Which phenomenon limits the x-ray tube current by crowding electrons near the cathode?

• A. Space-charge effect
• B. Bremsstrahlung
• C. Anode heel effect
• D. Photon attenuation

Answer: (A)

The key idea is the space-charge effect: when electrons are emitted from the heated cathode, they form a dense cloud of negative charge near the cathode. This space charge repels additional electrons trying to leave the cathode, so the net current cannot rise indefinitely even if the emission rate is high. The anode voltage helps pull electrons away, but at lower voltages the repulsive field from the electron cloud dominates and caps the current—the tube is space-charge limited. Bremsstrahlung, the radiation produced when electrons brake near nuclei, isn’t about limiting current. The anode heel effect describes how x-ray intensity varies across the beam due to the angle of the anode, not how many electrons reach the anode. Photon attenuation concerns how photons are absorbed in matter, not electron flow.