Drift-Wave Instability

The universal instability driven by density and temperature gradients, the seed of most plasma turbulence.

The Drift Wave

A drift wave is a low-frequency electrostatic oscillation propagating in the diamagnetic drift direction, driven by the pressure gradient perpendicular to the magnetic field. Its natural frequency is the diamagnetic frequency omega-star, proportional to the perpendicular wavenumber and the density gradient. In its stable form it is a benign wave; a small phase shift between density and potential perturbations turns it unstable.

The Destabilizing Phase Shift

If electrons responded instantly along field lines, density and potential would stay in phase and the wave would neither grow nor decay. Any mechanism that delays the electron response, such as collisions, electron Landau damping, or magnetic trapping, introduces a phase lag. That lag lets the E-cross-B convection transport plasma down the gradient in step with the wave, feeding energy from the gradient into the fluctuation.

Family of Modes

Drift waves are the parent class of the microinstabilities that dominate core transport: the ion-temperature-gradient mode, the trapped-electron mode, and the electron-temperature-gradient mode are all drift-type instabilities driven by different gradients and resonances. Because some gradient is always present, drift waves are called universal instabilities, and they set the anomalous transport that exceeds neoclassical predictions.

Relevance

Drift-wave turbulence, regulated by self-generated zonal flows, sets the turbulent diffusivity and thus the energy confinement time. Suppressing it, for example through sheared flow or reversed magnetic shear, produces transport barriers. For the Hyperion breeder concept, drift-wave stability and turbulent transport are evaluated with gyrokinetics at the design stage; the machine is a simulation study.