Transport reduction due to sheared E x B flow in the torsatron TJ-K

Plasma biasing was successfully applied to the TJ-K plasma and fields of the order of 1100 V/m have been induced with shearing rates up to 1 MHz, leading to a steepening of the density gradient by a factor of 5 and a reduction in particle transport. At the same time, turbulent density structures on larger spatial scales beyond validity of simple mixing-length estimates remain. However, these structures feature a shortened lifetime of the order of 20μs. A shortened lifetime would reduce the effective radial step size of a diffusive process. The maximum contribution to the turbulent transport as measured with a poloidal probe array was found on the LFS. In the presence of the E x B shear flow, the transport on the LFS was reduced due to both, a reduction in density fluctuations and a modification of the cross-phase. For the case of constant fluctuation levels, a reduction solely due to the cross-phase was reported in Ref. [14]. Local measurements on the LFS showed that a substantial fraction of the spectral power was redistributed from the low-frequency range into a high-frequency mode causing inward transport due to cross-phases of φvn > π/2. Furthermore, the poloidal measurements presented here showed that the particle flux can locally be reversed and, thus, significantly contribute to the reduction in flux-surface averaged transport. Since in TJ-K turbulence is dominated by drift-waves [11], these results are qualitatively consistent with theoretical predictions by Terry et al. [4, 5].