Spontaneous rotation sources in a quiescent tokamak edge plasma

Abstract

Spontaneous rotation sources in a quiescent tokamak edge plasma are studied without an external momentum source, such as, beam injected or wall-born neutrals. Discussions are based upon example neoclassical solutions from an edge gyrokinetic particle code. The main study is performed in a DIII-D plasma [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] with the ion Grad-B drift directed toward the single-null divertor. Comparison with a reversed Grad-B drift case is also shown. It is found that there is a robust spontaneous co-current toroidal plasma rotation source in the far scrape-off plasma due to the wall sheath effect. As the edge pedestal width becomes narrower, the co-current rotation in the far scrape-off becomes weaker, but there appears a stronger co-current rotation in the pedestal top/shoulder from the X-point orbit loss effect, possibly providing a co-rotation boundary condition to the core plasma. Reversal of the magnetic field and plasma current brings down the overall co-rotation, especially in the far scrape-off plasma.

Neoclassical physics in full distribution function gyrokinetics

“…III and IV complement the existing studies of neoclassical physics in the plasma edge [64][65][66] whilst providing a successful cross-comparison between this Monte-Carlo scheme and the previously detailed Eulerian-based operators implemented in GYSELA.…”

Compressed ion temperature gradient turbulence in diverted tokamak edge

“…The ambipolar radial electric field is established quickly within 1 it ͑=0.078 ms͒ from the start of simulation due to the strong neoclassical polarization effect in quasineutral plasma under radial orbital excursions of ions. 3 Once the quasisteady E r is established, E r and, thus, the E ϫ B mean flow in XGC1 does not decay without the onset of ITG turbulence. 3 What Fig.…”

“…3 Once the quasisteady E r is established, E r and, thus, the E ϫ B mean flow in XGC1 does not decay without the onset of ITG turbulence. 3 What Fig. 11 shows is a rapid transient neoclassical equilibration process from zero to about 1.5 it , followed by a neoclassical equilibrium between about 1.5-3 it which would be quasisteady with the turbulence, and another decay after 3 it with the half decay rate of about 50% of the collision rate.…”

“…3,4,10,11 In the present study, we use full-f marker particles for neoclassical/turbulent ions and neoclassical electrons. Adiabatic electrons are used for the ITG turbulence response.…”

“…3 The parallel Debye sheath, which is a small scale subgrid phenomenon and not resolved in a gyrokinetic simulation, is forced to be included in the large scale presheath for quasineutrality and ambipolarity. 3 Detailed information on obtaining the edge equilibrium solution can be found in Ref. 3.…”