Cross-separatrix simulations of turbulent transport in the field-reversed configuration

Abstract

Recent local simulations of the field-reversed configuration (FRC) have reported drift-wave stability in the core and instability in the scrape-off layer (SOL). However, experimental measurements indicate the existence of fluctuations in both FRC core and SOL, with much lower amplitude fluctuations measured in the core. With the updated cross-separatrix capabilities of the simulation code used in this paper, nonlinear turbulence simulations find that linear instabilities grow in the SOL, generating fluctuations which spread from SOL to core. After saturation of the linear instabilities, a balance of the inward spread and local damping in the core is achieved. The steady state toroidal wavenumber spectrum shows lower amplitude core fluctuations and larger SOL fluctuations with amplitude decreasing towards shorter wavelengths, which are consistent with experimental measurements.

Effects of zonal flows on ion temperature gradient instability in the scrape-off layer of a field-reversed configuration

“…Consistent with these experimental observations, local linear gyrokinetic simulations using the gyrokinetic toroidal code (GTC) [8] find that ion temperature gradient (ITG) mode can be unstable in the SOL with a critical pressure gradient comparable to the experimentally measured threshold, but the ion-scale ITG mode is mostly stable in the core [9][10][11]. Subsequent global nonlinear simulations using the ANC code [12] find that linear ITG instability first grows in the SOL, and then the turbulence spreads from the SOL to the core, resulting in a steady state spectrum characterized by lower amplitude core fluctuations and larger SOL fluctuations consistent with experimental measurements [13]. Finally, nonlinear simulations using the GTC-X code [14] find that equilibrium E × B flow shear can reduce the ITG instability growth rate, saturation amplitude, and ion heat transport in the SOL by reducing both the turbulence intensity and eddy size [15].…”

“…Previous studies show that the ion-scale turbulence is mainly driven by the ITG instability in the FRC SOL, which nonlinearly spreads to the core region [12,13]. For simplicity, the current simulations of the effects of zonal flows on the long wavelength ITG instability are carried out only in the SOL.…”

Simulation of equilibrium and transport in advanced FRCS

“…The toroidal wavenumbers are normalized such that wavelengths are 'short' (wavenumbers are 'large') relative to the local ion sound gyroradius, defined as ρ s ≡ √ T e m i eB . While the linear properties of the instabilities may differ from the C2/C2U simulations [20][21][22], the overall evolution of fluctuations, depicted in figure 3, remains similar. Consistent with previous simulations, the FRC core is found to be inherently stable while short toroidal wavelength (k ζ ρ se ∼ 20.15) instabilities grow in the SOL.…”

“…Perpendicular turbulent transport is modeled using the 3D PIC codes ANC [17,18], GTC [19] and GTC-X [20]. In previous simulations [20][21][22], a gyrokinetic particle push was used, necessitating the removal of magnetic null regions from the simulation domain. Nevertheless, nonlinear simulations find qualitative agreement in fluctuation spectrum with experimental Doppler back scattering (DBS) measurements [23].…”

“…Using this new particle model, linear and nonlinear results are found to be consistent with the previous gyrokinetic simulations. The new turbulence simulations also include kinetic electron effects which were neglected in the global gyrokinetic simulations [20,22] but kept in local simulations [21].…”