Part No. 404-0008 RPA ECCN: EAR99 HTS: 9031.80.8085 The Small-Area 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: 77% each grid (100 lines/inch, square hole width 0.0088 inch). Overall transparency ~36% for four grids. Grid material: SS 316 Body material: Graphite type DS4 ultrafine EDM is standard. Stainless 304 or aluminum are available. Lead Time: Typically 4 weeks if not in stock. The "Probe Only" option is the RPA without cables. The probe body has a 6-pin MS3474L10-6P jam nut receptacle. One mating MS3475L10-6S plug is supplied. The user will need to crimp leads (20 gauge max) and insert sockets. 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. The 6-conductor cable is type M27500-22RC6S06, typically specified to 600 VAC (800 VDC) by the cable manufacturer. 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.
