Infrared LaB6 Cathode Diagnostic for the Large Plasma Device

The Large Plasma Device (LaPD) is a 20-meter long magnetized plasma, generated by an arc from a lanthanum hexaboride (LaB6) cathode to a molybdenum mesh anode. This is a flexible tool for plasma science research. This study reports on the construction and application of an inexpensive Czerny-Turner infrared spectrometer, assembled using off-the-shelf components, primarily to measure the surface temperature of the cathode. A second goal will be to diagnose potential problems with the LaB6 cathode. The spectra of emitted wavelengths follows black body at shorter wavelengths, but various absorption mechanisms cause deviation from blackbody at wavelengths longer than about 1 micron. We hope to account for this deviation in the temperature measurement, but also identify impurities, such as carbon dust, generated from various sources in the machine and deposited on the cathode surface. Initial alignment and qualification of the spectrometer are presented. Alignment is accomplished in visible light using a 1200 line grating which is replaced with a 600 line spacing for infrared data collection. The cathode will be viewed through a Calcium Fluoride window. The goal is for accumulated spectral data to reveal evolution of the cathode and its condition over time scales of weeks up to the lifetime of the cathode (> 1 year at this point).