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 B_min contours can be shifted in the major radius direction with control of the bumpy field, where B_min is the minimum value of |B| in the toroidal direction within one field period. The area of closed mod B_min contours is a useful measure with which to evaluate global collisionless particle confinement as long as the mod B_min 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 B_min 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 B_min 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 B_min structure. This enhances the large poloidal drift of the guiding centre orbits, while the radial drift is unaffected, effectively improving the particle confinement.