Stellarators seek to confine fully ionized plasms by means of magnetic fields that are intricately shaped in all three dimensions of space. The concept was invented by Lyman Spitzer Jr at Princeton University in 1951 and has been studied theoretically and experimentally ever since. The largest investment in this branch of plasma physics was made by the Max Planck Society in the form of the Wendelstein 7-X project, the world’s largest stellarator, which started operation in 2015.

Stellarators can be designed in many different ways thanks to the approximately 50 degrees of freedom available in the shaping of the magnetic field. Wendelstein 7-X is the result of an optimization effort where neoclassical transport and the plasma current were minimized. This optimization seems to have worked. Experimental results confirm the theoretical predictions, giving great confidence in our ability to predict and optimize various aspects of plasma performance. 

Nowadays, it is possible to theoretically predict plasma behavior to a greater extent and to higher accuracy than at the time of the Wendelstein 7-X design. Moreover, computers have become faster and optimization algorithms have been improved, so it should be possible to design stellarators that perform even better than Wendelstein 7-X. This is one goal of the MPPC activity in this area. Another goal is to develop stellarator theory in general and to use this knowledge to better understand experimental results from Wendelstein 7-X. There is strong synergy with the Simons Collaboration on Hidden Symmetries and Fusion Energy (https://hiddensymmetries.princeton.edu/).