Development of a Lagrangian Gyrocenter Tracking Code (LGT1) for Application to CGYRO Calculated Turbulence*

A Lagrangian Gyrocenter Tracking code (LGT1) is developed for evaluation of the Lagrangian correlation function (L_c) and Lagrangian diffusion coefficient (D_L) of the turbulence. The saturated turbulence fields (φ, A_||), used in LGT1, are calculated in advance with the nonlinear Eulerian gyrokinetic code CGYRO[1]. The Lagrangian correlations are a crucial part in the theory of transport. However, due to the Eulerian nature of CGYRO they are not directly calculable. LGT1 provides the further calculations needed to access the Lagrangian statistics. In LGT1 an ensemble of test particles (currently ions) is launched into the evolving turbulence fields (from CGYRO). For compatibilty with CGYRO and simplifying the calculations, field-aligned coordinates are employed for tracking the gyrocentrers. The ensemble of particles is initiated with velocities taken from a Maxwellian distribution. Further, particles are launched on the outboard midplane, with initial radial and binormal locations chosen randomly from the simulation box, for maximum sampling of turbulence. Particle drift velocities are then used for the evaluation of L_c and D_L. Future developments will include adding collision models and different particle species.

[1] J. Candy, E.A. Belli, and R.V. Bravenec. A high-accuracy Eulerian gyrokinetic solver for collisional plasmas. J. Comput. Phys., 324:73, 2016