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Abstract:
In ASDEX Upgrade and JET, the ion temperature profiles can be described by R/LTi which exhibits only little variations, both locally, when comparing different discharges, and radially over a wide range of the poloidal cross-section. Considering a change of the local ion heat flux of more than a factor of two, this behaviour indicates some degree of profile stiffness. In JET, covering a large ion temperature range from 1 to 25 keV, the normalized ion temperature gradient, R/LTi, shows a dependence on the electron to ion temperature ratio or toroidal rotational shear. In particular, in hot ion plasmas, produced predominantly by neutral beam heating at low densities, in which large Ti/Te is coupled to strong toroidal rotation, the effect of the two quantities cannot be distinguished. Both in ASDEX Upgrade and JET, plasmas with internal transport barriers (ITBs), including the PEP mode in JET, are characterized by a significant increase of R/LTi above the value of L- and H-mode plasmas. In agreement with previous ASDEX Upgrade results, no increase of the ion heat transport in reversed magnetic shear ITB plasmas is found in JET when raising the electron heating. Evidence is presented that magnetic shear directly influences R/LTi, namely decreasing the ion heat transport when going from weakly positive to negative magnetic shear.
