Thursday, 19 December 2019
Dr Gunsu S. Yun
Department of Physics
Pohang University of Science and Technology
"What breaks the confinement of magnetically confined plasmas?"
It appeared as a simple idea to confine high-temperature plasmas for sustained nuclear fusion reactions using toroidally symmetric magnetic fields called “tokamak” configuration. Half a century later, that still remains a challenging goal albeit significant progresses in the underlying physics such as fluid/kinetic instabilities and particle/energy transport processes. To put it simple, the difficulty lies in the fact that improved confinement increases the probability of severe instabilities.
In modern tokamak experiments, the boundary of the plasma is characterized by a semi-periodic growth of steep pressure gradient owing to the “good” confinement provided by the tokamak configuration and its abrupt relaxation leading to a violent loss of energy and particles. It is well known that the relaxation is related to a class of eigenmode instabilities in the steep boundary but a question remains: What triggers the relaxation? An imaging diagnostics (like MRI in hospitals) developed on the KSTAR tokamak provided a clue to this question, showing that an eigenmode perturbation appears as the pressure gradient increases, and then a “non-normal” perturbation emerges overriding the eigenmode and soon bursts destroying the confinement.
In this talk, I will start with a brief overview on the physical constraints on the tokamak fusion concept and explain the importance of boundary conditions. I will then move onto the main topic, i.e., the dynamics of non-normal perturbation as one of boundary problems. I will use a toy model based on a generalization of the Ginzburg-Landau equation taking into account the shear flow in the plasma boundary, in order to explain a transition mechanism from eigenmode to non-normal perturbation.
WHEN: Thursday 19 December 2019
WHERE: Room 314, Science 3 Building
TIME: 3.00 pm–4.00 pm
All interested are welcome to attend
Light refreshments to follow in Common Room