Physical Review Special Topics. Accelerators and Beams (Nov 2012)

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

  • Thorsten R. Pusch,
  • F. Frommberger,
  • W. C. A. Hillert,
  • B. Neff

DOI
https://doi.org/10.1103/PhysRevSTAB.15.112801
Journal volume & issue
Vol. 15, no. 11
p. 112801

Abstract

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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 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 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 μm 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.