Retarding Field Analyzer

The Retarding Field Analyzer (RFA) is an electrostatic charged particle beam energy analyzer which allows for measuring the kinetic energy distribution and the energy spread of the particles. The system mainly consists of three meshes, the central one being set on high voltage to establish the retarding field, and a Faraday cup at the end detecting the beam current. Furthermore, the RFA features a collimator with changeable apertures of 1mm, 2mm, or 3mm diameter at a length of 50 mm to optimize the setup considering beam intensity versus required energy resolution.

The setup can be mounted onto a linear motion feedtrough with the beam axis perpendicular to the DN 160 CF support flange normal but also at the end of a beamline directly onto a DN 100 CF flange. The maximum mesh voltage is limited to 15 kV meaning that energy distributions of particles up to 15 keV per charge state can be analyzed.

Functional Principle

To analyze the energy distribution of a charged particle beam the potential of the retarding field mesh is increased stepwise while measuring the beam current on the Faraday cup. While increasing the retarding field, 5 phases of influence on the incident beam can be distinguished, as summarized in Figure 1:

• particles pass the mesh (retarding potential very low)
• particles are focused into the mesh holes (retarding potential increased)
• particles reflection begins (retarding potential reaches the range of the kinetic beam energy)
• some particles are stopped, some can still pass (retarding potential is near the mean energy)
• all particles are stopped (retarding potential is higher than the kinetic energy of the particles

The measured dependency I_ion versus U_RFA can be differentiated to obtain the beam energy distribution δU_ion/δI_RFA which can be fitted with a Gaussian, see Figure 2. Thereby, the mean energy of the charged particles E_m is defined as the maximum of the Gaussian. The energy spread of the beam is defined as the full width half maximum (FWHM) of the distribution.

An example showing the possibilities of the RFA in terms of resolution and sensitivity is presented in Figure 2. The measurement was carried out using a Dresden EBIS-A producing a beam of Ar⁸⁺ ions. In this experiment, the 3 mm collimator apertures were used. A properly adjusted high-quality ion beam from the EBIS-A does not require an additional collimation effect, thus, large apertures can be used to achieve the maximum signal intensity.