L-mode is the baseline low-confinement regime of tokamak operation — the state a magnetically confined plasma enters by default once external heating is applied, and the benchmark against which all higher-performance regimes are measured.
The name is retrospective. "L" stands for low confinement, a label that only became necessary after experiments on the ASDEX tokamak in Germany discovered a high-confinement regime, 📝H-mode, in 1982; everything below that threshold became L-mode. The regime's defining trait is turbulence-dominated transport: heat and particles leak from an L-mode 📝plasma far faster than collisions alone can explain, and its energy confinement degrades as heating power rises — a scaling that would force any reactor confined this poorly to be enormous.
L-mode is more than a waypoint. Every 📝Tokamak discharge starts up and shuts down in it, its confinement is captured by empirical scaling laws of its own, and it is free of the bursty edge instabilities that accompany high-confinement operation — which keeps L-mode scenarios in play wherever those bursts are unacceptable.