The Helias Reactor Concept: Comparative Analysis of Different Field Period Configurations

The Helias reactor (HSR) is an upgraded version of the Wendelstein 7-X (W7-X) experiment. A straightforward extrapolation of W7-X leads to a five-period configuration with a major radius of 22 m. To reduce the size of the reactor, another option with four periods has been investigated. Recent studies have focused on a three-period Helias configuration (HSR3/15i) (major radius 15 m, plasma radius 2.5 m, B = 5 T), which presents a more compact option than the five- and four-period configurations. In HSR3/15i, the resulting magnetic configuration is consistent with the island divertor concept. The stochastic region outside the last magnetic surface is imposed by the remnants of the 3/4 islands and the plasma flows along distinct channels toward the plates. The main problem is due to the high value of the bootstrap current (~1 MA) and alpha-particle losses (estimated as 6%). Further optimization of HSR3/15i can cause the maximum value of the magnetic field at the superconductive coils to be exceeded. There is a trade-off between physics goals (alpha-particle confinement and small bootstrap current) and technical realization (NbTi technology). The comparative analysis of different period configurations will be presented.