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Abstract
Diverting energy from energetic $\alpha$-particles to waves ($\alpha$-channelling) would be extremely beneficial for a magnetically confined deuterium tritium fusion reactor. If these waves were to damp on fuel ions, a hot ion mode would result, doubling the fusion power of the reactor at the same confined pressure. Alternatively, if these waves damp preferentially on electrons traveling in one direction, current would be driven. In both cases, the pressure profile could be modified and ash could be removed to advantage.
These potentially significant benefits motivate a detailed study of the implementation of $\alpha$-channelling. This thesis identifies and explores issues in realizing $\alpha$-channelling, making the following advances: (1) Calculating, through 0-dimensional reactor simulations, the substantial benefit in $\alpha$-channelling which motivates the subsequent work. (2) Framing the $\alpha$-channelling problem as a diffusion problem, with absorbing and reflecting boundaries. To develop insight, $\alpha$-channelling is considered in a 2-dimensional phase space associated with a simple slab geometry. To solve realistic problems we pose $\alpha$-channelling in the 3-dimensional constants of the motion space associated with particle orbits in tokamaks. (3) Developing a rapid Monte Carlo simulation in constants of motion space to keep track of wave-induced and collisional effects on the energetic particle distribution. This approach is equivalent to the full energetic particle dynamics in the limit of small changes during a single bounce time and diffusive wave-particle interactions. (4) Identifying the wave characteristics necessary to produce the channelling effect, which we discover are available in a combination of the mode converted ion-Bernstein wave (MCIBW) and the toroidal Alfven eigenmode (TAE). (5) Demonstrating how two waves can be combined in a reverse shear tokamak reactor to absorb 2/3 of the energy from the 93% of the $\alpha$-particles ejected! (6) Showing how the basic building blocks of the $\alpha$-channelling effect can be deduced from existing experimental data, including: (a) Validating that the simulation agrees qualitatively with the results of TFTR experiments which show strong interaction of MCIBW with fast ions. (b) Demonstrating the existence of the k$\sb{\parallel}$-flip of the MCIBW. (c) Using the simulation to infer from experimental data a MCIBW diffusion coefficient, which significantly exceeds that which is predicted by geometrical-optics estimates.
Taken together, the advances in this thesis show how experiments to date give us a measure of confidence in both the simulations themselves, the underlying physical assumptions, and ultimately the reasonableness of the application of these ideas to $\alpha$-channelling in a tokamak reactor.
