An ultra low noise AC beam transformer for deceleration and diagnostics of low intensity beams

The design of a broad band ultra-low noise ferrite loaded AC beam transformer is presented. It is designed for use in the CERN Antiproton Decelerator (AD), where beams of a few 107 charges must be decelerated from 3.5 GeV/c to 100 MeV/c. It is used in the RF beam-phase loop, and for intensity and bunch shape measurements during deceleration. When the beam is debunched for cooling on magnetic flat tops, the pick-up is used for measurements of intensity and momentum distribution by means of longitudinal Schottky scans. When used as Schottky pick-up, the signal to noise ratio should be better by about 40 dB than the existing stripline based longitudinal Schottky pick-up. The integrated design of pick-up and associated low-noise amplifier is presented. The achieved noise performance of a few from 1 to 3 MHz is obtained by attaching a low-noise, high-impedance silicon JFET (junction field effect transistor) amplifier to a high-Q resonant ferrite loaded cavity, and then eliminating the resonant response by low-noise RF feedback such that broad band response over almost 2 decades of frequency (0.3 - 15 MHz) is obtained. The longitudinal coupling impedance is close to 10 W in this frequency range, and the equivalent noise temperature of this resistance is about 0.4 °K mid-band although all components operate at ambient temperature. Finally the application of a similar design for single-pass bunch intensity measurements is discussed.