The cathode loading limit in circular beam electron devices

On the assumption that an electron gun of rotational symmetry is operating at its theoretical Langmuir limit, the work of Haine and Schwartz is extended and combined to develop a semi-universal curve which defines its maximum usable cathode loading before the Coulomb forces operate to degrade substantially the spot. This curve shows that quite low cathode loadings (order of 0.1 amp/cm2) are sufficient to cause severe beam focus degradation, due to space charge, for the spot sizes common in normal cathode ray tubes (order 1 mm diameter). This may explain why television cathode-ray tubes are subject to spot ¿blooming¿ at higher grid drive levels.