A medium-power helicon thruster (MPHT) is an electric propulsion device that uses a helicon plasma source to achieve specific impulses of up to 1500 s with argon propellant. 1 The Center of Studies and Activities for Space (CISAS) has created a prototype design, and the Georgia Institute of Technology (GA Tech) has designed and built a nominally 1.5-kW MPHT that sustains high density, steady-state plasma over a range of operating conditions. All tests are performed in the GA Tech vacuum test facility at an operating pressure less than 3.1 x 10-5 Torr-Ar. Ion number density, electron temperature, and electron energy distribution function measurements are taken with an RF-compensated Langmuir probe as a function of RF frequency (2-15 MHz), RF forward power (0-1.5 kW), magnetic field strength (0-1100 Gauss), and argon mass flow rate (0.74-4.45 mg/s). The maximum ion number density is 8.0 x 10 18 m-3. The measurements are compared with several known helicon plasma sources. Experimental characterization of this device allows validation of the 1-D and 2-D codes developed at CISAS, which simulate plasma acceleration during vacuum expansion.