Radiation asymmetry and MHD activity in rapid shutdowns on Alcator C-Mod
Author(s)Olynyk, Geoffrey Michael
Radiation asymmetry and Massive gas injection activity in rapid shutdowns on Alcator C-Mod
Massachusetts Institute of Technology. Department of Nuclear Science and Engineering.
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Disruptions, the sudden termination of tokamak fusion plasmas by instabilities, have the potential to cause severe material wall damage to large tokamaks like ITER.e mitigation of disruption damage is an essential part of any fusion reactor system. Massive gas injection (MGI) rapid shutdown is a technique in which large amounts of noble gas are injected into the plasma in order to safely radiate the plasma energy evenly over the entire plasma-facing first wall. However, it has been observed that this energy is not radiated evenly: it can have significant asymmetries, which could cause melting in large devices even in the case of a successful rapid shutdown. The first rapid shutdown experiments using multiple gas injectors on any tokamak were conducted on Alcator C-Mod. A dedicated toroidal array of fast ultraviolet photodiodes was installed in order to diagnose toroidal radiation asymmetries during the thermal quench (TQ). It is found that the radiation asymmetry is controlled by a low-n brightness mode in the TQ phase of rapid shutdowns. is mode sometimes rotates, and the rate of rotation sets the integrated radiation asymmetry in the TQ. It is proposed that this brightness feature is caused by the transport of energy from the hot plasma core to the radiative edge by the MHD flow at one phase of an n = 1 global MHD mode. is phenomenology is confirmed by extended MHD simulation using the NIMROD code. An exponentially growing n = 1 magnetic mode is observed during the pre-TQ phase of MGI rapid shutdowns; the saturation of this mode marks the beginning of the thermal quench. It is proposed that this mode is a magnetic island caused by a radiative tearing mode; the predicted growth rate is compared to the predictions of analytic theory. It is proposed that this mode is a magnetic island then couples to other global n = 1 MHD modes, causing the energy transport during the TQ. An important implication of this result is that simply adding more gas injectors cannot guarantee a symmetric rapid shutdown: the asymmetry is controlled by the behavior of the core MHD activity during the TQ. the implications of this rotating radiation asymmetry during the TQ of MGI rapid shutdown for the beryllium wall of ITER are discussed.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Department of Nuclear Science and Engineering.
Massachusetts Institute of Technology
Nuclear Science and Engineering.