Cryogenic pellet ablation modelling in a hot, magnetised plasma

Martin, Frederick Kyle (2021) Cryogenic pellet ablation modelling in a hot, magnetised plasma. PhD thesis, University of Glasgow.

Abstract

This thesis explores the possibility of using a simple "toy" model approach to characterising the ablation rate of a pellet once injected into a magnetised plasma. A model for the ablation rate and structure of the cloud in terms of its size and density is produced by assuming the ablation process is in kind to that of the evaporation of sessile pellets. An empirical law known as the D-squared law is revised and applied to determine these key quantities which are then used in a Bethe-type calculation assuming continuous slowing down of electrons within the neutral cloud. Any stopped charge then contributes to an electric potential field which further slows particles as they enter the cloud and a feedback system is established. Additionally, a floating sheath is generated whose potential field further mitigates the electron energy as they traverse the cloud and the creation of ions by ionisation within the cloud are permitted to diffuse and ions sufficiently close will be accelerated by the sheath field and allowed to impact the pellet. The energy arriving at the surface of the pellet releases neutral particles which then change the structure of the cloud and the system evolves self-consistently. The ablation rate and projected lifetime of the pellet in each of these scenarios are measured against experimental results to assess the efficacy of the model. Finally, the H-alpha emission is modelled and compared to emission profiles from experiment.
The modified evaporative model applied in this thesis agrees moderately well with existing measurements from tokamak devices. Order of magnitude agreement is found but more thorough agreement is hard to achieve given the temporal variation of the system the model predicts and the struggle to determine what point in the pellet's lifetime an experimental measurement was taken. A similar conclusion on the results is found for the neutral density of the cloud. The influence that these results have over the self-consistent field models is evidenced in ablation rate results for the four models developed in the thesis. The Fully Neutral model is the only model that predicts the ablation rate to even order of magnitude with the potential fields generated inhibiting any significant ablation. The neglect of charge deposition and the self-field is fundamentally incorrect and more complex models that boost the ablation with additional mechanisms or mitigate charge density in the cloud are needed in further bodies of work.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Plasma physics, nuclear fusion, cryogenic pellet, pellet injection, ablation,
• Subjects: Q Science > QB Astronomy; Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Diver, Prof. Declan
• Date of Award: 2021
• Depositing User: Mr. Frederick Kyle Martin
• Unique ID: glathesis:2021-82131
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 27 Apr 2021 15:26
• Last Modified: 27 Apr 2021 15:43
• Thesis DOI: 10.5525/gla.thesis.82131
• URI: https://theses.gla.ac.uk/id/eprint/82131