Measuring the intensity and position of a pA electron beam with resonant cavities

Abstract

In order to continuously monitor the intensity and position of an electron beam of a few hundred pA, a system of resonant cavities has been set up. The current measurement relies on signals of a few fW power extracted out of a cylindrical resonator, excited at its ${\mathrm{TM}}_{010}$ mode. The demodulated cavity pickup signal allows the reconstruction of the beam current with a precision of a few pA. For beam position measurements, we designed two resonators, one each for the horizontal and vertical plane. They are excited at their ${\mathrm{TM}}_{110}$ dipole modes, the signal strength vanishing with the beam passing on their symmetry axis. Commercial digital lock-in amplifiers perform a phase-sensitive detection of the position signals, separating them from background noise. A frequency mixing scheme was applied to transform the signals into the passband of the amplifiers. Great care was taken to prevent cross talk by using special shielding. With these techniques, a relative beam position resolution of $50\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ was achieved. The position readings are sampled with a maximum rate of 9 Hz. A standard PC is used to read out the lock-in amplifiers. It transfers the measured raw data as well as processed values to the accelerator control system for graphical display.