Langmuir probe is an electrode diagnostic inserted into the edge of a plasma, inferring local density, temperature, and electric potential from the current it draws as its bias voltage is swept.
The probe is named for Irving Langmuir, who developed it in the 1920s while studying ionized gases at General Electric — the same investigations in which he coined the word 📝plasma itself, making the probe the founding instrument of plasma physics. Its operating principle is direct: a small conducting tip is biased through a range of voltages, and the shape of the resulting current-voltage curve encodes the 📝electron temperature, the plasma density, and the local potential of the surrounding plasma.
That directness is also its defining limitation, and the distinction against every remote diagnostic from 📝Thomson scattering to 📝interferometry: a Langmuir probe must physically touch the plasma it measures. No material electrode survives contact with a 100-million-degree core, so probes are confined to the cooler boundary — the 📝scrape-off layer, and the target plates of the 📝divertor, where arrays of probes embedded flush in the armor measure the heat and particle fluxes arriving at the surface. In that role the oldest diagnostic in the field remains the workhorse of edge physics, characterizing the thin boundary plasma that sets how a machine handles its exhaust.