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Title: Magnetic effects on dust charging and dynamics in plasmas
Author: Simons, Luke
ISNI:       0000 0005 0287 3558
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2020
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This thesis documents the analytical and computational models developed for the purpose of understanding the influence of magnetic fields on dust behaviour in tokamak plasmas, vital for the safe and stable operation of future devices. The Monte Carlo code Dust in Magnetised Plasmas (DiMPl) designed to investigate the influence of magnetic fields on dust rotation and charging is presented. Comparisons of DiMPl with other simulation results revealed a surprising insensitivity to the form of the potential and the collisionality of the plasma for the purposes of determining the equilibrium potential and net surface current for dust smaller than the Debye length. The dependence of the normalised potential on the temperature ratio, flow speed, dust size and magnetic field strength was explored and used to constrain a semi-empirical model for the plasma currents, suitable for implementation in dust tracking codes. Simulations of the rotational dynamics of dust due to the collection of angular momentum revealed the importance of electrons at weak magnetic field strengths. These results and analytic estimates explain experimental observations in low temperature discharges of dust rotating anti-parallel to the magnetic field at ~100Hz. These improved models were incorporated into DTOKS-U and compared with fast-camera observations of carbon dust transport in DIII-D and breakup of molten tungsten observed over 103 discharges in JET. Both the importance of the strike point for limiting dust transport in DIII-D and the rotational breakup mechanism responsible for the observations in JET were identified. These results suggest that the higher magnetic fields, plasma temperatures and densities of ITER will alter charging but also enhance the breakup process, protecting the core plasma from acute impurity deposition, providing significant operational benefits.
Supervisor: Coppins, Michael Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral