The Low Frequency Edge Oscillation in Alcator C-Mod and ASDEX Upgrade I-Mode

Autor:	McCarthy, William C.
Rok vydání:	2022
Druh dokumentu:	Diplomová práce
Popis:	The I-Mode confinement regime in tokamaks is a promising regime for nuclear fusion reactor operation for a number of reasons. It exhibits high energy confinement (similar to H-Mode) and low particle and impurity confinement (similar to L-Mode), which allows it to reach high pressures without unacceptable impurity accumulation. It has been shown to be stable against Edge Localized Modes which are large bursts of plasma leaving the confined region of the tokamak, lessening the heat exhaust challenge. However, the physical mechanism behind I-Mode is still not well understood. In this thesis, one possible explanation for I-Mode phenomenology is explored. Namely, that a low frequency oscillation in a variety of plasma parameters near the Last Closed Flux Surface (LCFS) is responsible for setting the transport qualities of I-Mode. The experimentally observed fluctuation is referred to as the Low Frequency Edge Oscillation (LFEO). The LFEO is observed by most diagnostics that measure the edge of the confined plasma with a frequency between 10kHz and 40kHz on Alcator C-Mod (C-Mod) and 5kHz and 15kHz on ASDEX Upgrade (AUG). These include, but are not limited to: Reflectometry (density measurement), Gas Puff Imaging (density measurement), electron cyclotron emission (temperature measurement), Mirnov Coils (magnetic field fluctuation measurement), and Langmuir probes (simultaneous density, temperature, and electric potential measurements). The LFEO occurs at the same location as the Weakly Coherent Mode (WCM), another common feature of I-Mode, which is thought to exist at the bottom of the radial electric field well. Its frequency structure is observed to depend on electron temperature and is modulated by the sawtooth heat pulse. Previous work has established that the LFEO is a Geodesic Acoustic Mode (GAM). The GAM is a toroidally and poloidally symmetric electric potential perturbation that results in a poloidal (m=1) flow and pressure perturbation and poloidal (m=2) magnetic perturbation. A major goal of this thesis was to confirm this identification on both C-Mod and AUG. On both machines, the LFEO frequency was consistent with, albeit slightly lower than, the frequency of a GAM driven very close to the LCFS. The radial structure of the LFEO was investigated using a combination of Gas Puff Imaging and scanning Mirror Langmuir Probe, revealing an inwardly radially propagating potential structure consistent with a GAM. Magnetic mode structure analysis on C-Mod revealed an m=2 magnetic structure, consistent with a GAM. An observed correlation between LFEO frequency and impurity concentration was qualitatively consistent with GAM models that include impurities. However, the required impurity levels to explain the correlation were much higher than would be physical. Given the accumulated evidence, the LFEO was confirmed to be a GAM, although one likely modified by shaping, rotation, or other effects. The role of the LFEO in I-Mode phenomenology was explored first by reviewing existing literature and then via a database existence space study. Previous work by Manz on AUG and Cziegler on C-Mod has shown that the GAM couples with WCM and transfers energy from the WCM central frequency to frequency bands offset by the GAM frequency. Further, Cziegler has shown on C-Mod that the total nonlinear drive into Zonal Flows (believed to be a trigger for H-Mode) is reduced in the standard I-Mode “unfavorable” plasma configuration and that GAM drive only meaningfully appears following the L-I transition. Due to the reduction in zonal flow drive in the “unfavorable” configuration appearing in LMode prior to I-Mode, it is clear that the presence of the GAM is not the controlling factor in allowing IMode access, but perhaps does extend the I-Mode window. This was supported via a database analysis aimed at identifying the parameter space existence of the LFEO. Within this study, the LFEO was determined to be observable in only 62% of C-Mod I-Modes and 40% of AUG I-Modes. While no clear parameter space separation was found between LFEO and non LFEO shots, this does confirm that the LFEO is not necessary for I-Mode operation. Finally, a shot by shot analysis of I-Modes and LFEOs revealed a sharp, unexplained transition in LFEO and WCM frequency with increasing heating power. This hints at different degrees of confinement within I-Mode, with different controlling physics. It is possible that the LFEO is important to one such regime. While the LFEO is not necessary for I-Mode, it may still play a role in regulating particle transport while present. The electrostatic particle and energy fluxes near the LCFS, associated with the LFEO, were calculated using a scanning Mach Mirror Langmuir Probe. This revealed significant levels of flux just inside the LCFS, peaking at the maximum probe plunge depth. The energy flux, assuming a uniform flux around the LCFS, was compared with the total energy crossing the LCFS from a balance of heating and radiative powers. We found that using fluxes at peak probe plunge depth, the flux accounted for 80% of the power crossing the LCFS. However, if the flux calculated directly at the LCFS is used, the percentage reduces to 5%. Thus, the calculated flux is not unphysical, but can be significant. The particle flux was compared to the flux calculated at the divertor using an array of divertor Langmuir probes. It was found that the flux associated with the LFEO at the divertor was significantly reduced compared to flux at the LCFS. These results suggest that the flux is not poloidally symmetric around the LCFS. A surprising result was the observation of a significant and sudden reduction in particle flux to the divertor in all fluctuations below 40kHz, at the L-I transition. The GAM is thus, an important, but not necessary part of I-Mode phenomenology. While it is clear that I-Mode can exist without the LFEO, the significant particle fluxes at the edge indicate that the LFEO can help to regulate particle fluxes. Further exploration of the existence space of the LFEO is necessary to determine where it can exist and how it could influence different I-Mode regimes. Ph.D.
Databáze:	Networked Digital Library of Theses & Dissertations
Externí odkaz:	https://hdl.handle.net/1721.1/144665 Zobrazit plný text záznamu