X-ray source

An X-ray source comprising:

• an X-ray emissive target (70);
• a high voltage power source (20, 90, 290);
• a cathode filament (30, 230) and an anode electrode (60) coupled to said high voltage power source;
• said cathode filament and said anode electrode being adapted to establish, in response to a beam current drawn by said cathode filament from said power source, a beam (50) of electrons directed at said target;
• a control grid electrode (40, 240);
• self-biasing means for generating a bias voltage for application to said control grid electrode to control the magnitude of said electron beam and to produce a focussing electric field for the electron beam;
• the self-biasing means including an active variable conductance device (130, 330) and sensing means (160, 325/361) for sensing said beam current and generating an indication of the magnitude thereof;
• means (355, 355a) for conveying said indication to a remote location;
• means at said remote location for monitoring said beam current and determining adjustment required thereto;
• and control means (140, 366/365) for generating a control signal indicative of said adjustment and for applying said control signal to said active variable conductance device to control its conductance to vary the beam current, and thereby the magnitude of said electron beam, in accordance with said adjustment.