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

026
progress report
2010
S(μ W/sr nm)
1.5
0.5
-0.5
0
200 400
Time (ns)
Figure 1.32 – Emissions observed by
four spectral channels (blue 1020–1050
nm, green 950–1020 nm, orange
850–950 nm, cyan 670–850 nm)
Bremsstrahlung radiation. Accurate evaluations suggest that the
tungsten line emission at 401 nm (selected by an interference filter
with full width at half maximum (FWHM) ≤1 nm) by the impact
vapour cloud is expected to be larger than the background radiation
by two orders of magnitude during a time interval of the order of a
few hundred ns after the impact. The construction of the mechanical
parts of the probe was completed in 2010. The installation of the
probe in the FTU port 5, as well as the final assembly of the optical
and electrical detection systems (including data acquisition) should
be completed by 2011. It is worth noticing that the project was
developed with a strong support, both financial and scientific,
provided by the IFP of the CNR, Milan, also in the frame of a
collaboration with the University of Naples “Federico II”, the University of Molise and the Royal Institute of
Technology (KTH) of Stockholm. The EO probe development was also supported by Ministero
dell’Università e della Ricerca (MiUR) under grant PRIN–2007L4YEW4 and by EFDA baseline and priority
supports.
Another important issue considered in the framework of dust particle studies in tokamak environments was
the detection of dust mobilized after disruptions. Such dust particles were observed by the new high resolution
Thomson scattering system on Joint European Torus (JET) [1.20]. The system consists of filter spectrometers
that analyze the Thomson scattering spectrum from 670 to 1050 nm in four spectral channels. The laser source
was a 5 J Q–switched Nd:YAG laser. Without a spectral channel at the laser wavelength, the dust elastic
scattering cannot be observed and only dust particles that emit broadband light could be detected. The time
behaviour of their emission is clearly different from that expected for a Thomson scattering pulse. As shown
in figure 1.32, the emission peak is actually delayed by about 10 ns as compared to the laser pulse (15 ns
duration). Moreover, the typical emissions observed by the spectral channels have almost the same amplitude,
consistent with a black body emission of dust particles with a diameter of 10 μm at a temperature of 4000 K.
The fast decrease of the signal after the peak (10 ns time scale) suggests the occurrence of ablation
phenomena, since the radiative cooling is expected to be much slower. These studies concerning the detection
of dust during disruptions at JET are preliminary, further investigations are planned for the next JET
campaigns C28 and C29.
It is worth noticing that the characterisation of size, composition and origins of dust in fusion devices is the
subject of a specific International Atomic Energy Agency (IAEA) Coordinate Research Project (CRP), and the
FTU group is involved in this project by providing the chief scientific investigator. The report of the second
research coordination meeting of this project has been published in November 2010 [1.21].
These researches can benefit from collaboration with scientists involved in different fields, such as the study of
dust in laboratory or space plasmas [1.22].
Ion cyclotron antennas. The collaboration between IPP – Garching and ENEA–Frascati started more than one
year ago, but has been effective since spring 2010. It is essentially focused on the solution of one major open
issue related to ion–cyclotron (IC) antennas, namely the capability to reach both good plasma coupling and
low sheath potentials in front of the launchers. In particular, the sheath potentials are responsible for high
impurity production and their reduction is still a huge challenge for the design of high power ion cyclotron
resonance frequency (ICRF) antennas, with specific relevance to ITER and FAST experiments.
In this context Axially Symmetric Divertor EXperiment Upgrade (Asdex–U) stands out to be one of the best
candidates to test new IC antenna solutions due to its H–modes and to the fully tungsten wall. Furthermore,
the Asdex–U team has a remarkable experience in terms of antenna design and sputtering yield analysis, as
proved by its long list of references and international collaborations ([1.23, 1.24] to cite only a few) on these
topics.
The present work is focused on two main activities, i.e. the modelling and simulation of the actual AUG IC
antennas, and the design of a new launcher to be installed on the machine by mid 2012. Two main prediction
tools are being used for this purpose, HFSS and TOPICA [1.25], whose results have been compared as far as
rectified potentials are concerned (the integral of the parallel electric field component along magnetic field
lines in front of the antenna).
HFSS is a 3D full–wave frequency domain electromagnetic field solver based on the finite element method,