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X-ray cargo inspection systems for the detection and verification of threats and contraband must address competing performance requirements. In addition to high X-ray energy, high X-ray intensity is needed to penetrate dense cargo, while low intensity is desirable to minimize the radiation footprint, i.e. the size of the controlled area, required shielding and the dose to personnel. We report here on a collaborative effort between HESCO/PTSE Inc., Schonberg Research Corporation and Rapiscan Laboratories, Inc. to design an Intensity-Modulated Advanced X-ray Source (IMAXS) that allows such cargo inspection systems to achieve up to two inches greater penetration capability, while on average retaining the same radiation footprint as present fixed-intensity sources. Alternatively, the new design can be used to obtain the same penetration capability as with conventional sources, but reducing the average radiation footprint by about a factor of three. The key idea is to anticipate the needed intensity for each X-ray pulse by evaluating signal strength in the cargo inspection system detector array for the previous pulse. Requirements therefore include an X-ray source capable of changing intensity from one pulse to the next by electronic signal, as well as electronics inside the cargo inspection system detector array determining the required source intensity for the next pulse. The feasibility of pulse-to-pulse intensity modulation of both S-band (2998 MHz) and X-band (9303 MHz) linac sources has been demonstrated.