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Title: Global gyrokinetic simulations of kinetic ballooning modes
Author: Martin Collar, James P.
ISNI:       0000 0004 8498 0092
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2018
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When a tokamak plasma is heated beyond a certain threshold, a pedestal, a region of large pressure gradient, is formed, which is limited by instabilities including kinetic ballooning modes (KBMs). In order to predict the pedestal height and width more accurately, KBMs must be further understood. Simulations of the pedestal region require both global and kinetic effects be present. The code used during this thesis, the global gyrokinetic Particle-In-Cell code ORB5, can accomplish this, whereas magnetohydrodynamic (MHD) simulations do not include kinetic effects and local gyrokinetic simulations are only correct in the limit of large system size. In order to study the physics of KBMs, such as drive strength and mode structure, a simplified circular outer-boundary equilibrium was created and ORB5 simulations compared to MHD and local gyrokinetic simulations in the appropriate limits. These simulations show the error that arises from the neglect of the magnetic field strength fluctuations in ORB5 (AII formulation). With the corrected drive, ORB5 simulations are shown to agree with other codes in the appropriate limits and analytical theory. The growth rates, in gyrokinetic simulations, of high toroidal number modes are then shown to be equal to MHD growth rates with diamagnetic drift stabilisation. The other kinetic effects are not important. Simulations of KBMs in the pedestal region were then undertaken, in a JET equilibrium. Firstly, a method is provided for extrapolating equilibria beyond the last closed flux surface, avoiding unphysical suppression due to the simulation boundary. Then, the critical-β is found to be the same in ORB5 as in local gyrokinetic simulations without the bootstrap current. Therefore, local simulations, without the bootstrap current, can be used to provide the KBM constraint in the EPED model, used for predicting pedestal parameters.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics