Experimental observation of ion beams in the Madison Helicon eXperiment

Argon ion beams up to Eb=165 eV at Prf=500 W are observed in the Madison Helicon eXperiment (MadHeX) helicon source with a magnetic nozzle. A two-grid retarding potential analyzer (RPA) is used to measure the ion energy distribution, and emissive and rf-filtered Langmuir probes measure the plasma potential, electron density, and temperature. The supersonic ion beam (M=vi/cs up to 5) forms over tens of Debye lengths and extends spatially for a few ion-neutral charge-exchange mean free paths. The parametric variation of the ion beam energy is explored, including flow rate, rf power, and magnetic field dependence. The beam energy is equal to the difference in plasma potentials in the Pyrex chamber and the grounded expansion chamber. The plasma potential in the expansion chamber remains near the predicted eVp~5kTe for argon, but the upstream potential is much higher, likely due to wall charging, resulting in accelerated ion beam energies Eb=e[Vbeam-Vplasma]\u3e10kTe