An optical set-up for spectroscopic measurements able to resolve spatially the volume inside a graphite hollow cathode (7 mm internal bore) of a discharge run in argon in the glow regime, allowed the calculation of the radial distribution of the population densities of the 5d₁2/⁰, 6d₇2/⁰ states (jl-coupling) of Ar I. These upper states of Ar I are found to be in equilibrium inside an axially homogeneous plasma column filling the cathode and having a diameter of 2 mm. This plasma column acts as a reservoir and the discharge extinguishes when the column formation is prevented. The equilibrium inside the column can be understood as a partial LTE with cool ultimate electrons obtained from the fast electrons emitted by the cathode wall through a process of energy degradation.