Stellarators and the path from ITER to DEMO

A demonstration of fusion power, called DEMO, requires information on a number of physics issues that are addressed by non-axisymmetric shaping of the plasma. Stellarator experiments have shown that non-axisymmetric shaping provides the control needed to address the maintenance of the magnetic configuration, robustness against disruptions and restrictive upper limits on the plasma density. Shaping is the primary means of control of a toroidal fusion plasma. The importance of axisymmetric shaping is recognized and exploited. The remaining freedom is in non-axsiymmetric shaping, which has physics benefits but technical challenges. The axisymmetric vision of DEMO as a self-organized plasma with weak external control is a design choice and not a requirement. DEMO cannot be perfectly axisymmetric, so non-axisymmetric magnetic fields must be controlled. The questions are at what level, of what type and for what purpose.