This image, from the Oak Ridge National Laboratory, shows a fusion reaction contained by a force field. The magnetic fields force the plasma – the stuff being fused – into a donut shape, colored here to show the strength of the magnetism.
As the lab explains:
This figure shows the ITER plasma surface and toroidal magnetic field coils; color contours indicate local magnetic strength. The wavy structure of the contour lines is caused by deviations from axisymmetry due to the finite number of toroidal field coils and the presence of ferritic materials in three test blanket modules, which are located on one side of the torus