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GX is a code for solving the nonlinear gyrokinetic system for low-frequencyturbulence in magnetized plasmas, particularly tokamaks and stellarators. InGX, our primary motivation and target is a fast gyrokinetic solver that can beused for fusion reactor design and optimization along with wide-ranging physicsexploration. This has led to several code and algorithm design decisions,specifically chosen to prioritize time to solution. First, we have used adiscretization algorithm that is pseudo-spectral in the entire phase-space,including a Laguerre-Hermite pseudo-spectral formulation of velocity space,which allows for smooth interpolation between coarse gyrofluid-like resolutionsand finer conventional gyrokinetic resolutions and efficient evaluation of amodel collision operator. Additionally, we have built GX to natively targetgraphics processors (GPUs), which are among the fastest computational platformsavailable today. Finally, we have taken advantage of the reactor-relevant limitof small $\rho_*$ by using the radially-local flux-tube approach. In this paperwe present details about the gyrokinetic system and the numerical algorithmsused in GX to solve the system. We then present several numerical benchmarksagainst established gyrokinetic codes in both tokamak and stellarator magneticgeometries to verify that GX correctly simulates gyrokinetic turbulence in thesmall $\rho_*$ limit. Moreover, we show that the convergence properties of theLaguerre-Hermite spectral velocity formulation are quite favorable fornonlinear problems of interest. Coupled with GPU acceleration, which we alsoinvestigate with scaling studies, this enables GX to be able to produce usefulturbulence simulations in minutes on one (or a few) GPUs. GX is open-sourcesoftware that is ready for fusion reactor design studies.

GX: a GPU-native gyrokinetic turbulence code for tokamak and stellarator design