Design and calibration of a retarding field energy analyzer for the LTX-β scrape off layer and modeling of electrostatic potential in a collisionless SOL

•Li coated walls produced a collisionless scrape off layer.•Ions are mirror-trapped in a collisionless scrape off layer.•Ambipolar potential is significantly lower than Debye sheath potential.•Potential gradient extended into the whole scrape off layer.•Retarding Field Energy Analyzer successfully characterized a deuterium beam.