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Electron cyclotron resonance (ECR) plasma reactors are being developed for etching and deposition of thin films during integrated circuit fabrication. To control critical parameters such as the flux and energy distribution of ions impacting surfaces, it is necessary to understand how these parameters are influenced by physical construction, electromagnetic design, and chemical kinetics. In this work, we report detailed measurements of ion velocity distributions in both the source and reactor regions of an ECR system using mixtures of Ar and He. Using Doppler‐shifted laser‐induced fluorescence spectroscopy, we measure metastable Ar‐ion velocity distributions parallel and perpendicular to the magnetic field direction as a function of magnetic field amplitude, pressure, rf bias voltage, and microwave power. The measurements, in turn, are used to estimate the magnitude of electrostatic potentials and fields parallel and perpendicular to the magnetic field. Indicative of ion trapping, we find nearly isotropic ion velocity distributions when the source is operated as a magnetic mirror and the He partial pressure is low; higher He pressures tend to cool the parallel velocity distribution. Downstream, we consistently observe bimodal ion velocity distributions: the fast component, created in the source, follows magnetic flux lines into the reactor; the slow component, created mostly where the plasma expands from the source into the reaction chamber, is more isotropic. The relative amplitudes of these two components, the average ion energy, and the ion energy distribution are easily controlled by changing pressure and magnetic field.