Possibility of improvement in the four-period quasihelically symmetric stellarator
The method of paraxial expansion has established the possibility of different types of quasihelical symmetry (QHS). Besides Helias-like systems with n(1) = -1/2 number of revolutions of the magnetic surface cross-section relative to the magnetic axis principal normal, the Heliac-like systems with n(1) = 0 can also possess quasihelical symmetry properties. The geometry of the Heliac-like systems is more favourable for the creation of a magnetic well, thus could provide higher locally stable beta. Starting with the plasma boundary determined from the QHS condition in near-axis approximation, we computed 3-dimensional equilibria and evaluated the magnetic field strength harmonics in Boozer coordinates using the VMEC and TERPSICHORE codes. After that we modified the parameters that describe the boundary to decrease the B harmonics that violate the QHS condition and to improve the local ideal MHD stability. The Mercier beta limit here is about 4%. In contrast to the n(1) = -1/2 systems like HSX, the beta value in the case considered is limited more strongly by the ballooning modes rather than by the Mercier ones.
Bibliographic Reference: Article: Nuclear Fusion Letters (1997)
Record Number: 199710183 / Last updated on: 1997-04-01
Original language: en
Available languages: en