Description
The optimized stellarator Wendelstein 7-X is currently demonstrating the viability of the stellarator concept for a fusion reactor. The strength of the confining magnetic field has a decisive role in various key metrics for stellarators, with higher magnetic field being generally beneficial: improved confinement according to ISS04 scaling, reduced fast ion losses due to smaller orbits, smaller plasma beta and therefore less MHD activity and less stochastization of the island divertor scrape-off layer (SOL), and reduced asymmetries from ExB drift flows in the SOL. Current designs for future stellarators on the path to reactors universally rely on operation at significantly higher magnetic field than W7-X (BW7-X=2.5T), typically between 6T and 9T.
It is therefore obvious to experimentally assess the role of magnetic field strength in W7-X. However, experiments at B<2.5T are not trivial as the electron cyclotron resonance heating (ECRH), the main heating system at W7-X, is tied to operation at 2.5T via the resonance of the 140GHz gyrotrons in 2nd harmonic heating. In the 2024/2025 W7-X campaign, a reduced field plasma scenario based on neutral beam-assisted startup and 3rd harmonic ECRH has been established.
The first series of experiments at reduced field were conducted in a wide range of magnetic configurations (varying iota, magnetic shear, and MHD stability properties). Record plasma betas of close to 3% (volume-averaged) and 10% (on axis) have been achieved at reduced field. Confinement quality reduces as expected at lower field. An earlier onset of MHD crashes compared to full field is observerd. Beta-dependent Shafranov shift of core flux surfaces modifications of the edge islands preliminarily agree with expectations. In the SOL, a larger effect of drift flows is observed, in line with increased ExB velocities at reduced field.