Development of a high dynamic range retarding potential analyzer for electric propulsion plume diagnosis

A new high dynamic range retarding potential analyzer design is proposed to measure the ion energy distribution in electric propulsion plasma plumes. The main difference between the proposed probe and previous designs is an ion-optical system with a high negative accelerating potential. Applying the principle of acceleration and focusing of the ion flow provides the correct operation of the probe in a dense plasma in which the Debye length can be ten times smaller than the diameter of the grid apertures. As a result, the new probe has high transparency with an extended operating range of measured plasma densities. Another advantage of the probe is its accurately known and time-stable transparency for ions, so the probe can measure the absolute values of the ion current density like the Faraday probe. The analyzer geometry, which provides measurement in the range of ion current densities up to 100 A/m² at ion energies up to 2000 eV with an error of no more than 2%, was determined using numerical simulation. We present the measurement results of the plasma plume of a Hall effect thruster with a nominal discharge power of 2.3 kW.