JET high resolution bent crystal spectrometer

In this paper we describe the bent crystal spectrometer of the Johann type built for the JET tokamak. This high resolution (λ/Δλ∼20 000) instrument is characterized by a very long focal length (25 m) allowing the crystal and the position sensitive detector to be placed outside the JET torus hall, in order to keep the noise induced by the background radiation to an acceptable level. By means of the shape analysis of x‐ray lines emitted by medium Z, highly ionized impurities present in the plasma, the ion temperature can be evaluated. Relevant information about other plasma parameters such as the electron temperature, the impurity ionization equilibrium and concentration, and the plasma toroidal velocity can be obtained by means of the analysis of different lines present in the spectra and of their wavelengths. Examples of the results obtained during the first year of operation of this instrument are also illustrated.