Roles of bumpy field on collisionless particle confinement in helical axis heliotrons

Abstract

The roles of `bumpy' field on collisionless particle confinement in helical axis heliotrons are investigated with model magnetic field and particle orbit calculations in Boozer co-ordinates. The mod Bmin contours can be shifted in the major radius direction with control of the bumpy field, where Bmin is the minimum value of |B| in the toroidal direction within one field period. The area of closed mod Bmin contours is a useful measure with which to evaluate global collisionless particle confinement as long as the mod Bmin contours connect toroidally. The negative value of the ratio between the bumpy component and the helicity component contributes to obtaining the largest area of closed mod Bmin contours for a finite ratio between the toroidicity component and the helicity component. The radially increasing bumpy field contributes to the realization of a toroidally localized mod Bmin structure. It also produces a region with a large derivative of |B| with respect to the toroidal magnetic flux, which is well aligned to the toroidally localized mod Bmin structure. This enhances the large poloidal drift of the guiding centre orbits, while the radial drift is unaffected, effectively improving the particle confinement.