Performance of the buncher and chopper at the Daresbury Laboratory

A beam buncher and chopper have been installed on the Daresbury tandem to provide very short clean pulses of beam for use in nuclear physics experiments. The buncher is a two-harmonic double drift klystron type operating over a frequency range of 3.5-11.0 MHz. Selecting the operating frequency to be proportional to the ion velocity allows fixed klystron voltages and relative phase to be used for a wide variety of ions making setting up very easy. The chopper consists of parallel electrostatically deflecting plates operating at half the buncher frequency, the bunched beam arriving at the plates at each zero cross of the applied rf voltage. Tests of the system with sulphur ions showed that chopped beam widths of 0.7 ns FWHM could be achieved. Particles between pulses were completely eliminated while maintaining an overall transmission efficiency of 50%. Details of the construction, operation and performance of the buncher and chopper are presented. (orig.)