This article describes the overall design of the Joint European Torus ͑JET͒ motional Stark effect ͑MSE͒ diagnostic. The principle of this diagnostic has been proven on other tokamaks ͑TFTR and DIII-D͒ but its implementation on JET involves specific difficulties associated with optical access and neutral beam geometry. The solutions adopted for the access problem include the use of a novel "Air Amici" prism for beam deviation and low Verdet coefficient glass for the relay optics. MSE signals

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... the different beam sources are distinguished by their Doppler shift; the diagnostic uses the ϩ components of the Stark spectrum to avoid contamination with signals from different beam sources. The optimum viewing location for good space resolution across the plasma profile is also one that allows good accuracy in the reconstruction of the q profile from the measurements. An assessment of the optical throughput predicts a measurement accuracy of the polarization angle of better than 0.15°with 1-10 ms time resolution.