Undergraduate Research at Jefferson Lab

Modeling Magnetic Flux Expulsion in SRF Cavities

Student: Owen Meilander

School: Westminster College

Mentored By: Dr. Pashupati Dhakal

Superconducting radio frequency (SRF) niobium cavities are used by modern particle accelerators to accelerate electrically charged particles to near the speed of light. Superconducting niobium is used in these cavities because of its extremely low resistance, leading to minimal power dissipation in the form of heat. However, even in the superconducting state, there is still a residual resistance that persists. At temperatures far below niobium's critical temperature, it has been experimentally determined that the main contributor to this resistance is trapped magnetic flux. This flux is pinned during the cooldown process by impurities, grain boundaries, and other imperfections leading to an incomplete transition into the Meissner state. The focus of this project was to explore the effects of the geometry of the cell and the temperature gradient during the transition of the niobium to the superconducting state on the magnitude of the trapped flux. COMSOL Multiphysics was used to simulate the flux expulsion from both TESLA and JL C75 cavities by varying the relative permeability of the cavity and changing the angle of the applied field. Experimental data was then fit to this model, thereby incorporating the dependence on the temperature gradient. It was found that the trapped flux was proportional to the temperature gradient during cooldown through the critical temperature. The quantification of these properties confirms the other factors contributing to the residual resistance can be minimized with a significant temperature gradient during cooldown. This simulation also confirms that due to the three-dimensional nature of the magnetic field in the environment, magnetic shielding around the cavity is necessary to minimize flux trapping during the cooldown process and increase the performance of a given cell. These findings will allow for further optimization of these SRF cavities leading to increased performance and efficiency.

[Watch the presentation on YouTube]