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Physics, Techniques and Procedures

Saturation voltage

the voltage between the X ray tube filament and the X-ray tube anode at which all of the electrons thermionically emitted by the filament are attracted to the anode. Without a voltage difference applied, the electron cloud formed when the filament is heated remains in the vicinity of the filament wire, and an equilibrium exists between electrons emitted by the wire and electrons attracted back to the wire by the local positive charge due to the loss of emitted electrons. When a voltage is applied, some of the electrons in the cloud are attracted to the positive charge on the anode. If the applied voltage is not sufficient to attract all of the electrons to the anode, a residual space charge remains in the vicinity of the filament, which tends to limit the number of electrons available for flow to the anode. As the voltage is increased, more electrons flow to the anode. When the point is reached that all electrons flow to the anode, then a further increase in voltage should have no effect on increasing current flow. The current is then limited by the number of electrons available and the current flow is emission limited. In practice, X-ray tubes may be operated below the true saturation voltage where the effects of a small residual space charge can be observed (Fig.1, upper curve). Modern X-ray circuits are designed to compensate for this small effect in order to attain precisely controlled tube currents. At voltages above the operative "saturation voltage", the filament heating is automatically reduced to keep the tube current constant (Fig. 1, lower curve).

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Fig.1

Diagram of tube current vs. voltage, with and without automatic adjustment of filament heating. See text.
Saturation voltage, Fig.1