A diagnostic has been developed to simultaneously determine the magnetic field and space potential in fusion‐like plasmas. Initial tests have demonstrated the ability to measure an absolute change of 17 G in the magnetic field strength; under the same test conditions a 6‐V change in space potential could have been detected. The spatial resolution in these tests is a volume of 0.25 cm3. The magnetic field is determined by a measurement of the Zeeman resonance absorption pattern with a frequency scanning narrow‐band laser of an energetic (<50 keV), low current (∼10 &mgr;A) barium ion beam. The laser is injected tangentially to the ion beam to take advantage of the narrow Doppler spread in the direction of the beam. Space potentials are determined by a measurement of the Doppler shift due to a change in beam energyE=E0−e&Jgr;. The diagnostic will be used initially on the TRW STM device to measure diamagnetic depression of the ELMO rings in STM.