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To faithfully simulate ITER and other modern fusion devices, one must resolveelectron and ion fluctuation scales in a five-dimensional phase space and time.Simultaneously, one must account for the interaction of this turbulence withthe slow evolution of the large-scale plasma profiles. Because of the enormousrange of scales involved and the high dimensionality of the problem, resolvedfirst-principles global simulations are very challenging using conventional(brute force) techniques. In this thesis, the problem of resolving turbulenceis addressed by developing velocity space resolution diagnostics and anadaptive collisionality that allow for the confident simulation of velocityspace dynamics using the approximate minimal necessary dissipation. With regardto the wide range of scales, a new approach has been developed in whichturbulence calculations from multiple gyrokinetic flux tube simulations arecoupled together using transport equations to obtain self-consistent,steady-state background profiles and corresponding turbulent fluxes andheating. This approach is embodied in a new code, Trinity, which is capable ofevolving equilibrium profiles for multiple species, including electromagneticeffects and realistic magnetic geometry, at a fraction of the cost ofconventional global simulations. Furthermore, an advanced model physicalcollision operator for gyrokinetics has been derived and implemented, allowingfor the study of collisional turbulent heating, which has not been extensivelystudied. To demonstrate the utility of the coupled flux tube approach,preliminary results from Trinity simulations of the core of an ITER plasma arepresented.

Trinity: A Unified Treatment of Turbulence, Transport, and Heating in Magnetized Plasmas