Breakdown on Linac electrodes

After being in operation for a year, the new 500 kV accelerating column, at the input of the linear accelerator which feeds the proton synchrotron, was opened for inspection during the shutdown. These photographs of the inside of the column was taken looking from the position of the proton source. The beam passes through the circular aperture at the centre of the electrode (called the intermediate electrode; it carries a potential of 250 kV, half the total potential across the column). The electrode is constructed of titanium, which is an excellent material from the point of view of high voltage properties in vacuum. Electrical breakdowns do occur, however, from time to time and the small pock marks, which can be seen surrounding the aperture, were produced by the sparks.