Abstract :
Summary form only given. A time dependent model of the negative glow plasma of an electroded, low pressure, Hg/rare gas discharge is used to study the behavior of the cathode/anode fall in fluorescent lamps. The model is used with, and results compared to, measurements from a mm-wave (150 GHz) interferometer and from probes located inside specially-constructed lamps. The behavior of cathode fall is of particular interest since it is responsible for sustaining the plasma in the electrode region by 1) accelerating thermionically-emitted electrons from electrode to plasma (so-called beam electrons) to energies sufficient to maintain ionized particle density and 2) accelerating ions from plasma to electrode to energies sufficient to maintain the high electrode temperature necessary for thermionic emission. A consequence of the latter is that the ions may induce sputtering of the workfunction-lowering emitter material, a process which may limit lamp life and cause undesirable end-darkening effects. The model is represented by coupled 1-D (radially averaged) electron particle and energy balance equations and a 0-D model of the sheath. The plasma model is a time dependent generalization of previous models whereby Maxwellian plasma electrons are created through ionization and excitation of atoms by a separate mono-energetic, beam electron species (with energy corresponding to the sheath potential). Inclusion of the electron energy balance allows for describing a smooth transition to the positive column plasma. Eight excited state levels of mercury are included. The sheath model characterizes the sheath potential and the electric field at the electrode surface as functions of the plasma parameters at an idealized plasma/sheath boundary. The surface electric field provides for the full, Schottky-corrected, thermionic emission current. The sheath model is similar to that of a standard Langmuir sheath, but includes the space-charge effects of the beam electrons. Total current,- given as the sum of beam electron, plasma electron, and ion currents, is conserved and set equal to the (specified) discharge current
Keywords :
electron beams; excited states; fluorescent lamps; glow discharges; ionisation; mercury (metal); plasma boundary layers; plasma density; plasma kinetic theory; plasma probes; plasma sheaths; plasma temperature; plasma transport processes; plasma-beam interactions; positive column; space charge waves; thermionic emission; 150 GHz; Hg; Langmuir sheath; Maxwellian plasma electrons; Schottky-corrected current; anode fall; atom excitation; atom ionization; beam electron species; cathode fall; discharge current; electrode temperature; end-darkening effects; energy balance equations; excited state levels; fluorescent lamp discharge; ionized particle density; mm-wave interferometer; negative glow plasma; particle balance equations; positive column plasma; probes; sheath potential; space-charge effects; sputtering; surface electric field; thermionic emission; thermionically-emitted electrons; Electrodes; Electron beams; Fluorescent lamps; Plasma accelerators; Plasma density; Plasma materials processing; Plasma measurements; Plasma sheaths; Plasma temperature; Thermionic emission;
