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A special type of vacuum-photodiode detector is being developed for x-ray imaging of plasma in fusion-producing tokamaks such as the international thermonuclear experimental reactor (ITER), where the radiation environment will be too hostile for conventional x-ray detectors. The vacuum photodiode has modest efficiency, but it is intrinsically immune to radiation damage if built in such a manner as to expose only metal components to radiation. A design based on appropriately chosen materials (including high-Z cathodes) and geometries (including a small angle between cathode surface and incident x rays) can provide good signals from the 1–100 keV x rays that are of particular importance for imaging the plasmas in the Joint European Torus (JET) and ITER. It should also provide better rejection of signal distortion and noise due to unwanted detection of neutrons and hard gamma rays than conventional detectors. A prototype design is described, along with performance parameters predicted for JET and ITER. In addition, we show results of laboratory experiments that confirm some of the predicted behaviors of the design. © 1997 American Institute of Physics.