Advanced Light Source linac subharmonic buncher cavities

The two lower subharmonic bunchers for the Advanced Light Source linac are described; one operates at 124.914 MHz while the other operates at 499.654 MHz. Each electron bunch exiting the electron gun and entering the 124.914-MHz subharmonic buncher has a pulse width of 2.5 ns. This first buncher compresses the 2.5-ns bunch to a width of 800 ps. The 499.654-MHz subharmonic buncher further compresses the electron bunch to a width of approximately 200 ps. After going through the seventh gap of the 3-GHz buncher, the electron bunch attains a width of 20 ps. Prototypes were constructed and various measurements have been made to confirm the calculated parameters. However, tests under high vacuum have not been made because the models were not made to operate under high-vacuum conditions. With the tests performed so far the design parameters and the actual performance are in good agreement. Approximately 35-kV spark down voltage has been achieved in air across the 2-cm gap spacing in both buncher cavities