Congratulations to Hannah Willett a York CDT student, who successfully defended her thesis at viva in August 2018. Her thesis is entitled “Applications of linear plasma device studies to the improvement of power injection and handling in tokamaks” and her supervisor is Professor Kieran Gibson. An abstract from Hannah’s thesis is below:
“A more advanced understanding of basic plasma physics processes is essential to the success of commercial fusion energy. We study two processes of considerable importance to power handling and injection in tokamak reactors, using two linear plasma devices.
Plasma ‘detachment’ is vital for the reduction of heat and particle fluxes to the divertor (the exhaust region of a tokamak) below the 10 MW/m^3 limit imposed by the material properties of the target plates. However, the physics of detachment is not fully understood. An area of particular concern is the potential influence of intermittent plasma transport perpendicular to the confining magnetic field.
In the first study, using the York Linear Plasma Device, we employ fast frame imaging and Langmuir probe diagnostics to identify fluctuations in the plasma column that are associated with the onset of detachment. Evidence is found for the intermittent outward radial transport of filamentary structures, which then cool to initiate the recombination necessary for detachment. A hypothesis suggesting the centrifugal instability as a mechanism for this transport is proposed.
The second study focuses on the potential use of helicon plasma devices as efficient, caesium-free negative ion sources for tokamak neutral beam injection (NBI) systems. A caesium catalyst is currently necessary to increase the negative ion production rate, but eliminating this is important for simplifying the maintenance requirements of these sources.
We examine the negative ion population behaviour in the helicon device MAGPIE. Peak densities of 1.25 x 10^18 m^-3 (an order of magnitude above the required NBI threshold) are measured using laser photodetachment, and a simple model of the time evolution of the negative ion population is in agreement with the experimental data. Neutral depletion is proposed as a mechanism governing the evolution of the plasma, and is consistent with additional experimental and simulation results”.