A compact high-power-density helicon plasma source operating on abundant medium atomic weight gases is under examination for a potential electric propulsion application. A high specific impulse, which is easily varied in the range of ~1-2 ks for Ar and ~2-4 ks for N 2 propellant, was achieved. A well- collimated plasma plume with emittance ~10deg is produced in a single-magnet configuration. Analysis of the data obtained using in situ and noninvasive spectral diagnostics indicates almost complete gaseous propellant ionization and thrust efficiency eta T ~ 20% with the lowest net ionization cost of ~(80 V) -1 per ion. Direct weighing has yielded a thrusting force of T ~ 10 mN.