OMNIGENITY AND QUASIHELICITY IN HELICAL PLASMA-CONFINEMENT SYSTEMS

Omnigenous stellarators, those with bounce averaged drifts lying withi n the magnetic surfaces, form a much larger class than quasihelical st ellarators (i.e., with magnetic-field strength depending on only a sin gle linear combination of the toroidal flux angles) while nevertheless having very good transport properties. It is shown by construction th at exactly omnigenous magnetic strengths can be very far from quasihel ical, but that all such magnetic strengths are nonanalytic. However, b y truncation of the Fourier representation one can obtain analytic mag netic strengths that are very close to being omnigenous while still ve ry far from quasiherical. This indicates that the condition of quasihe licity can be significantly relaxed and yet retain good neoclassical t ransport. Finally, it is conjectured that reasonably good transport ca n be obtained by requiring omnigenity for only the deeply trapped and marginally trapped particles. Such fields will have no transition part icles and, hence, no separatrix crossing chaos. These conditions reduc e to the requirements that the held line maxima and minima of the magn etic field and the separatrix action be constant on a magnetic surface . (C) 1997 American Institute of Physics.