The plasma sheath is essential for understanding the plasma-material interaction (PMI) since it regulates the plasma particle and energy fluxes to the wall. The key concept in sheath theory is the Bohm criterion that gives the lower bound of the plasma exit flow speed, also known as the Bohm speed. Traditionally, the Bohm speed is evaluated in the asymptotic limit of an infinitely thin sheath and ignores the transport physics in the plasma-sheath transition problem. Whereas in practical applications, the sheath has a finite thickness and the transport in the neighborhood of the sheath entrance is complicated. The focus of this thesis is on performing Bohm speed analysis for different applications that are away from the asymptotic limits, with an emphasis on the critical role of transport physics on the Bohm speed formulation.
The classical sheath problem with a wide range of Coulomb collisionality is revisited. Here, we derive an expression for the Bohm speed from a set of anisotropic plasma transport equations. The thermal force, temperature isotropization and heat flux enter into the eval- uation of the Bohm speed. Away from the asymptotic limit, it is shown that there exists a plasma-sheath transition region, rather than a single point at the sheath entrance. In the transition region, the quasineutrality is weakly perturbed and the Bohm speed is predicted for the entire transition region. By comparison with kinetic simulation results, the Bohm speed model in our work is shown to be accurate in the sheath transition region over a broad range of collisionality.
The Bohm speed analysis developed above can be applied to plasma-sheath transition prob- lems with more complex transport physics, such as a high recycling divertor in a fusion reactor. In the high recycling regime, the plasma particles hitting on the divertor surface will be recycled through reflection or desorption and return to the plasma in the form of neutrals. The plasma will interact with the recycled neutrals through atomic collisions such as ionization, excitation, or ion charge-exchange collision, complicating the plasma transport in the transition layer. A new Bohm speed model is proposed to account for the effect of the anisotropic transport and atomic collisions in the transition layer. A first principle ki- netic code VPIC with the atomic collision package is used to investigate a 1D self-consistent slab plasma with a high recycling boundary for tungsten and carbon divertors. The results demonstrate the accuracy of the Bohm speed model in predicting the ion exit flow speed in the transition region, as well as the reduction of the Bohm speed due to the ion-neutral friction. / Doctor of Philosophy / Controlled thermal nuclear fusion is a promising candidate for future energy supply. In a fusion reactor, a vast amount of energy is created and confined in the main plasma, while the boundary plasma can carry a certain amount of energy from the main plasma and deposit it on the surface of the plasma-facing component (PFC) of the reactor. The edge plasma and the material surface are strongly coupled through the plasma-material interaction (PMI). It is widely understood that PMI is a critical issue in realizing controlled thermonuclear fusion.
The PMI problem involves complex physics phenomena that cover a wide range of spatial and temporal scales, posing a significant challenge to its modeling. This work mainly focuses on physics at the intermediate scale, where sheath/presheath physics dominates.
The plasma sheath is a thin, positively charged layer that forms in front of the material surface to equalize the electron and ion fluxes. In classical sheath theory, an idealized point, the sheath entrance, connects the quasineutral plasma and non-neutral sheath. The ions can be accelerated by the presheath electric field and reach the Bohm speed (equal to the sound speed in classical sheath theory) at the sheath entrance. That is the Bohm criterion, a necessary condition for a stable sheath to form.
The plasma sheath in a fusion reactor is exposed to a complex environment where the atoms and molecules are abundant and can interact with the plasma inelastically. As a result, many assumptions made in the classical sheath theory may not be valid for practical applications, such as a divertor sheath. The classical sheath theory is derived in the asymptotic limit of an infinitely thin sheath. In a real plasma, a sheath transition layer, rather than a singular sheath entrance, exists, and it connects the plasma and sheath smoothly.
In the transition region, the quasineutrality is weakly perturbed, and the plasma transport is significant. Previous evaluation of the Bohm speed invokes drastic simplification of the transport physics, resulting in a Bohm speed equal to the sound speed. Here, we propose a new Bohm speed model that considers the dominating transport phenomena-anisotropic transport and collisional transport. The Bohm speed analysis is performed in two cases:(i) a classical sheath problem with absorbing boundaries and (ii) a high recycling divertor where the plasma-neutral interaction is significant. In the first case, we extend the classical sheath analysis to a regime that is away from the asymptotic limit. The counterpart of important concepts in the two-scale analysis, such as the sheath entrance and Bohm speed, is established and well explained. The transport dependent Bohm speed model is derived from a set of anisotropic transport equations, where the heat flux, thermal force, and Coulomb collisional isotropization are considered. The model can predict the ion exit flow speed in the transition region over a broad range of Coulomb collisionality, as shown by comparison with the kinetic simulation results. The second case is more practical, where the Bohm speed analysis is performed at the edge of a fusion reactor. The plasma transport in the transition region is complicated by the plasma-neutral interactions. As a result, the Bohm speed model includes atomic collisions, such as ionization, excitation, and ion charge-exchange collision.
Among all the collision processes, the ion charge-exchange collision has the most significant influence on the Bohm speed. It acts as a significant momentum sink for the ions and makes the Bohm speed subsonic in the transition region.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/114948 |
| Date | 05 May 2023 |
| Creators | Li, Yuzhi |
| Contributors | Aerospace and Ocean Engineering, Srinivasan, Bhuvana, Scales, Wayne A., Adams, Colin, England, Scott L. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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