Optical emission spectroscopy, Langmuir probe, and microwave interferometry measurements have been used to characterize the electron temperature and plasma density of electron‐cyclotron‐resonance (ECR) heated Ar plasmas in a magnetic‐mirror plasma tool. The magnetic field was operated in a symmetric mirror, asymmetric mirror, and minimum‐B configuration. The dependency of the plasma parameters upon microwave power, pressure, and magnetic field configuration have been examined. Plasma densities up to 2×1012cm−3at the midplane and 5×1012cm−3at the ECR zone for a symmetric mirror configuration of the magnetic field have been measured. Operation of the minimum‐B magnetic field configuration was found to increase the plasma stability with comparable plasma density while an asymmetric mirror magnetic field configuration, similar to other devices recently studied, yielded densities in the 2–8×1011cm−3range. Radial uniformity was found to be ±25% over the 4 cm radius of the microwave window.