Gradient drift coupling instability in plasmas as an artifact

Instabilities in plasmas can occur in a variety of settings where a gradient is present for properties such as temperature or density, and can evolve into a variety of phenomena, including turbulence. In Physics of Plasmas, investigators identify the gradient drift coupling (GDC) instability in current plasma literature as a spurious artifact instead of a real physical effect. The discovery is important for all studies of magnetically confined plasmas.

In the report, the authors present a gyrokinetic linear stability analysis for a simple collisionless slab geometry for a plasma and derive a general dispersion relation for the GDC mode. They show that the artifact arises from inconsistent assumptions applied during the linearization process for the stability analysis, and from terms in the linearized stability equations being erroneously dropped. This error was found in earlier work when previous investigators did not apply the total pressure balance condition correctly.

Besides the negative result of the GDC instability being a spurious artifact, the investigators also found small scale non-magnetohydrodynamic (MHD) instabilities, called “universal instabilities.” Universal modes require the detailed gyrokinetic model to correctly predict plasma dynamics at length scales comparable to the Larmor radius associated with cyclotron motion. They point out that these modes can be expected to exist in virtually any magnetized plasma system found in space or the laboratory because they only require a particular combination of plasma inhomogeneities in the ion temperatures, electron temperatures and density to become unstable.

Future research will explore nonlinear behaviors, such as turbulence, that these newly-discovered non-MHD instabilities are expected to generate.

Source: “Gyrokinetic theory of slab universal modes and the non-existence of the gradient drift coupling (GDC) instability,” by Barrett N. Rogers, Ben Zhu, and Manaure Francisquez, Physics of Plasmas (2018). The article can be accessed at https://doi.org/10.1063/1.5024748 .