I know typically the hot cathode of an electron gun or vacuum tube is heated either directly or indirectly by electric resistance heating. What if the tungsten (cathode) were heated by another means, say focused sunlight? Electrons would still boil off through thermionic emission. If a small accelerating current where applied to motivate the free electrons to the anode would and the anode were connected in a closed loop circuit across a load could useful power be generated? Look forward to your input.