Measurements of periodic ripple transport in the ISX-B tokamak

Abstract

The effect of periodic toroidal field (TF) ripple on ion confinement has been studied in the ISX-B tokamak by comparing neutral-beam-heated plasma performance with 9 and 18 TF coils. Three ripple physics issues were treated by these experiments: (1) enhanced ion thermal conductivity, (2) enhanced loss of energetic ions, and (3) ripple damping of beam-induced toroidal plasma rotation, which may affect the plasma losses. Under a wide variety of plasma conditions, ripple reduced the central-ion temperature by a factor of approximately two (600 eV → 300 eV). Ion temperature was found to be nearly independent of applied neutral-beam power in the large ripple configuration (9 TF coils). These results are shown to be in reasonable agreement with theoretical models of ripple transport. Charge-exchange measurements of the fast-neutral flux indicated no loss of fast passing ions due to ripple, but a large depletion of the fast ions trapped in local ripple wells, as expected theoretically. The central toroidal rotation velocity was reduced by a factor of six by ripple, yielding a momentum confinement time substantially less (factor of about seven) than that expected from standard theoretical expressions for ripple-enhanced ion viscosity.