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

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 TM010 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 TM110 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 50μm 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.