Trapped structures in drift wave turbulence

Abstract

The development of trapped structures in decaying and saturated drift wave turbulence is studied via computer simulation. A two-dimensional electrostatic fluid model is used. The turbulence that evolves in the pure decay runs (i.e., no nonadiabatic electrons) is characterized by tightly bound monopole vortices and a very narrow frequency spectrum. For the studies of saturated turbulence, a new nonadiabatic electron model is introduced, which gives the qualitatively correct response to a coherent trapped structure. This model takes into account the effects of broadening and shifting of the frequency spectrum. These effects are found to be quite important. Trapped structures are observed in many of the saturated simulations, even in the presence of moderately broad frequency spectra. The extent of the trapping varies dramatically, becoming a much stronger effect as the average electric field increases.

Drift waves and transport

“…(106) has been extended by Crotinger and Dupree (1992) and Naulin and Spatschek (1997) Signal processing techniques for directly testing for the quadratic mode coupling as in Eq. (85) have been developed (Kim and Powers, 1978).…”

Nonadiabatic electron response in the Hasegawa-Wakatani equations

“…Since iteration is not required, the predicted functional form ofb n (a) is much simpler than the results of analogous calculations that do not employ the a, b variables. 31,32 For the C 1 limit, this relation provides a one-field approximation to the HWEs, enhancing physical intuition and greatly easing the burden on any further analytical manipulations, such as statistical closures. Direct numerical simulation of the HWEs demonstrated that the approximationb n (a) indeed rapidly converges to the dynamically evolvedb for C ≤ 1 (Figs.…”

“…41 Helpfully for physical intuition, a is scale-by-scale proportional to the cold-ion limit of the ion gyrocenter density used in gyrofluid models, 15,42,43 a correspondence that follows from the fact that the polarization drift is absorbed into the coordinate transform from particle to gyrocenter position. 44 The nonadiabatic electron density is extensively used in near-adiabatic approximations, [25][26][27]31,32 as well as in other contexts. Note also that a and b are the amplitudes that result from projection onto the C → 0 limit of the drift-wave (ϕ = n) and damped (n = ∇ 2 ⊥ ϕ) eigenmodes of the HWEs, respectively.…”

“…30 Crotinger and Dupree (CD) nonlinearly extended a linear collisionless densitypotential relation by replacing the linear frequency with nonlinear time derivatives evaluated using the HasegawaMima equation, effectively broadening the frequency as well as shifting it. 31 Using a similar iterative procedure, Naulin and Spatschek (NS) obtained a one-field nearadiabatic reduction of the 2D HWEs. 32 While these reductions to one-field models represent significant simplification, the iteration involved in the more faithful models results in lengthy, complicated equations with terms that are difficult to interpret physically.…”

The effect of sheared diamagnetic flow on turbulent structures generated by the Charney–Hasegawa–Mima equation

“…This implies that when L n Ϫ1 is zero, a constant or periodic, an infinite plane can be simulated by choosing the boundary conditions periodic in both the x and y directions. In the literature, L n Ϫ1 is treated almost exclusively as a constant [16][17][18][19][20][21][22][23][24][25][26][27] or as equal to zero. 23,[28][29][30][31][32][33] Here, we treat s L n Ϫ1 Ӷ1, where s ϭc s /⍀ ci , c s being the ion sound speed and ⍀ ci the ion cyclotron frequency.…”