Multi-objective Bayesian optimization of tokamak disruptions using fluid and kinetic models
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Complex adaptive systems (MPCAS), MSc
Publicerad
2023
Författare
Ekmark, Ida
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
The generation of highly energetic runaway electron beams during tokamak disruptions
is a major challenge facing tokamak fusion reactors. One of the most studied
disruption mitigation schemes is massive material injection. Finding injected material
densities, such that the consequences of the resulting disruption – runaway
electron impact, localized heat losses and mechanical stresses – are tolerable, is still
an open question, and it represents a multi-objective optimization problem. We have
used a Bayesian optimization framework to optimize the injected densities of deuterium
and neon in a non-activated ITER-like tokamak set up. The cost function was
constructed systematically to maximize information gain, combining the maximum
runaway current, final ohmic current, current quench time and conducted thermal
losses. The simulations of plasma evolution were performed using the disruption
modelling tool Dream. Optimization of the developed cost function was performed
in two layers of physics fidelity, using both fluid and kinetic plasma models. The
fluid model is computationally less expensive, which is advantageous for exploring a
large parameter space. Once promising parameter regions are located using a wide
search with fluid models, these are further studied in higher physics fidelity using
kinetic simulations. These simulations resolve the energy distribution of the fast
electrons allowing us to also account for fast electron impact ionization and energy
transfer. Using two layers, the advantages of each model can be utilized resulting in
an efficient optimization with a reliable examination of relevant areas. Additionally,
a qualitative comparison of the two models was made to illuminate the differences
between the two layers. In general, the kinetic model generated more optimistic results
for the disruption consequences. More specifically, the kinetic model favoured
higher neon densities and slightly lower deuterium densities compared to the fluid
model. In both models, the optima are fairly insensitive to the radial distribution of
neon as long as there is a higher neon density at the edge. Furthermore, the optima
occurred for a moderately core-localized deuterium density. The explanation for the
differences between the fluid and kinetic models was concluded to be that the fluid
model overestimates the hot-tail runaway generation for certain injected material
densities, resulting in larger runaway currents.
Beskrivning
Ämne/nyckelord
fusion plasma , disruption mitigation , runaway electron , material injection , Bayesian optimization , fluid kinetic model