Understanding the Mechanisms that Determine the Edge
Electron Density Profile in Tokamaks
Name
Miller-millerma-PhD-NSE-2025-thesis.pdf
Description
Thesis PDF
Size
15.71 MB
Format
Adobe PDF
Checksum (MD5)
cc88d3a8acf60692d957ede1cd47db07
Author(s)
Miller Hernández, Marco Andrés
Advisor(s)
Hughes, Jerry W.
Date Issued
May 2025
Publisher
Massachusetts Institute of Technology
Abstract
The interaction between the physics of plasma turbulence and that of atomic neutral dynamics, intrinsic to the tokamak edge, makes prediction of edge profiles difficult. It is unclear to what extent neutral ionization, as opposed to particle transport, is responsible for the build up of edge density gradients. To this end, this thesis combines electron and neutral measurements across the edge region with high-fidelity simulations of neutrals to study these processes in high density and high magnetic field plasmas on Alcator C-Mod. This is enabled by measurements of Lyman-α (Lyₐ) emission made by the LYMID camera, as well as measurements of electron density, nₑ, and electron temperature, Tₑ, by the edge Thomson scattering (ETS) system. These result in a large database of inferred neutral density, n₀ and ionization source, S_ion, as well as radial particle flux, Γ_D, and effective diffusivity, D_eff, for stationary periods. For selected discharges, these are used to impose additional constraints to simulations of neutral dynamics in the plasma edge using SOLPS-ITER. This methodology is used to examine stiffness in the edge gradients forming the so-called “pedestal" in the high-confinement mode (H-mode) in response to increased ionization. This phenomenon is found to be associated with changes to local particle transport, and is observed to be correlated with a local parameter governing the influence of turbulence from interchange instabilities as opposed to that resulting from drift-waves. Reaching the threshold in this parameter may be avoided through improved particle control and is found to also be highly dependent on the 2D distribution of neutrals in the unconfined plasma region. The competition between interchange modes and the drift-wave is probed on Alcator C-Mod through validation of a semi-empirical model for tokamak operational boundaries. The separatrix operational space (SepOS) model [Eich & Manz, Nuclear Fusion (2021)] predicts boundaries for the L-H transition, the L-mode density limit, and the ideal MHD ballooning limit in terms of plasma quantities evaluated using separatrix parameters for a wide range of Alcator C-Mod plasmas. These boundaries are expressed in terms of dimensionless quantities borrowed from electromagnetic fluid drift turbulence (EMFDT) theory. The combined workflow of ETS and LYMID also allows for evaluation of quantities associated with plasma transport in connection with the plasma operational space. Experimental evidence of changes to particle transport near the boundaries is provided for the first time. Organization of Γ_D at the separatrix is observed in both H-modes and low-confinement modes (L-mode) for key dimensionless parameters. The model is also used to elicit the physics of high confinement regimes free of Type-I edge localized modes (ELMs). Databases of the transition to the improved-confinement (I-mode) and that between the Type-I ELMy H-mode and the Enhanced Dα (EDA) H-mode are studied using the SepOS framework. An empirical model for particle transport in the EDA H-mode to explain pedestal saturation in this regime is developed and tested. The findings are then leveraged for modeling of next-generation devices, with priority on core-edge integration and improved power handling. high confinement regimes free of Type-I edge localized modes (ELMs). Databases of the transition to the improved-confinement (I-mode) and that between the Type-I ELMy H-mode and the Enhanced Dₐ (EDA) H-mode are studied using the SepOS framework. An empirical model for particle transport in the EDA H-mode to explain pedestal saturation in this regime is developed and tested. The findings are then leveraged for modeling of next-generation devices, with priority on core-edge integration and improved power handling.
MIT Department
Massachusetts Institute of Technology. Department of Nuclear Science and Engineering
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