How nixie tube works?
When a high voltage is applied between the wire-mesh anode and one of the cathodes, the gas inside the nixie tubes becomes ionized making a sort of "soup" of plasma. Exactly like shown on the GIF on the left hand side. Much of the light that glows from a nixie tube is produced by collisions between gas atoms, ions, and electrons, just like in a neon tube. When they collide with atoms, they produce photons of visible light—giving that warm glow just beyond the cathode.