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We are constructing a diagnostic system to measure the electric field (≳100 kV/cm) of a free‐electron laser (FEL) beam when injected into the plasma of the Microwave Tokamak Experiment (MTX). The apparatus allows a crossed‐beam measurement, with 2 cm spatial resolution in the plasma, involving the FEL beam (with 140 GHz, ≊1 GW ECH pulses), a neutral helium beam, and a dye laser beam. After the laser beam pumps metastable helium atoms to higher excited states, their decay light is detected by an efficient optical system. Because of the Stark effect arising from the FEL electric field (E), a forbidden transition can be strongly induced. The intensity of emitted light resulting from the forbidden transition is proportional to E2. Because photon counting rates are estimated to be low, extra effort is made to minimize background and noise levels. It is possible that the lower E of an MTX gyrotron‐produced ECH beam with its longer duration pulses can also be measured using this method. Other applications of the apparatus described here may include measurements of ion temperature (using charge‐exchange recombination), edge‐density fluctuations, and core impurity concentrations.