Retarding Field Analyzer

Categories: Beam energy measurement Beam diagnostics
Manufacturer: DREEBIT GmbH
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 up to a beam energy of 15 keV per charge state.


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

Figure 1 - RFA phases

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 Iion versus URFA can be differentiated to obtain the beam energy distribution δUion/δIRFA which can be fitted with a Gaussian, see Figure 2. Thereby, the mean energy of the charged particles Em 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.

Figure 2 - RFA measurementAn 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.


RFA Parameters
beam energy acceptance up to 15 keV per charge state
collimator aperture diameters 1 mm, 2 mm, or 3 mm
General Parameters
beamline attachment flange DN 160 CF (on vertical linear feedthrough) or DN 100 CF (as horizontal end cup)
vertical travel (on vertical linear feedthrough) min. 50 mm (other travel distances on request)
dimensions RFA diagnostic unit 130 mm x 80 mm x 105 mm
weight RFA diagnostic unit incl. flange 10 kg (22 lbs)
max. bakeout temperature 150 °C
Infrastructural Requirements
vacuum conditions during operation suitable from 1e-10 mbar up to 1e-6 mbar

Scope of Delivery

  • Retarding Field Analyzer mounted on DN 160 CF flange with vertical linear feedthrough / DN 100 CF as horizontal end cup
  • 15 kV power supply incl. HV cables
  • beam current/charge measurement device (Picoamperemeter/Electrometer)
  • computer control unit with measurement and analysis software

Optional Equipment

  • remote controllable linear motion feedthrough
  • vacuum chamber with beamline connection flanges according to customer specifications

Reference Facilities

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