A PROBLEM IN THE INTERPRETATION OF TOKAMAK LANGMUIR PROBES WHEN A FAST-ELECTRON COMPONENT IS PRESENT

in the edge plasma of magnetically confined devices such as tokamaks, the electron distribution is not necessarily Maxwellian, and may consist of a small number of electrons characterized by a hot temperature, T-eh, With the bulk of the electrons at a colder temperature, T-ec. In a strong magnetic field, there is no satisfactory theory for interpreting the current-voltage characteristic above Boating potential, and so usually only the high-energy electrons in the distribution are used to deduce T-e. It is shown that the value of T-e extracted from the Langmuir probe characteristic in this way yields T-en, unless the hot component is extremely small. Langmuir probes therefore tend to overestimate T-e in magnetically confined devices. This effect may also be relevant to observations of (apparently) very low sheath heat transmission coefficients in tokamaks.