2. Application of high voltage across cathode and anode (kVp). Anode acquires positive charge and attracts the free electrons that were just boiled off the filament in the thermionic emission process. Electrons fly towards the anode. Collision with the anode causes the electrons decceleration ( slowing down). X-ray production process is very inefficient. When 50 kVp is used kinetic energy of projectile electrons is converted to 0.4 % of x-ray photons and 99.6% of heat. When 100 kVp is used kinetic energy of projectile electrons is converted to 0.7 % of x-ray photons and 99.3% of heat. When 200 kVp is used kinetic energy of projectile electrons is converted to 1.5 % of x-ray photons and 98.5% of heat.