Beam temperature, discharge lag and target biasing in some television pick-up tubes

Discharge lag in pick-up tubes, particularly of modern cathode-potential-stabilized type, and its relation to the acceptance of the electron beam by the target are analysed. Measurements of beam acceptance as a function of target potential reveal effective beam temperatures substantially equal to cathode temperature at low beam currents, but higher and multi-valued at high currents. These temperatures set a limit to the possible reduction of lag without target biasing; this limiting lag, referred to as Maxwellian lag, is calculated and presented graphically. Direct measurements of lag are compared with the limiting values. Target biasing, the application of a small positive potential to the target face, reduces this limitation. Results of electron-optical measurements, throwing some light on the anomalous beam temperatures, are presented.