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 (n(e)c approximately equal-to 5 x 10(11) cm-3) and temperature (0.1 eV less-than-or-equal-to k(B) T(e)c less-than-or-equal-to 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 (n(e)h approximately equal-to 10(9) cm-3) and temperature (k(B) T(e)h approximately equal-to 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.