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 floating 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 eh , 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.