Understanding the pedestal MHD turbulence structure and plasma response to magnetic perturbations using microwave imaging diagnostics on the DIII-D tokamak
- Yu, Guanying
- Advisor(s): Luhmann, Neville Clinton
Ever since men discovered nuclear fusion being the sun’s power source, there has been relentless and ongoing efforts to bring this power to the earth by building a magnetic fusion reactor. However, even with its mighty gravity, the sun still has a serious surface problem, where coronas at ~ 10^6 K throws energetic particles to the space. On earth, the surface problem of a magnetic fusion plasma is the Edge Localized Mode (ELM). ELM is one plasma edge instability which efficiently pumps out ashes of fusion waste. However, it also releases the energy explosively to the divertor, sputtering impurities back to the plasma and damaging the divertor material. Quiescent H-mode (QH) and Resonant Magnetic Perturbation (RMP) are two important techniques to control ELMs in the ELM community. Powerful diagnostics are needed to characterize the plasma during ELM suppression, which improves our understanding towards the physics mechanism and gain insights when projecting the techniques to reactor level plasma confinements. This dissertation uses Electron Cyclotron Emission Imaging (ECEI) as the chief diagnostic to study the MHD turbulence fluctuation at the pedestal and the plasma response to magnetic perturbations. Both are the key physics elements that have been enthusiastically studied in the QH and RMP reseaches. The interpretation of the ECEI observation gives validation and provides new insights to ELM suppression physics understanding.The non-local radiation effect of ECE at the pedestal and Scape Of Layer (SOL) is applied to characterize the turbulence (MHD) fluctuation during ELM suppression. Three turbulences (MHDs) respectively in the standard QH, wide pedestal QH, and RMP plasma pedestals are characterized with ECEI. The ECE radiation signals all display a phase-inversion at the separatrix. Forward modeling of the ECE radiation shows that the SOL radiation is negatively correlated with the electron density and temperature fluctuation at the pedestal foot, while the pedestal radiation is sensitively proportional to the electron temperature. Thus, the radiation inversion is consistent with the fluctuation caused by radial displacements, where electron temperature and density fluctuation are in-phase at the pedestal foot. This interpretation is consistent with the edge ECEI observation of core MHD modes, which excite non-zero displacement motions of the pedestal. Further evidence supporting the interpretation of displacements is found with the cross-phase measurement between Mirnov and ECEI. Interestingly, unlike the above three MHD turbulences, the Edge Harmonic Oscillation (EHO) does not display radiation phase-inversion on ECEI. Supported by radiation modeling, this radiation pattern is consistent with the EHO’s radial structure peaking at the pedestal top. Plasma kink response is applied to interpret the increased pedestal mode pitch observed with ECEI and Mirnov probe. Plasma kink response to external magnetic perturbation is predicted by fluid modeling in the RMP community. However, such a response is hard to be diagnosed in experiments. ECEI and Mirnov observed that the pedestal mode pitch of a 3/2 tearing mode is significantly increased at H-mode plasmas. Single fluid M3D-C1 modeling is employed to understand this phenomenon and found that this increased mode pitch is consistent with the plasma kink response to internal magnetic perturbations. The Mirnov measurement shows the mode pitch increases with edge safety factor and normalized β, which is also reproduced in the M3D-C1 parameter scans to understand the kink response. The magnetic perturbation’ effect on the divertor heat deposition is also theoretically studied with M3D-C1 and field line tracing TRIP3D. Due to the rotation shielding, tearing modes do not break the layered magnetic surfaces at the pedestal or produce a footprint on the divertor. However, the low m component of the perturbation can effectively modulate the pre-existed divertor footprint due to error fields or RMP. The modulation on the footprint allows the heat to be deposited at different locations in one cycle of the tearing mode, which equivalently increases the heat deposition area and reduces the heat flux on the divertor. An OMFIT module is developed for ECEI data post-processing and interpretation. Advanced algorithms and techniques are developed to fully utilize the multi-dimensional measurement capability of ECEI, such as characterizing the turbulence dispersion relation, decorrelation length, MHD radial structure and poloidal mode number. A 1D ECEI technique is specifically designed to characterize MHD in the frequency and radial space domain. The technique effectively suppresses the plasma thermal noise, which allows it to acquire high quality spectrums using a short time window and avoid biased fitting of the MHD radial structure. The OMFIT module also allow users to perform radiation modeling with analytical fluctuation structures, assisting the ECEI data interpretation when non-ideal effects like finite resolution and optical grey become significant.