In the characteristics shown, the normal glow mode is from F to G, the voltage is stable with increasing current. I understand the explanation of how the electrode current density stays constant(the plasma contact area with electrode increases as the current increases). However, I don't understand how the constant electrode current density is keeping the voltage drop constant. What is the relationship between the current density and the voltage drop?
in that range of current, adding more current "lights up" more electrode area, and voltage stays constant. subtracting current in this zone causes parts of the glow discharge to be extinguished, reducing the active area of the electrode. voltage stays constant during that process too.