Collisionality dependence of the quasilinear particle flux due to microinstabilities

Abstract

The collisionality dependence of the quasilinear particle flux due to the ion temperature gradient (ITG) and trapped electron mode (TEM) instabilities is studied, by including electron collisions modeled by a pitch-angle scattering collision operator in the gyrokinetic equation. The inward transport due to ITG-modes is mainly caused by magnetic curvature and thermodiffusion and can be reversed as electron collisions are introduced, if the plasma is far from marginal stability. However, if the plasma is close to marginal stability, collisions may even enhance the inward transport. The sign and the magnitude of the transport are sensitive to the form of the collision operator, to the magnetic drift normalized to the real frequency of the mode, and to the density and temperature scale lengths. These analytical results are in agreement with previously published gyrokinetic simulations. Unlike the ITG-driven flux, the TEM-driven flux is expected to be outwards for conditions far from marginal stability and inwards otherwise.

Impurity transport driven by ion temperature gradient turbulence in tokamak plasmas

“…For the GA standard case, this expression gives a/L nzc = 0.58 for Z = 6 and a/L nzc = 0.67 for Z = 10, which are in excellent agreement with the values that are obtained by keeping all the terms in the expression for the impurity flux (here we usedγ = 1.2 and ω n 0 = 0.2). Equation (21) shows that for higher impurity temperature gradient or higher k θ ρ s the zero-flux impurity density gradient is lower, a trend which is in good agreement with our numerical results.…”

“…The effect of collisions on the impurity flux only enters through the eigenfrequency, which means that the impurity flux is only very weakly dependent on collisionality. This is in contrast to the very sensitive dependence of the electron particle flux on collisionality, for which in general a sign change from inward to outward is expected at very small collisionalities [1,2,20,21,25].…”

“…The solution in (11) reproduces the numerical solution of the gyrokinetic equation more accurately than the WKB-solution used in [20,21], especially in the case of complex electrostatic potentials. However, comparing the resulting perturbed electron density response (12) with the corresponding expression in Eq.…”

“…Following the approach of Ref. [20,21], we use a model electrostatic potential motivated by a variational analysis and gyrokinetic simulations, and present analytical expressions for the perturbed density of the electrons, ions and impurities. These are used in the quasi-neutrality equation that is solved numerically to obtain the frequencies and growth rates of the unstable modes, including the effect of impurities on these modes, and the quasilinear impurity particle fluxes.…”

Collisional model of quasilinear transport driven by toroidal electrostatic ion temperature gradient modes

“…[6][7][8][9][10] On the theoretical side, it has been shown that the transport fluxes are dependent on the choice of the collision operator. 11 Numerical simulations of ion temperature gradient ͑ITG͒ and trapped electron ͑TE͒ modes have shown that collisions may influence the sign and the magnitude of the quasilinear fluxes driven by these instabilities. 12 Without collisions, the quasilinear particle flux driven by ITG modes is usually inward due to curvature and thermodiffusion.…”