Propagation of a Light-Emitting Wave-Front in a Fine Tube Positive Column Discharge

The propagation velocity of a light-emitting wave-front is observed to be up~2×10+5 m/s before Townsend breakdown and up~5×10+6 m/s after Townsend breakdown along a discharge tube of inner diameter ro~1.2 mm and length of 900 mm relevant to liquid crystal display television backlighting. Before Townsend breakdown, the origin of this wave is the ambipolar diffusion of plasma flux with the propagation speed up∝Da/ro for the plasma bounded by the radius ro with the diffusion coefficient Da along the positive column. After Townsend breakdown, the light-emitting wave-front propagates with the electron plasma wave generated by the pulses of driving voltage. The electron plasma wave propagates such a long distance along the tube without damping due to the effect of localized plasma generation by electron impact ionization collisions. The propagation velocity is described by up~2ue2/ud, which is larger than the electron thermal velocity ue as well as the electron drift velocity ud.