A micro-instability driven by the resonance between drift waves and magnetically trapped electrons, contributing to both particle and electron thermal transport.
The Trapped Electron Mode (TEM) is a drift-wave micro-instability in toroidal plasmas that draws its free energy from the density gradient and, in its kinetic variant, from the electron temperature gradient. It is driven by the precessional resonance of electrons that are magnetically trapped in the low-field side of the torus by the mirror force arising from the 1/R variation of the toroidal magnetic field.1
In a tokamak, a fraction of electrons—roughly √(r/R) at minor radius r and major radius R—are confined to banana orbits and cannot complete full poloidal circuits. These trapped electrons precess toroidally at a characteristic drift frequency. When a drift wave’s frequency matches this precession frequency, energy is transferred from the equilibrium gradients to the wave, destabilizing it.2
The TEM tends to dominate transport in regimes where the density gradient is steep relative to the ion temperature gradient, or where the electron temperature gradient is strong. It is particularly relevant in plasmas heated predominantly by electron cyclotron resonance heating (ECRH), where the electron channel carries the bulk of the input power and ion temperature gradients remain below the ITG threshold.3
Nonlinear gyrokinetic simulations reveal that TEM turbulence generates both particle and electron heat transport. The associated particle flux can drive a particle pinch under certain conditions, influencing density peaking—a phenomenon with direct consequences for fusion reactivity. TEM turbulence also interacts with zonal flows, though the regulation mechanism differs quantitatively from the ITG case.4
Collisionality plays a critical role: increasing electron collisionality de-traps electrons before they can complete their precessional resonance, damping the TEM. This collisional stabilization explains why the TEM is more prominent in low-collisionality, reactor-relevant regimes. Experimental identification relies on fluctuation measurements showing propagation in the electron diamagnetic direction, combined with parameter scans that track the predicted collisionality dependence.