A high resolution X ray task is the measurement of the ion temperature and rotation velocity from Doppler broadening and shifts. Stringent operation requirements considerably conditioned its versatility as a survey instrument or a tool for fundamental research. The unique conditions of the JET plasmas, however, allowed observations in high resolution and identification of lines from the highest ionization stages B like of the oneand two,electron systems of this element. These spectra and their relevance to the study of JET physics are briefly presented. The comparison with atomic physics theoretical computations of He like intense among these lines could regularly be monitored with adequate time resolution as to producte an extensive data base. The experimentalspectra are simulated by integrating the theoretical ones along the spectrograph line of sight well as rotation velocity measurements in ohmic and additionally heated plasmas are discussed and their relationship with other physicalquantities and operational parameters are reviewed. Details of the spectrometer design, its lay out are also given.