Disruption Mitigation in Tokamaks with Shattered Pellet Injection
Typ
Examensarbete för masterexamen
Program
Publicerad
2021
Författare
Vallhagen, Oskar
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
The sudden loss of confinement of the energy content of fusion plasmas in off-normal
events, called disruptions, are among the most severe threats to the future of fusion
energy based on the tokamak design. An efficient disruption mitigation system will
therefore be of utmost importance for future large, high-current devices such as
ITER. The potentially greatest threat to be mitigated is posed by currents carried by
highly energetic electrons, called runaway electrons, which may cause severe damage
upon wall impact. The disruption mitigation system must also ensure a sufficiently
homogeneous deposition of the thermal energy on the plasma-facing components,
and avoid excessive forces on the machine due to currents flowing in the surrounding
structures. The currently envisaged mitigation method is to make a massive material
injection when an emerging disruption is detected, attempting to better control
the plasma cooling and energy dissipation. In this work, we perform numerical
simulations assessing the performance of the most up to date mitigation schemes
based on shattered pellet injections in an ITER-like setting, with a particular focus
on the generation of runaway electrons. The main mitigation scheme investigated is
a two-stage shattered pellet injection, with a diluting deuterium injection followed
by a neon injection aiming to radiatively dissipate the plasma energy content.
Our studies indicate that the diluting deuterium injection can efficiently reduce
the runaway generation due to the hot-tail mechanism, by allowing for an intermediate
equilibration of the superthermal electron population between the injections. The
fraction of the initial thermal energy content conducted to the plasma-facing components
is also reduced compared to a single-stage injection with the same composition,
reducing the localised heat loads. During non-nuclear operation, the maximum
runaway current was found to be reduced to acceptable levels with realistic two-stage
injection parameters. On the other hand, during nuclear operation, the unavoidable
runaway seed from tritium decay and compton scattering was found to be amplified
to several mega-amperes by the avalanche mechanism for all investigated injection
parameters. The reason is that the intense cooling from the injected material leads
to a high induced electric field and a substantial recombination, resulting in an
enhanced avalanche multiplication.
Beskrivning
Ämne/nyckelord
fusion plasma , disruption mitigation , shattered pellet injection , runaway electron