Full-f gyrokinetic particle simulation of centrally heated global ITG turbulence from magnetic axis to edge pedestal top in a realistic tokamak geometry

Abstract

Global electrostatic ITG turbulence physics, together with background dynamics, has been simulated in realistic tokamak core geometry using XGC1, a full-function 5D gyrokinetic particle code. Adiabatic electron model has been used. Some verification exercises of XGC1 have been presented. The simulation volume extends from the magnetic axis to the pedestal top inside the magnetic separatrix. Central heating is applied, and a number, momentum, and energy conserving linearized Monte-Carlo Coulomb collision is used. In the turbulent region, the ion temperature gradient profile self-organizes globally around R/L T = (Rd log T /dr = major radius on magnetic axis/temperature gradient length) ≃ 6.5 − 7, which is somewhat above the conventional nonlinear criticality of ≃ 6. The self-organized ion temperature gradient profile is approximately stiff against variation of heat source magnitude. Results indicate that the relaxation to a self-organized state proceeds in two phases,namely a transient phase of excessively bursty transport followed by a 1/f avalanching phase. The bursty behaviors are allowed by the quasi-periodic collapse of local E × B shearing barriers.

Compressed ion temperature gradient turbulence in diverted tokamak edge

“…These core plasma turbulence behaviors are being reported elsewhere. 29 We have not seen either a clean 1 / f or bursty behavior in the density slope from the present edge simulation at 2 MW heat source. A heat source scan is planned in the future to examine dependence of edge turbulence and transport characteristics on heating power.…”

“…10. Indeed, a whole device simulation with the edge and core together shows a radially inward turbulence front propagation from the pedestal region at a speed v s s / R. 29 Again in this figure, the residual GAM oscillation is strong at both locations, but its magnitude is stronger in the slope region where q is higher. A likely explanation for the collisionless zonal flow growth in time in the density slope region is the stronger GAM activities, with the inverse cascade of GAM energy into the zonal flows in the absence of collisions.…”

Validation metrics for turbulent plasma transport

“…A variety of such codes have been developed, which can generally be divided between those taking a continuum approach [52][53][54][55][56] and those that use particle-in-cell methods. [57][58][59][60][61][62] As in neutral fluid DNS, both approaches initialize the simulation with some very small amplitude fluctuations, which first grow exponentially at the linear growth rate(s) of the instabilities being considered (the "linear" phase), and then saturate at a finite amplitude set by the balance of these linear drives and nonlinear couplings between different wavenumbers (the "saturated" phase). The statistics of various quantities (such as mean energy flux or fluctuation power) from this saturated phase are then used for transport modeling predictions, 63,64 as well as comparisons with other models and experiments in V&V studies.…”

Bootstrap current for the edge pedestal plasma in a diverted tokamak geometry

“…XGC0 is a drift-kinetic particle-in-cell code, 24 a turbulence-free version of the gyrokinetic particle code XGC1, 25,26 in which the five-dimensional (3D in position and 2D in velocity) time advance of the marker ion and electron positions is described by the well-known Lagrangian equation of motion, 27 which conserves mass, momentum, and energy:…”