VACUUM BREAKDOWN MECHANISMS, AND X-RAY PULSES IN ACCELERATORS

During the conditioning of evacuated accelerator tubes, groups of disc rete X-ray signals with apparent energies of greater than 300 keV are observed, A mechanism to explain the X-ray emission is proposed in ter ms of a series of transient vacuum are discharges. A review is given o f vacuum breakdown mechanisms, and of the most likely processes occurr ing in accelerator tubes juring 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 suffic ient to lead to the formation of a vacuum are if there are fine whiske rs, impurities, of accelerated dust particles present. Such a vacuum d ischarge will have a very short life (similar to 1 mu s), as the curre nt of the arc will cause the collapse of the electrode voltage which i s maintained by a small current through 600 M Omega resistors. During the discharge, electrons will escape into the accelerator tube, and fa ll through several sections to be finally arrested, producing the disc rete X-ray signals. The vacuum are will release charged molten droplet s of anode material into the high-field region; after some delay, thes e 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.