Speaker
Afra Romano
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.1053.pdf
Effects of pellets and impurity injection on runaway control experiments on
FTU
A.Romano1, G. Apruzzese1, F.Bombarda1, L.Boncagni1, P. Buratti1, D.Carnevale2, S.
Ceccuzzi1, F. Cordella1, C. Di Troia1, G. Ferrò2, L.Gabellieri1, E. Giovannozzi1, M.
Gospodarczyk2, G. Ramogida1, S. Sibio1, B.Tilia1, O. Tudisco1 and the FTU team*
1
ENEA, Fusion and Nuclear Safety Department, C. R. Frascati, Via E. Fermi 45, 00044
Frascati (Roma), Italy
2
Dip. di Ing. Civile ed Informatica, Università di Roma “Tor Vergata”, Italy
*See G. Pucella et al., Nucl. Fusion 57 102004 (2017)
Generation of runaway (RE) beams following disruptions is a serious concern in tokamak devices.
FTU is conducting an extensive program aimed at controlling and mitigating the purposely
generated RE beams (natural and induced disruptions) [1, 2]. During the last experimental
campaign different mitigation techniques have been tested, which involved injection of impurity
gas (Neon), of multiple Deuterium pellets (1-2⋅1020 atoms), and of Laser Blow Off metal
impurities (Tungsten, Molybdenum, Iron and Zirconium). The injections have been performed
both during the plasma current flat top with seed REs embedded in a hot plasma and after current
quench with current mainly carried by REs. The experimental results point to a complex picture
where MHD effects, impurity transport and radiation, temperature and density variation all play
different roles depending on the target plasma condition.
As far as the LBO is concerned, the element providing the most useful information was Fe.
Injections during the plasma current flat-top in RE discharges show up in the spectroscopic
diagnostics, bolometry and Soft X-ray. The signal time correlations appear to be related to the
amount of RE electrons in the plasma, qualitatively estimated from the ratio of the
(neutron+gamma)/neutron signals. Pellets injected during this phase caused an increase of plasma
density; the effects on the RE beams were analyzed. LBO injections performed on the RE beam
formed after the current quench do not produce any effects on any diagnostics nor on the beam
itself, possibly because of the very low electron temperatures.
Deuterium pellets injected into a RE beam display complex behaviors. When injected in the early
phase of the beam the pellet is ablated but ionization does not take place, presumably because the
background plasma is too cold; the electron density surprisingly diminishes in this case. When the
pellets are injected a later time into a warmer plasma and a less energetic RE beam, they are
ionized increasing the electron density.
[1] D. Carnevale, et al., Runaway Electron Beam Control, EPS 2018.
[2] B Esposito et al 2017 Plasma Phys. Control. Fusion 59 014044.
