As the current global energy economy focuses on alternatives to fossil fuels, there is increasing interest in nuclear fusion, the power source of the sun and other stars, as an attractive possibility for meeting the world’s growing energy needs. Properly understanding turbulent transport losses, which demands the application of computational resources at the extreme scale, is of the utmost importance for the design and operation of future fusion devices, such as the multi-billion dollar international burning plasma experiment known as ITER – a top priority investment in the Department of Energy’s Office of Science. This Early Science project will achieve significantly improved understanding of the influence of plasma size on confinement properties in advanced tokamak systems such as ITER. This will demand a systematic analysis of the underlying nonlinear turbulence characteristics in magnetically confined tokamak plasmas that span the range from current scale experiments, which exhibit an unfavorable “Bohm-like” scaling with plasma size to the ITER scale plasma that is expected to exhibit a more favorable “gyro-Bohm” scaling of confinement. The “scientific discovery” aspect of such studies is that while the simulation results can be validated against present-day tokamaks, there are no existing devices today that are even one-third of the radial dimension of ITER. Accordingly, the role of high physics fidelity predictive simulations takes on an even more important role—especially since the expected improvement in confinement for ITER-sized devices cannot be experimentally validated until after it is constructed and operational. In dealing with this challenge, researchers will deploy GTC-P and GTS, which are highly scalable particle-in-cell gyrokinetic codes used for simulating microturbulence-driven transport in tokamaks.