Plasma loss and associated diffusion coefficients to an isolated internal octupole ring

Measurement of particle transport is made in the magnetohydrodynamically stable region near a completely isolated internal ring in a levitated octupole by telemetering the collected plasma signal. With a toroidal field of 10% of the poloidal field and with the parameters of gun-injected plasma (Ti=20–40 eV, Te=5 eV, n=109/cc) a derived diffusion coefficient of D=60 cm2/sec results. With only the poloidal field, and thus without flux surfaces or shear, D=750 cm2/sec. Both diffusion coefficients are independent of magnetic field strength between 1.2 and 3.0 kG. This independence suggests Okuda–Dawson convective diffusion, but the observed magnitude with a toroidal field is 40 times greater than the minimum given by theory including mirroring effects. With only the poloidal field the observed result is 20 times the theoretical estimate. Transport with a toroidal field is very sensitive to levitation. Supports or potential disturbing connections must be removed from the ring or transport is an order of magnitude larger.