Power balance of negative-glow electrons

Laser-induced fluorescence and absorption spectroscopy are used to study the mixing of populations of excited He atoms due to electron collisions in the negative glow of a dc He discharge. These measurements yield the density (${\mathit{n}}_{\mathit{e}}^{\mathit{c}}$≃5×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}3}$) and temperature (0.1 eV≤${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{e}}^{\mathit{c}}$≤0.2 eV) of the low-energy (or ‘‘cold’’) electrons in the negative glow. The cold electrons are trapped in a potential-energy well. In a complementary investigation, Monte Carlo simulations are used to determine the density (${\mathit{n}}_{\mathit{e}}^{\mathit{h}}$≃${10}^9$ ${\mathrm{cm}}^{\mathrm{-}3}$) and temperature (${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{e}}^{\mathit{h}}$≃3 eV) of the high-energy (or ‘‘hot’’) electrons in the negative glow. Results from the experiments and Monte Carlo simulations are combined to study the power balance of the cold trapped electrons. The cold-electron temperature is established by a balance between cooling from recoil during elastic collisions with neutral atoms, and heating due to Coulomb collisions with hot electrons. The linear variation of hot-electron density with discharge current density causes a linear variation in cold-electron temperature with current density. Hot electrons also excite metastable atoms to higher radiating levels to produce most of the light from the negative glow.