Modern Langmuir probe diagnostics

During the last decades, it has become evident that many fundamental concepts of low-temperature plasma physics must be revised considering its highly non-equilibrium nature. Most definitions of many plasma parameters and their diagnostics are based on the assumption of a Maxwellian Electron Distribution Function (EEDF. However, it is known for a long time that the assumption of Maxwellian EEDF is not the case for typical conditions in gas discharges and space plasmas.  For non-Maxwellian EEDF, the electron temperature T_e is losing its usual meaning as a parameter of electron distribution but usually implies a mean electron energy T_eff. The key plasma parameters and rates of electron-induced chemical reactions calculated with T_e found from a narrow portion of the real EEDF (with probes or spectroscopy) may strongly disagree with those calculated with realistic EEDF. It is known that T_eff, plasma conductivity, Debye length_, and the ion sound speed are mainly defined by the low-energy part of the EEDF, while inelastic collision rates and floating wall potential (responsible for electron energy loss to the wall) are extremely sensitive to the EEDF shape at high electron energies. The errors in the classical probe diagnostics, based on electron or/and ion probe currents, due to non-Maxwellian EEDF and others non-accounted in classical probe theories effects are discussed in this presentation.  It is shown that measurement of EEDF according to Druyvestein procedure is the only reliable probe technique for diagnostics of low-temperature plasma. The rest of the presentation is devoted to errors in EEDF measurement, their origination, and ways of their mitigation. Examples of professionally executed EEDF measurement performed by different authors in laboratory setups and in plasma processing reactors demonstrate practical feasibility for accurate EEDF measurement in a wide range of plasma devices, providing the fundamental requirement for classical probe diagnostics are satisfied.