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Lars van Ruremonde - graduation project at Tokamak Energy Ltd.

For my master graduation project I got to be part of the diagnostics group at Tokamak Energy Ltd., in Oxfordshire, England. I was happy to do my research together with colleagues in the group, under the supervision of dr. Vladimir Shevchenko.

The project

Tokamak Energy’s mission is to provide fusion energy by 2030. The company builds on the so-called spherical tokamak reactor concept, pioneered in the START experiment and later in MAST. The spherical tokamak concept has a more ‘squashed’ shape than a conventional tokamak – a lower aspect ratio – making it look more like a cored apple than the typical donut-shaped conventional tokamak. This concept has some advantages over the conventional tokamak design, mostly improving on efficiency through a high bootstrap current fraction, and lower sensitivity to instabilities due to ‘good curvature’. However, a problem with spherical tokamaks is the lack of space for a central solenoid, starting up the plasma current through induction. This sparked the research into solenoid-free start-up, and the so-called ‘merging-compression’ process was developed.

This method uses two in-vessel coils making two plasma rings. These rings then mutually attract and current is driven by the process of magnetic reconnection. This process efficiently converts magnetic energy into plasma kinetic and thermal energy, heating it to fusion-relevant temperatures. This makes the merging-compression technique very suitable for starting up a plasma current in the spherical tokamak. My job was to measure these high ion temperatures expected from magnetic reconnection start-up using Doppler spectroscopy. The goal of the research was to investigate where and how the reconnection process heats the plasma, and whether it is in agreement with theoretical predictions from resistive MHD laws. During my project, I found indications for a relation between the reconnecting magnetic energy and the ion temperature.

My contributions were providing spectroscopic simulations on predicting signal-to-noise ratios for both passive and active measurements, and to improve on the current spectroscopic set-up, providing an intense enough spectrum from passive radiation to provide not only lower errors on the determination of the temperature, but also to allow a spectral inversion to be applied, allowing the resolution of the radial ion temperature. Research into magnetic reconnection is not only relevant for efficient fusion reactor start-up, but it is also the process governing sawtooth crashes in tokamaks. Moreover, it is the fundamental cause of many astrophysical processes such as solar flares and geomagnetic sub-storms in Earth’s magnetosphere.

This graduation project has been a very nice opportunity for me to combine fusion and plasma physics. Moreover, the experience in a company abroad provided a good preparation for a future corporate career. My experiences in the UK, especially at Tokamak Energy were made possible by FuseNet, for which I am grateful.

- Lars van Ruremonde