An x-ray generating device includes a field emission cathode formed at least partially from a nanostructure-containing material having an emitted electron current density of at least 4 A/cm2. High energy conversion efficiency and compact design are achieved due to easy focusing of cold cathode emitted electrons and dramatic reduction of heating at the anode. In addition, by pulsing the field between the cathode and the gate or anode and focusing the electron beams at different anode materials, pulsed x-ray radiation with varying energy can be generated from a single device. Methods and apparatus for independent control of electron emission current and x-ray energy in x-ray tubes are also provided. The independent control can be accomplished by adjusting the distance between the cathode and anode. The independent control can also be accomplished by adjusting the temperature of the cathode. The independent control can also be accomplished by optical excitation of the cathode. The cathode can include field emissive materials such as carbon nanotubes.