Adjustment of a double drift harmonic buncher and bunch shape measurements

The longitudinal beam matching of the new 50 MeV linac is achieved with a double drift harmonic buncher. For optimum bunching efficiency the RF phase difference between the two cavities should be known and controlled to about 1 degrees (at 202.56 MHz). The adjustment of the RF levels and phases is based on observations of the bunch form via a broad-band probe connected to a travelling wave oscilloscope. The time base is tightly locked to the RF by a specially developed circuit allowing a systematic study of different bunching conditions. A comparison is made between some preliminary measurements and the waveforms predicted by a simplified theory. (3 refs).