Electron energy distributions in the low-pressure mercury-vapour discharge: the Langmuir paradox

Abstract

Electron energy distributions have been measured in mercury-vapour discharges by means of the Langmuir-Druyvesteyn probe method. These measurements show that the discharge can be conveniently divided into two contrasting regions-the first, very near the cathode, in which strong beam-plasma interaction occurs, resulting in the excitation of strong plasma oscillations and the scattering of the electron beam from the cathode to produce a uniform monotonically decreasing distribution function; the second region, or positive column, occupies almost the whole length of the discharge tube and in this region the electron energy distribution is not Maxwellian, there being no energy-exchanging process operative in the column, in the wall sheath or in the probe sheath. Non-uniformities in the energy distribution originating near the cathode persist along the 170 cm length of the column. In a part of this column a simple Langmuir probe curve (In i - V ) has been obtained which is superior to any which has previously been published as evidence for the existence of a Maxwellian distribution. However, the more sensitive Druyvesteyn plot of d 2 i /d V 2 against V shows that even this distribution is non-Maxwellian. It must, therefore, be concluded that the Langmuir probe method has never satisfactorily established the existence of a Maxwellian distribution in a low current discharge.