Power balance of negative-glow electrons.

Abstract

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 (${\mathit{n}}_{\mathit{e}}^{\mathit{c}}$\ensuremath{\simeq}5\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) and temperature (0.1 eV\ensuremath{\le}${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{e}}^{\mathit{c}}$\ensuremath{\le}0.2 eV) of the low-energy (or ``cold'') electrons in the negative glow. The cold electrons are trapped in a potential-energy well. In a complementary investigation, Monte Carlo simulations are used to determine the density (${\mathit{n}}_{\mathit{e}}^{\mathit{h}}$\ensuremath{\simeq}${10}^{9}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) and temperature (${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{e}}^{\mathit{h}}$\ensuremath{\simeq}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.