RPA Measurements in the Plume of an RF Magnetron Discharge with and Without a Magnetic Nozzle Expansion

We present retarding potential analyzer (RPA) measurements of ion acceleration in a magnetic nozzle expansion of an RF magnetron discharge. The source consists of concentric cylindrical magnets creating strong radial magnetic fields, with axial RF excitation at 50-120 MHz. Using RPA and Langmuir probe measurements, we record ion energy distributions throughout the magnetic nozzle expansion region. Ion acceleration is attributed to the usual double layers generated in a partially-magnetized magnetic nozzle, enhanced partially by bulk plasma potential rectification in some operating regimes. Measurements reveal bimodal ion energy distributions with a high-energy population corresponding to ions accelerated from the source (energies up to 40 eV) and a secondary population at 15-20 eV attributed to charge-exchange and ionization collisions in the expanding plume. Higher magnetic field strengths are seen to enhance ion beam currents. Ion energy distributions evolve axially due to ion-neutral collisions, with charge-exchange processes creating background thermal populations while preserving the energetic beam component over characteristic distances determined by the collision mean free path. These results further our understanding of ion acceleration physics in these magnetron discharge-driven expansions.