ρ s scaling of characteristic turbulent structures in the torsatron TJ-K

The scalings of correlation lengths and times with the drift-scale parameter ρ s and other dimensionless parameters are investigated. Using a novel 8×8 Langmuir probe matrix, the two-dimensional structure of drift-wave turbulence has been measured inside the confinement region of the toroidal low-temperature plasma in the TJ-K torsatron [N. Krause, Rev. Sci. Instrum. 73, 3474 (2002)]. Using five different gases from hydrogen to argon, ρ s could be varied by a factor of 10. For small ion masses, the scalings of the microscopic parameters turn out to be close to the predictions from drift-wave turbulence, which lead to a gyro-Bohm scaling of the diffusivity. Including heavier ions turns the scaling to more Bohm-like. It is shown that ρ s scaling studies carried out on the diffusivity can be misleading if residual dependencies on other parameters are present and the cross phase between poloidal electric field and density fluctuations is not constant as in the present case. The measured turbulent diffusivity is consistent with typical values reported for the edge of fusion plasmas.