In DC glow discharge, there is a fixed cathode and a fixed anode. Electrons will be accelerated from cathode to anode and acquire more energy. Along the way to the anode, electrons will experience many collisions with other ions and neutrals. During the impacts, electrons will transfer their energy to bonded electrons within ions and neutrals. Once the electric field strength reaches a certain level, free-moving electrons can acquire enough energy to knock out bonded electrons from neutral particles. A process similar to avalanche breakdown will happen in the gas chamber. Electron and ion density will be multiplied and eventually the whole space will be filled with positive, negative ions and electrons. Sometimes a heated filament is used as cathode electrode to emit constant flow of free electrons to help to ignite the plasma and improve the stability. Unfortunately, evaporated metals from heated filament can sometimes contaminate the sample. There is an optimum voltage range between cathode and anode to generate plasma. If the voltage is too low, free electrons won't have enough energy to ionize the neutrals. If the voltage is too high, electrons will move too fast to avoid any collisions with neutrals. The optimum voltage range is a function of gas species and pressure.