Osborne, Nicholas 
ORCID: 0000-0002-3773-6113
(2024)
A novel approach to understanding the role of plasma-molecular kinetics in detachment in the MAST-U and TCV tokamak divertors, and its impact on power exhaust.
PhD thesis, University of Liverpool.
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Abstract
This work investigates the role of plasma-molecule collisions in the divertors of the MAST-U and TCV tokamaks during detachment across a wide range of conditions (divertor shape, baffling, fuelling location, isotope species and power). Experimental data is obtained from both devices and, in particular, high resolution Fulcher band spectroscopy is used to examine the evolution of the rotational and vibrational distributions of the molecules during detachment. The rotational temperature achieved by the D2 (and H2) molecules is related to the length of time during which they undergo collisions (particularly with the ions) in the plasma. In “hot” ionising plasma conditions (Te >∼ 5eV), this is limited by their lifetime before dissociation and, in "cold” conditions (Te <∼ 3−5eV), their transit time through the plasma. As such, the temperatures reached by the molecules may increase significantly, by ∼1000s of K during detachment, and decay again in the most deeply detached (cold) conditions. It is found to be possible to use this rotational temperature, along with measurements of the ionisation front position (volume of the neutral cloud below the front) and measurements of the divertor neutral pressure (molecular density), to estimate power losses from the plasma due to elastic collisions between the ions and the molecules. These losses are significant (∼ 10% of PSOL) in long-legged, strongly baffled, detached, MAST-U Super-X divertors. Momentum losses can also be inferred from these measurements, and may account for highly significant (majority) pressure loss in strongly detached plasmas. These estimates are also found to be consistent with SOLPS-ITER simulations. The measured vibrational distribution is found to be non-Boltzmann in the ground state. It has the same characteristic shape under all conditions on MAST-U and TCV, and undergoes a similar elevation of the ν = 2,3 bands (in the upper Fulcher state) at, or soon after, detachment. Attempts to reconstruct the vibrational distribution using an Eirene-like collisional-radiative model setup shows some agreement with the data, but also some disagreements indicating possible gaps in the model and the need for further research. In magnetic confinement fusion, the power exhaust problem, and the necessity to be able to achieve and maintain reliable plasma detachment in the running of future reactors in order to resolve this, remains a significant issue. Predicting power and momentum loss will be crucial in this endeavour. It is known that the role of molecules in detachment is likely to be highly significant in these devices, especially those with long-legged and/or strongly baffled divertors such as SPARC, ARC and STEP. This work shows this experimentally and provides a much clearer understanding of this role, highlighting the importance of molecules as energy and momentum sinks in the detachment process.
| Item Type: | Thesis (PhD) |
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| Uncontrolled Keywords: | Detachment, Fulcher band, Fusion, MAST-U, Rotational temperature, Spectroscopy, TCV, Tokamak, Vibrational distribution |
| Divisions: | Faculty of Science and Engineering Faculty of Science and Engineering > School of Electrical Engineering, Electronics and Computer Science |
| Depositing User: | Symplectic Admin |
| Date Deposited: | 13 Aug 2025 14:31 |
| Last Modified: | 13 Aug 2025 14:31 |
| DOI: | 10.17638/03190259 |
| Supervisors: |
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| URI: | https://livrepository.liverpool.ac.uk/id/eprint/3190259 |
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