Current PhD Project - Sean Knott
The objective of my Ph.D. project is to determine the density and temperature of electrons in a helium plasma based on their optical emission. This is of interest since material probes such as a Langmuir probe, if immersed in a tokamak plasma, will be quickly destroyed by the high power fluxes.
In the plasma experiment in the Department of Physics, UCC, a negatively biased tungsten filament is heated to incandescence to generate primary electrons by thermionic emission with energies up to 120 eV which ionize the helium working gas in the vacuum vessel to create a plasma.
The plasma is confined by a magnetic mirror formed from two stacks of Neodymium Iron Boron (NdFeB) magnets on opposite sides of the vessel. The plasma parameters are determined experimentally using a Langmuir probe diagnostic. This is a small wire (in this case tungsten) immersed in the plasma, which is subject to a voltage sweep resulting in an electrical current being drawn (as shown in the first graph). The current drawn by the probe consists of hot primary electrons generated by the filament as well as cold electrons and ions generated by the ionisation of helium atoms.
The optical emission of the plasma is also measured (see typical helium line spectrum in the second graph). This is carried out for a wide variety of plasma parameters from which a predictive model for the electron temperature and density based on helium line ratios can be constructed.
Tokamak Equilibrium Interpretation
Knowledge of the current density distribution J is crucial to understanding magnetohydrodynamic stability in an axisymmetric tokamak plasma. J cannot be measured directly, but must be inferred from a variety of magnetic and other diagnostic measurements. This constitutes a complex, ill-posed inverse problem that requires both physics insight and modelling expertise. To fully exploit the constraints on the J profile shape that are provided, in particular, by B-sensitive polarimetric measurements based on the Motional Stark Effect, a major new MHD equilibrium code has been developed and is maintained by the group. The CLISTE code (CompLete Interpretive Suite for Tokamak Equilibria) is used for reconstruction of the J profile on the ASDEX Upgrade tokamak at Garching, where it makes an essential diagnostic contribution to the ongoing research programme, in particular to the understanding of low-turbulence, high confinement discharges with transport barriers.
Stellarator Equilibrium Parameterization by Database Methods
The identification of 3-D MHD equilibrium configurations in a helical stellarator plasma by a conventional equilibrium code is a computationally intensive task, requiring several hours on a workstation. The number of free parameters in a stellarator equilibrium is modest, however (of order 10), and the experimentally accessible parameter space may be adequately sampled by the order of 1000 randomly selected simulated equilibria. The inverse mapping method known as Function Parameterization (FP) entails (i) Construction of a simulated database of experimental states of a system whose parameters are to be identified from measurements of the system, (ii) A once-off offline training phase to construct simple functional relationships between measurements and system parameters and (iii) Rapid, including realtime system identification by using these numerically determined functions in predictive mode. Steps (i) and (ii) are performed rarely (on a timescale of a year or longer) but step (iii), which is computationally trivial, is used routinely. FP has been successfully applied by the group to speed up equilibrium identification on the Wendelstein 7 advanced stellarator at Garching by close on a factor of 10000, so that the equilibrium configuration is recoverable in seconds rather than hours. The method has been under further development in recent years to meet the challenges of rapid identification of equilibrium flux topology with island structures in the case of the Wendelstein 7-X (W7-X) experiment, uin operation since 2015 in Greifswald, Germany,
Self-consistent determination of MHD equilibrium during ELM recovery
The transient heat and particle fluxes triggered by an ELM crash pose a potentially serious challenge to the reliable operation of ITER. This research topic involves the analysis and exploitation of high resolution edge kinetic data which has recently become available on the ASDEX Upgrade tokamak experiment to self-consistently determine the evolution of the plasma equilibrium state following an ELM crash with the help of relevant modelling tools, in particular the CLISTE equilibrium reconstruction code.
The Influence of Fast Particles on the Beta Limit in a Tokamak.
Fusion occurs in a deuterium-tritium fuel mixture in the plasma state consisting of equal densities of positively and negatively charged matter. An issue that will need to be addressed involves determining the best way to confine plasma so that fusion can be ignited and sustained. Confinement of the hot plasma is achieved magnetically and an important measure of its efficiency is determined by a parameter called the beta limit. This is the ratio of the plasma pressure to the magnetic field pressure. It is desirable to be able to measure and control this ratio. However there are various physical processes that can disrupt the equilibrium that exists between these quantities. This project is concerned with the question of how energetic alpha particles produced by fusion reactions are confined by the toroidally shaped magnetic field and also how they interact with the Maxwellian background distribution of particles. The fast population will have an effect on the beta limit in the tokamak which, if too high, can lead to instabilities and magneto-hydrodynamic ballooning modes which will result in a loss of confinement and energy. MHD stability at high beta will be vital for producing a cost-effective reactor and in determining the achievable fusion power density.