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

Abstract

The edge bootstrap current plays a critical role in the equilibrium and stability of the steep edge pedestal plasma. The pedestal plasma has an unconventional and difficult neoclassical property, as compared with the core plasma. It has a narrow passing particle region in velocity space that can be easily modified or destroyed by Coulomb collisions. At the same time, the edge pedestal plasma has steep pressure and electrostatic potential gradients whose scale-lengths are comparable with the ion banana width, and includes a magnetic separatrix surface, across which the topological properties of the magnetic field and particle orbits change abruptly. A driftkinetic particle code XGC0, equipped with a mass-momentum-energy conserving collision operator, is used to study the edge bootstrap current in a realistic diverted magnetic field geometry with a self-consistent radial electric field. When the edge electrons are in the weakly collisional banana regime, surprisingly, the present kinetic simulation confirms that the existing analytic expressions [represented by O. Sauter et al., Phys. Plasmas 6, 2834] are still valid in this unconventional region, except in a thin radial layer in contact with the magnetic separatrix. The agreement arises from the dominance of the electron contribution to the bootstrap current compared with ion contribution and from a reasonable separation of the trapped-passing dynamics without a strong collisional mixing. However, when the pedestal electrons are in plateau-collisional regime, there is significant deviation of numerical results from the existing analytic formulas, mainly due to large effective collisionality of the passing and the boundary layer trapped particles in edge region. In a conventional aspect ratio tokamak, the edge bootstrap current from kinetic simulation can be significantly less than that from the Sauter formula if the electron collisionality is high. On the other hand, when the aspect ratio is close to unity, the collisional edge bootstrap current can be significantly greater than that from the Sauter formula. Rapid toroidal rotation of the magnetic field lines at the high field side of a tight aspect-ratio tokamak is believed to be the cause of the different behavior. A new analytic fitting formula, as a simple modification to the Sauter formula, is obtained to bring the analytic expression to a better agreement with the edge kinetic simulation results. V C 2012 American Institute of Physics.

Radially globalδfcomputation of neoclassical phenomena in a tokamak pedestal

“…The benchmarking paradigm used here is important and novel in that it depends centrally on the finite orbit width, as we examine departures from zero-orbit-width (local) theory. The benchmarking procedure may be useful to other kinetic simulations of edge plasma [3][4][5][6][7][8][9][10][11][12][13][14][15].…”

“…The radial coordinate is only a parameter in a local code, whereas it is more challenging to solve a global problem due to radial coupling. Global full-f numerical calculations therefore require orders of magnitude more processor-hours [14] than local δf calculations.…”

“…For improved pedestal neoclassical calculations, some researchers [3][4][5][6][7][8][9][10][11][12][13][14][15] have considered a model that may be termed "global full-f ", in contrast to the conventional approach [1,2,[16][17][18] which may be called "local δf ." In a full-f code, the entire distribution function f is solved for, whereas in a δf code, one only solves for the departure from a Maxwellian, with the Maxwellian constant on each flux surface.…”

Global and pedestal confinement and pedestal structure in dimensionless collisionality scans of low-triangularity H-mode plasmas in JET-ILW

“…This is done to ensure that all equilibria have the same global beta. In this work, the bootstrap currents are calculated with the Koh model [77]. Figure 8(a) shows the pedestal pressure estimated with EPED1 versus the experimental p e ped .…”

“…As described in section 2.3, the profiles are shifted in order to have T e sep ≈ 100 eV. The edge bootstrap currents are calculated using the Koh model [77], which at high collisionality gives a more reliable result (i.e. closer to the results obtained with the drift kinetic code NEO [80,81]) than with the more common approach described in [82].…”