The Retarding Potential Analyzer (RPA) measures the energy-per-charge distribution downstream of an ion thruster, Hall-effect thruster, ion source, or other plasma device.
The RPA has four electrically isolated grids/meshes and one collector electrode. The current to the collector is measured as a function of the discriminator grid bias, the derivative is the energy-per-charge distribution.
Grid-to-grid spacing: 1.59 mm (0.0625 in)
Entrance aperture diameter: 1.27 cm (0.5 in)
Entrance aperture area: 1.27 cm^2 (0.20 in^2)
Grid open area: 85% each grid (100 lines/inch, square hole width 0.2342 mm). Overall transparency ~52% for four grids.
Lead Time: Typically 4 weeks if not in stock.
The "Probe Only" option is the RPA having a 6-pin connector.
The "Probe + Feedthrough + Cables" option includes the probe, in-vacuum and air-side teflon-based cables, and a double-ended 6-pin feedthrough. By default, each cable is 10 feet (3 meters) long, and the feedthrough is mounted on a KF40 flange - we will confirm these options prior to assembly.
Accessories: Plasma Controls can provide both in-vacuum and air-side cables at customer-specified lengths, as well as 6-pin feedthroughs mounted on KF16, KF40, KF50, Conflat CF1.33, CF2.75, CF3.375, NASA 4.5"OD standard, or customer-specified flanges.
An ion energy distribution function (IEDF) is measured by grounding or letting the first grid float to minimize plasma disturbance. The second "repeller" grid is biased negative to reject electrons. The third "discriminator" grid is swept positive to reject ions of increasing energy. Similar to the repeller, the fourth "suppressor" grid is biased negative of the collector to return any secondary electrons emitted from the collector due to high energy ion bombardment. Ions that pass through all four grids reach the collector electrode.
The information gleaned from the RPA is similar to that from an Electrostatic Energy Analyzer (ESA). The RPA is physically smaller than the ESA and has fewer parts; however, the ESA trace does not have to be differentiated to obtain the energy distribution function.