SNR comparison experiments of DC heated LaB₆ rod and tungsten filament cathodes diagnosed by laser-induced fluorescence

Multi-dipole confined hot cathode discharges are extensively used in low temperature plasma studies to investigate sheath-presheath formation, develop plasma diagnostics and study plasma-wave interactions, etc. Typically, high-energy primary electrons in multi-dipole devices are generated by direct hot cathode discharge from very thin (d< 0.3 mm) tungsten filaments, which are readily heated via a DC current to the temperature emitted by electrons to produce plasma, but is very luminous such that the filaments themselves may become a sizable source of noise in laser induced fluorescence (LIF) measurements [1]. Recently, tungsten filaments are increasingly being replaced by less luminous alternatives, such as barium oxide and lanthanum hexaboride (LaB₆) cathodes. These materials can emit electrons at temperatures close to 1000 K lower than the tungsten material, so the background light emitted will be greatly reduced, which involves an important improvement in active spectral diagnostic studies such as laser-induced fluorescence, i.e. the decrease in background black body radiation under the same discharge parameters will be conducive to the improvement of the signal-to-noise ratio of effective fluorescent signals. However, it is more difficult to heat these cathodes via a DC current, complicating their associated cathode assembly designs. In this presentation, we present a simple design to directly heat a LaB6 cathode manufactured at suitable dimensions, and performed comparison of LIF signal-to-noise ratio of this LaB6 hot cathode discharge with that of a typical tungsten filament discharge.