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PhD Thesis Prize 2020

Many congratulations to George King, Ben Chapman and Connor Mosley, for their success in the 2020 PhD Thesis Prize competition. A full list of recent thesis prize winners, and other accolades earned by the Department's PhD students, can be found here.

George King

George King was awarded the Winton Prize for Astrophysics, for his doctoral work on "Observations of Exoplanetary Systems at X-ray Wavelengths". George explained:

My work focused on exoplanets - planets around other stars - orbiting much closer to their star than Mercury orbits the Sun. At such close distances the output of high-energy X-rays from a star can remove material from the atmospheres of planets. I investigated the rate and energy of X-rays hitting a range of nearby exoplanets, to assess both current ongoing changes these are causing and how it has influenced the planets' evolution in the past. I also worked on the first detailed observation of a exoplanet transit (the short drop in the light from the star due to a planet passing in front of it) in X-rays. Observing this in X-rays could allow us to directly investigate gas being stripped from the planet.

Connor

Connor Mosley was a joint winner of the Faculty of Science Thesis Prize, and also won the Springer Thesis Prize. His thesis on terahertz spectroscopy of anisotropic materials will be published in book form by Springer. In Connor's work he created a new source of terahertz (far infrared) radiation with excellent polarisation control, which he used to study exotic magnetic materials. Connor writes:

The work presented in this thesis may have immediate impact on the study of anisotropic media at THz frequencies, with photoconductive emitters and detectors being the most commonly used components for commercially available terahertz spectroscopy and imaging systems, and by providing a new way to study the nature of magnetic phase transitions in multiferroics. In the longer term the increased understanding of multiferroics yielded by ultrafast spectroscopic methods, including terahertz time-domain spectroscopy, may help develop new magnetoelectric and multiferroic materials for applications such as spintronics.

BenBen Chapman was a joint winner of the Faculty of Science Thesis Prize, for his thesis titled "Ion cyclotron emission from energetic ion populations in fusion plasmas". Ben writes:
Future fusion reactors, such as ITER, will fuse deuterium-tritium to produce 3.5 MeV alpha-particles. It is crucial we understand the underlying physics of these fast ions if they are to be exploited to sustain thermonuclear burn. These fast ions are able to excite Ion cyclotron emission (ICE) waves, which, in fusion reactors, are in the radio frequency range. The detection of ICE is non-invasive, making it an attractive way forward for energetic ion measurements in future tokamaks such as ITER.

Ion cyclotron emission (ICE) comprises suprathermal radiation whose spectrum peaks at successive local cyclotron harmonics of the emitting energetic ion population. The driving mechanism behind ICE is the magnetoacoustic cyclotron instability (MCI). In this thesis, we simulate the MCI using the EPOCH particle-in-cell (PIC) code to solve the self-consistent Maxwell-Lorentz system of equations for fully kinetic electrons, thermal background ions, and the minority energetic ion distribution that drives the primary ICE.

By comparing PIC simulation results with experimental data from the KSTAR tokamak, we examine the nature of ICE excited by different energetic ion driving populations during edge localised mode (ELM) events. Simulations of ICE observed in the JET and ASDEX-Upgrade (AUG) tokamaks are then discussed. Motivated by recent observations of ICE in the core region of several tokamaks, we then compare MCI simulations that use two types of energetic ion distribution function, a spherical shell of varying thickness, and a ring-beam of varying width. We build on this work by demonstrating that observations of core ICE in AUG can be explained in terms of a driving population of fusion-born protons relaxing via the MCI. Bicoherence analysis shows that this is made possible through nonlinear wave-wave interactions between linearly unstable cyclotron harmonics.