We report here the use of measurements of the time dependence and intensity of emission of rare-gas spectral lines in the negative glow as a cathode fall diagnostic in fluorescent lamps. For argon buffer gas, the relevant energy levels and spectral lines are shown in Fig 1. The 420 nm violet line is emitted in a transition between the 5p state at 14.50 ev and the metastable state at 11.55 ev. The 811.5 nm infra-red line is emitted in a transition between the 4p state at 13.08 ev and the metastable state at 11.55 ev. We have discovered that under conditions in the negative glow of a fluorescent lamp the 5p state at 14.50 ev is primarily excited step-wise from the metastable state. Primary electrons accelerated through the cathode sheath are responsible for excitation of the metastable state, while the plasma electrons of the negative glow have ample energy to excite the 5p state from the metastable. Thus, the time-resolved onset of emission of 420 nm radiation from the negative glow immediately in front of the cathode spot signals the phase at which cathode fall exceeds 11.55 volts. There is under certain conditions a second onset threshold marking the direct excitation of the 5p state from the ground state, signaling the phase angle at which cathode fall exceeds 14.50 volts.
展开▼
