Full-f gyrokinetic simulation of turbulence in a helical open-field-line plasma

Abstract

Curvature-driven turbulence in a helical open-field-line plasma is investigated using electrostatic fivedimensional gyrokinetic continuum simulations in an all-bad-curvature helical-slab geometry. Parameters for a National Spherical Torus Experiment scrape-off-layer plasma are used in the model. The formation and convective radial transport of plasma blobs is observed, and it is shown that the radial particle-transport levels are several times higher than diffusive Bohm-transport estimates. By reducing the strength of the poloidal magnetic field, the profile of the heat flux to the divertor plate is observed to broaden.

Electromagnetic full- gyrokinetics in the tokamak edge with discontinuous Galerkin methods

“…A floor of one-tenth the peak particle source rate is used near the midplane to prevent regions of n n 0 from developing at large x. Unlike in [25] we do not use numerical heating to keep f > 0 despite the fact that our DG algorithm does not guarantee positivity. While the simulations appear to be robust to negative f in some isolated regions, lowering the source floor in the SOL region leads to simulation failures due to positivity issues at large x.…”

“…We have also run a corresponding electrostatic simulation in this configuration for direct comparison. This simulation is identical in configuration to the L z = 8 m case from [25] except for the increased particle source rate and lack of cross-species collisions. In Figure 9 we show a comparison of radial profiles of density, temperature, and beta for the electromagnetic (EM) and electrostatic (ES) cases.…”

Gyrokinetic continuum simulations of plasma turbulence in the Texas Helimak

“…Therefore, we chose initial conditions with a narrow density gradient scale length and a non-zero flow velocity Scans of the ion-to-electron mass ratio were performed in previous helical open-field-line simulations with Gkeyll and resulted in no significant changes to turbulence statistics. 30,31 The difference between the true and reduced mass ratio for argon is much larger in the case of the Helimak. The large parallel ion transit time requires longer simulation times to reach saturation and is computationally expensive.…”

“…The unit tangent vectors in the figure are defined aŝ e y = ∂R/∂y/|∂R/∂y| andê z = ∂R/∂z/|∂R/∂z|. Conducting-sheath boundary conditions [29][30][31] are used in the z direction, where field-lines intersect conducting plates, for the distribution function f . These permit local parallel current fluctuations into and out of the wall.…”

Conservative discontinuous Galerkin scheme of a gyro-averaged Dougherty collision operator

“…We now present a benchmark test of the full 5D (long-wavelength) gyrokinetic system, given by (3)- (6). With the moment-conserving GkLBO, we simulated plasma turbulence on helical, open field lines, using a nonorthogonal fieldline-following coordinate system as in [44,45]. In this coordinate system, z is parallel to magnetic field lines, x is the radial coordinate, and y is the "bi-normal" coordinate.…”