I don't think anyone has ever performed a test with mercury. A heated mercury bath creating a heavy, saturated mercury vapor in helium gas, could be easily excited with radio-frequency for a 'glow discharge,' thereby simulating some of the aspects of a gasseous or vapor core reactor. However, the problem there is in the neglecting of the ionizing aspects of fairly intense neutron and gamma radiation, which would no doubt transfer a significant amount of energy to the walls as well. This is one reason why I am a bit skeptical of the quartz 'crystal sphere' lightbulb reactor--no one has really looked at neutron irradiation to the barrier. Neutron irradiation would dislocate and thus disturb the optical clarity of the fused SiO2 sphere, causing bare silicon inclusions to form, which would definately become opaque to UV radiation from the fissioning plasma.

Further, total reliance upon centrifigal forces of spinning gasses to keep the fuel (uranium gas, or uranium-flouride gas) seperated from the hydrogen working fluid is a tall order. In order to improve the efficiency of the centrifuge, then multiple stages are needed. This drastically increases the complexity of such a machine, especially if it is supposed to be for propulsive purposes.

This is why I tend to think that a less complex single centrifuge system using a closed cycle with recirculating fuel-gasses running an MHD generator to generate lots of electricity is the way to go. Of course, this means that the reactor probably cannot be an efficient nuclear thermal rocket, however it could be a fairly efficient nuclear-electric rocket.