I-mode impurity removal and energy confinement*

Using high-z wall materials attempts to switch the fusion challenge from heat load handling to removing impurities. We propose a means of measuring the radial impurity flux from currently available diagnostics, providing insight on optimal tokamak operation to prevent impurity accumulation [PoP 24, 055904 (2017)]. Our description is a modification of Per Helander's high Z impurity treatment [PoP 5, 3999 (1998)]. It uses poloidal impurity flow measurements rather than a main ion kinetic calculation of screening effects. High confinement mode operation was discovered 35 years ago to almost quadruple fusion power, and later explained by turbulence reduction by sheared flows. Less than a decade ago, improved mode operation was discovered to have the same desirable property, while removing impurities and providing fueling. Thanks to the impurity radial particle flux measuring technique developed, we explain the outward radial impurity flux without invoking a (sometimes undetected) turbulent mode [PPCF 60, 094001 (2018)]. This theory is supported by the observed ExB flow shear, which also explains the desired energy confinement via turbulence reduction.