During the conditioning of evacuated accelerator tubes, groups of discrete X-ray signals with apparent energies of greater than 300 keV are observed. A mechanism to explain the X-ray emission is proposed in terms of a series of transient vacuum arc discharges. A review is given of vacuum breakdown mechanisms, and of the most likely processes occurring in accelerator tubes during the conditioning process. In the case of the vacuum accelerator tube, there is initially no gaseous medium, but the applied voltage of 30 kV between the tube electrodes is sufficient to lead to the formation of a vacuum arc if there are fine whiskers, impurities, or accelerated dust particles present. Such a vacuum discharge will have a very short life (∼1 μs), as the current of the arc will cause the collapse of the electrode voltage which is maintained by a small current through 600 MΩ resistors. During the discharge, electrons will escape into the accelerator tube, and fall through several sections to be finally arrested, producing the discrete X-ray signals. The vacuum arc will release charged molten droplets of anode material into the high-field region; after some delay, these droplets will trigger further arcs in different sections of the tube. The triggered arcs will be at the inner edges of the electrode where the droplets hit the electrode, and where electrons are most readily released into the accelerator tube. Thus a whole series of high energy X-ray signals are observed.