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The poet should say what sorts of thing might happen, that is, the things possible according to likelihood or necessity.

- Aristotles, Poetics Ch. 9

Nuclear Fusion

Imagining our world without any electrical power just for a few seconds, we come to a non-negotiable conclusion: we require electrical power to sustain and flourish in our lives. This power has been generated from natural resources, such as coal, oil, natural gas, and they inevitably produce carbon dioxide which may be argued as a cause of global warming. Moreover, the limited amount of such resources is another vital concern. Thus, it will be undoubtedly beneficial if we can generate electrical power without further enhancing global warming and without depleting the limited amount of resources so that we can return the planet to our children without corrupting it further. Nuclear Fusion power satisfies these criteria.

We do not inherit the planet from our parents, we borrow it from our children.

- Native American

Energy can be generated by fusing two nuclei of deuterium and tritium, which produces a harmless helium nucleus and a neutron with a total energy release of 17.6 MeV. To fuse the two nuclei they must vercome the repulsive electrostatic force which can be achieved by heating them to (large thermal energy) around ~ 10-20 keV at which reasonable performance of a fusion reaction can be achieved. At this high temperature, particles are ionized, i.e., become plasma.

Given that such hot plasmas with a sufficiently large density are created, the performance of a fusion power plant depends on how long they can be confined within a finite spatial domain. One way to confine the plasma is using the Lorentz force, a basic concept of ‘magnetic confinement’:

where m, and Ze are the mass, the velocity and the charge of the particle, respectively, and c is the speed of light. E and B are the electric and magnetic fields, respectively. By this law, a single charged particle in a strong magnetic field in the absence of an electric field is constrained to move along the magnetic field line with a helical trajectory, i.e., plasmas are confined in the perpendicular plane with respect to the magnetic field.

If the confined plasmas were collisionless in infinitely long parallel straight magnetic field lines, then a perfect confinement would be achieved. But, such field lines cannot be generated in practice, and furthermore, plasmas should not be collisionless if the goal is to extract power from fusion reactions, i.e., particles need to collide with each other occasionally.

Even though infinitely long straight magnetic field lines are not feasible, effectively ‘infinitely long’ lines are certainly possible: a closed field line is infinitely long in the sense that starting and ending points are indistinguishable.

This is the basis for a concept of the ‘TOKAMAK’ a transliteration of a Russian acronym a toroidal chamber with axial magnetic fields. A tokamak basically creates nested tori, where each torus is referred to as a flux surface, such that no magnetic field lines are connected between the two tori within the Last Closed Flux Surface (LCFS) unless there exist radial magnetic perturbations.

As the final practical goal of the fusion community is to light up the whole world with economical fusion power plants, we must be able to ignite plasmas, i.e., generate self-sustained burning plasmas. Known as the Lawson riterion, the ignition condition can be written as a triple produce of density n, temperature T and energy confinement time τE of plasmas:

The International Thermonuclear Experimental Reactor (ITER) endeavors to satisfy the condition.

- From Y.-c. Ghim’s D.Phil. thesis Ch1.

Research Interests

Turbulence is a major factor limiting the achievement of better tokamak performance as it enhances the transport of particles, momentum and heat which hinders the foremost objective of tokamaks. Hence, understanding and the possibly being able to control turbulence in tokamak is of paramount importance, not to mention our intellectual curiosity of it.

Our research focuses on the fusion-grade plasma turbulence observed in both KSTAR at National Fusion Research Institute (NFRI) in the Republic of Korea and MAST at Culham Centre for Fusion Energy (CCFE) in United Kingdom. As the understanding of plasma turbulence requires both theoretical/numerical and experimental researches, we make a strong connection with the KSTAR team, theory group from University of Oxford and simulation group from CCFE. We aim to understand the plasma turbulence better as it tests our wits and foils our attempts to control it in addition to the practical purpose of achieving high performance fusion reactor.