Radiation Asymmetry during disruptions in JET device Oxford, United Kingdom

The plasma stored energy in a burning plasma pulse, as in ITER, will significantly exceed that in present tokamaks. The rapid release of this energy during a plasma disruption, which is a dramatic event during which confinement is suddenly destroyed [TokW], has the potential to cause melting of the first wall and cause high electromagnetic loads close to the design limits. It is thus of crucial importance establish reliable and effective strategies to avoid and, as a final line of defence, to mitigate disruptions. In this Thesis project, a mitigation technique named the Shattered Pellet Injection system (SPI), has been investigated considering experiments carried out at the JET device, located at Culham Centre for Fusion Energy (CCFE, Oxfordshire, UK). JET is a suitable machine to perform this study being equipped with a Beryllium and Tungsten as wall materials, as in ITER. The mitigation via SPI relies on fast impurity injection in the form of frozen pellets, usually Deuterium and Argon or Neon, which allow the plasma energy to be released as radiation. SPI is the disruption mitigation system adopted in ITER and to provide an effective SPI action, without damaging first-wall components, the radiation shall be spread toroidally [Jachmich22]. This project has focused on understanding the mitigation dynamics via SPI and calculating from experimental data and a 1D radiation model the toroidal peaking factor, a metrics that describes the level of radiation asymmetry. This work followed the studies reported in [Lehnen15],[Jachmich22] and considered a database of Ohmic and H-mode experiments where the SPI has been fired. The distribution of radiation has been inferred from the change of the radiated power measured at a given toroidal location as a function of the location of the phase of the locked mode [TokW]. The location of the locked mode has been tailored by using the Error field correction coils. A workflow has been established, which includes data analysis from multiple diagnostics and 1D modelling implemented in the Python framework [GitHub]. The main findings obtained in this project are: the radiation asymmetry content is maximal when the mode locking occurs close to the injection location, and plasmas with higher thermal content, such as H-mode plasmas, result in lower asymmetries hence more manageable load, which is consistent with previous results ([Lehnen15],[Jachmich22]).