The pellet rocket effect in magnetic confinement fusion plasmas

Sammanfattning

Nuclear fusion power generation is widely regarded as a promising technology fora clean and sustainable future. To achieve positive energy gain from fusion, thefuel (deuterium and tritium) has to be sustained at temperatures around ten timeshotter than the core of the sun. Most of the power plant designs are currently basedon magnetic confinement fusion (MCF), where this fusion plasma is isolated andcompressed by strong magnetic fields. Refuelling, control of the plasma densityprofile and mitigation of off-normal events (disruptions) are done by injecting fasttiny pellets of dense frozen material, which quickly disintegrate from the extreme heatin the fusion plasma. While the efficiency of this material deposition in the plasmadepends on the pellet trajectory, the dynamics involved are poorly understood. Thisthesis develops a semi-analytical model for the so-called pellet rocket effect, whichaccelerates and deflects pellets in a fusion plasma. Asymmetries in the heat fluxonto the pellet surface enhance the ablation on one side of the pellet. Consequently,the pellet is pushed in the opposite direction to the ejected material, similarly toa rocket. This effect was shown in experiments to significantly modify the pellettrajectory. Projections for reactor scale devices indicate that it may even stop thepellet before it reaches the plasma core, which would severely limit the effectivenessof pellet injection methods. Our model predicts magnitudes of the pellet rocketacceleration (10^5 to 10^6 m/s^2) similar to the results of more sophisticated simulationsand experimental observations. Thus, the essential physics of the pellet rocketeffect are successfully captured in our model. With further validation and potentialimprovements, we expect that this approach can be used to inform the planning andoperation of pellet injection in future power plants.