Faradaycup

A Faraday cup is an instrument for measuring the current of charged particles by collecting them in a conductive container and routing the resulting charge to an electrometer or current-measuring amplifier. It is widely used in vacuum systems, particle accelerators, ion beam analysis and mass spectrometry as a direct and absolute method of determining particle flux or beam current.

The operating principle is simple: incident charged particles deposit their charge on a collector electrode that is electrically isolated from ground. The collected current, proportional to the particle rate and charge, is measured with high sensitivity. To improve accuracy, designs often include suppression electrodes or biased grids to prevent escape of secondary electrons and reduce backscattering; guard rings and shielding are used to control leakage and capacitance.

Typical Faraday cups are constructed from metals such as stainless steel or copper and may be single cups or arrays for spatial profiling. Variants include retarding-field and time-of-flight configurations, segmented collectors for beam position sensing, and integrated picoammeter assemblies for low-current measurements (picoampere to microampere range).

Advantages of Faraday cups are their simplicity, robustness and traceability to fundamental charge measurement. Limitations include their destructive nature (they absorb the beam), sensitivity to secondary-electron effects, and potential perturbation of the beam. Proper calibration, biasing and mechanical design mitigate these issues and ensure reliable quantitative measurements. The device is named for Michael Faraday in recognition of his foundational work on electric charge and current.