Measurements and implications of Zeff profiles on the DIII-D tokamak

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Abstract
Here Zeff profiles have been obtained across the entire minor radius of the DIII‐D [Fusion Technol. 8, 441 (1985); Plasma Physics and Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. 1, p. 159] tokamak from measurements of visible bremsstrahlung continuum radiation. The Zeff profiles are presented for Ohmic divertor and limiter plasmas as well as for neutral beam heated H‐mode and L‐mode plasmas. The measured Zeff profiles are found to have small gradients over the entire plasma cross section. This implies that resistivity gradient driven electrostatic modes are unlikely to influence the confinement properties in the edge region of these discharges. The H‐mode plasma has a centrally peaked Zeff profile and a relatively flat electron density profile that suggests the existence of a negative value for the parameter ηi.