Global gyrokinetic simulations with strong flows

Abstract

We report on the investigation of strong toroidal rotation effects in a global tokamak code, ORB5. This includes the implementation of a strong flow gyrokinetic Lagrangian, allowing a complete treatment of centrifugal and Coriolis effects in the laboratory frame. In order to consistently perform the linear analysis in this system, an axisymmetric gyrokinetic equilibrium distribution function is defined using the constants of motion: we show it corresponds to the standard choice in the local limit and is close to the neoclassical solution in the banana regime. The energy and momentum transport equations are presented in an analogous form to those for the weak flow system. Linear studies of Ion Temperature Gradient (ITG) modes in rotating plasmas are performed to determine how the global effects interact with the effects of strong rotation. We also determine the geodesic acoustic mode dispersion with respect to plasma rotation rate in this gyrokinetic model and compare it to MHD theory.

Generalised zonal modes in stationary axisymmetric plasmas

“…In the limit of  M 0 the GAM root can be recognised as the beta induced accumulation point of shear Alfvén continua ( [39] and references therein). We also note that the rotationinduced upshift of the GAM frequency predicted by the fluid treatment above has recently been confirmed in kinetic studies, both analytically [11,12] as well as numerically [40].…”

Orb5: A global electromagnetic gyrokinetic code using the PIC approach in toroidal geometry

“…The effects of strong rotation on the equilibrium have been discussed earlier for Orb5 [33], so we focus on demonstrating the operation of the code in the nonlinear regime; at the moderate levels of rotation tested here the effects are not expected to be dramatic. As in non-rotating simulations, there is some overall relaxation of the heat profiles as the turbulence driven transport commences, Figs.…”

“…Implementing this ordering in Orb5 enables the treatment of plasmas rotating toroidally at close to the Mach velocity. More details of this formalism have been published earlier [33]. In that case, a further approximation is performed on the background distribution function.…”

“…Those improvements are targeting the physical models, with e.g. drift-kinetic electron dynamics, electromagnetic (EM) perturbations [26], multiple gyrokinetic ion species, inter and intraspecies collisions [27], hybrid electron model [28,29], removal of the long wavelength approximation [30], various heating sources [31,32] and strong flows [33], and the numerical side with e.g. the enhanced control variate [34,35,36], and, more recently, the mixedrepresentation "pullback" scheme [37] resolving the so-called cancellation problem for EM simulations, various noise control operators (generalized moment-conserving Krook operator [31], coarse graining [38], and quadtree [39]), and a thorough refactoring with multithreading using OpenMP and OpenACC which will be detailed in a separate publication.…”