2.6°. The same measurement has been
done with the half lens: set at 25 kV and 150 μA respectively. A radiography of the samples has been done at
first (fig. 1.30) by putting it just in front of the detector, to check structure, contrast and dimensions and so on,
as reference. The image through the lens is obtained by putting the samples just before (roughly 5 mm) the
optics. These samples have been perfectly reproduced (fig. 1.31). There is only a decrease in the intensity at
the edges of the lens, but it could be corrected by using an appropriate factor.
Imaging properties have been therefore demonstrated for full–lens, with a resolving power at least of about
100 and for distances much longer (15 times) than the focal distance. Efficiency is found to be progressively
lower at the edge, as expected, and dependent on the energy of the x–ray photons. Half–lens revealed as an
excellent light collector, having an output beam with a very low divergence (quasi parallel).
New diagnostic for dusty plasmas. The research activity on dust in tokamak plasmas during the year 2010 was
in part devoted to the development of a final design and construction of a specific diagnostics for the detection
of micrometer size dust particles, which impinge on the vacuum chamber wall at velocities larger than a few
km/s, i.e. larger than the velocity of the compressional waves of the wall material. Such particles are particular
harmful for fusion reactors because, depending on their size and concentration, they may significantly
contribute to the wall erosion due to the impact craters produced (the erosion rate evaluated on the basis of
the FTU results is indeed of the order of 10 –4 –10 –3 mm/s). Their presence in FTU was suggested by the
detection of spikes in the ion saturation current collected by an electrostatic probe, interpreted in terms of
impact ionization events, and by the impact craters found on the molybdenum probe tip [1.17, 1.18]. The new
diagnostics, developed to confirm and extend such observations, is based on the simultaneous detection of the
spikes in the ion saturation current collected by an electrostatic, tungsten probe and the light flashes emitted
by the tungsten vapour cloud formed by the impact. This coincidence allows indeed to discriminate between
events due to plasma fluctuations, which are not accompanied by line emission of tungsten, and events due to
hypervelocity impacts. A detailed study of feasibility of the new electro–optical (EO) probe has been
performed [1.19]. The main concern was represented by the background plasma emission, mainly due to the
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Figure 1.30 – Radiography of a
plastic foil with holes
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Figure 1.31 – Image with a full lens
of a plastic foil with holes