Diagram of the retarding potential analyzer (RPA). The dual retarding grids (G2) act as the energy filter. The instrument moves to the left, leading to a net flow of particles into the RPA.
Two different geometric arrangements of a double-grid energy filter. The grids in model A are perfectly aligned, giving it the optical transparency of the first grid. The second grid in model B is offset in each direction, giving it a lower optical transparency than model A. Both models have a periodic symmetry over one grid cell in the second grid.
Simulation of ion tracks for three different energies through a real energy filter using ANSYS®. The RV grids that comprise the energy filter are perfectly aligned. The vertical lines represent the grid planes in the RPA. G1 and G3 are modeled as idealized grids, while G2 is modeled with real flat wires. The circles on G2 mark the location grid wires normal to the plane of the plot (the circle diameter is much larger than the actual wire diameter for purposes of illustration). The plot is positioned symmetrically, equidistant between two vertical wires out of the plane of the plot in the left G2 grid. The grid alignment is based on model A (see Fig. 2).
Transmission ratios for ions with velocities normal to the grid plane compared to the ideal assumptions for three different energy filter models and for different suppression voltages. The maximum optical transparency is used for the ideal case.
Transmission ratios for the two double-grid energy filter models. The notches are due to the lensing effects of the grids and the relative locations of the grid wires. A suppression voltage of is used for both models.
Sample simulated curves for a 3D drifting Maxwellian distribution of using the ideal analytic expression and the two simulation models. The temperature, composition, and ram velocity are representative of low Earth orbit conditions.
Inferred ram velocities and ion temperatures from least-squares fit of simulated curves and the ideal expression as a function of input temperature. All simulated data are for an input drift velocity of .
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