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

016
progress report
2010
Ceramic
window
Input WR284
(72.14 x 34.04 mm)
Reference plane
position
Short circuit
WR 284
Figure 1.8 – Pillbox ceramic window
Output
(58 x 34.04 mm)
Figure 1.9 – The 3dB hybrid junction
Figura 1.10 – Electric field in the
TE 10 to TE 30 mode converter
(SiC), on the TE mn
modes (m≠0). The
propagation of TE 0n
modes with n>1 is
avoided by an accurate dimensioning of the
base waveguide diameter.
In the following figures the most critical microwave components of the transmission system are shown, as, e.g.,
the 500 kW, CW pillbox ceramic window (fig. 1.8) separating the pressurized MTL from the vacuum region
of the launcher; the 3 dB hybrid junction (fig. 1.9) to initially split by two the rf power generated by each single
klystron; the TE 10
to TE 30
mode converter (fig. 1.10), which splits into three each output branch of the
previous hybrid junction in order to suitably feed the upper and lower parts of a single PAM module. The final
power distribution among the four active waveguides of each row of a PAM module is obtained by classical
cascaded bijunctions.
Alfvén Eigenmodes: modeling, experimental results and future plans
Electron–fishbones studies. Electron–fishbones are internal kink mode instabilities induced by supra–thermal
electrons. The knowledge of the supra–thermal electron dynamics, in presence of fishbone fluctuations, is very
important in burning plasmas. Indeed, energetic particles in the MeV range are present in ignited plasmas, as
either fusion products (alpha particles) or supra–thermal tails induced by additional heating systems. In
particular, good confinement and slowing down of alpha particles (electron collisional heating) are very
important issues. Perpendicular transport of magnetically trapped supra–thermal electrons is similar to that
of fusion alphas, since both have small orbit as compared to the plasma minor radius. Thus, the investigation
of such dynamics may shed new light on physics issues related to alpha particles. The stability analysis of
e–fishbones can be carried out within the framework of the so called general fishbone–like dispersion relation,
which can be thought of as a kinetic energy principle. Previous experiments on e–fishbone in FTU have been
conducted with lower hybrid radio frequency heating and q min
>>1. They suggest that the level of the rf power
determines the energy content of the supra–thermal electron tail and is crucial in controlling the transition
from nearly steady state non–linear oscillations to bursting regime; moreover, experimental observations
confirm the theoretical conjecture that the e–fishbone in the bursting regime is a continuum resonant mode
[1.6]. FTU experiments, as well as experiments conducted at Tore–Supra [1.7], underline the necessity of
systematic comparisons between experiments and theory by means of the general fishbone–like dispersion
relation and appropriate simulations codes. MARS is a resistive–magnetohydrodynamic (MHD) stability code
that can be used to this purpose to calculate the part of the plasma response entering the fishbone–like
dispersion relation, i.e. the MHD potential energy contribution. With this information, systematic stability
studies are underway, which can give insights about the FTU plasma conditions where the kinetic response due
to supra–thermal electron tails is most likely to yield the fishbone instability. In particular, q–profile sensitivity
studies will be carried out as well, for they will clarify the role of current profile in the e –fishbone dynamics.