That slot in the graphite rod (about 1/4 in dia) gives an emissivity much higher than the surface of the rod. You just put 180 to 200 amps through it to make it hot.
I forgot to turn on the argon flushing supply once and the source box showed HCN spectra (hot hydrogen, nitrogen or carbon always manages to find the other two to make HCN, and sticks to the box walls practically forever, and you never get completeley rid of it. There presumeably wasn't any carbon in what eventually proved to be the HCN laser.)
Well, I figured I'd just pump out the HCN poluted air, then turn on the argon flow. That's when the fun started. The saphire window immediately turned black. The carbon evaporated. Oh well, just dust off the lampblack and put in a new rod and start over. It took me two days polishing with Linde Alumina A (fine sapphire powder) to get rid of the thin sheet black diamond I'd made on my saphire window. [You don't just toss single crystal saphire windows 2 in dia. and a 1/2 inch thick in the trash can and put in a new one.]