A New Radiation-Hard Endoscope for Divertor Spectroscopy on JET
In preparation for ITER, JET has been upgraded with a new ITER-like wall (ILW), whereby the main plasma-facing components, previously made of carbon, have been replaced by Be in the main chamber and W in the divertor. A new endoscope with optimised divertor view has been developed in order to survey and monitor the emission of specific impurities such as tungsten, beryllium and the possibly remaining carbon in the tungsten divertor of JET-ILW. It operates in the wavelength range from 390nm to 2500nm with high optical transmittance (30%) as well as high spatial resolution, that is 2mm at the object plane and 3mm over the whole depth of field (±0.7m). The endoscope is a prototype for testing an ITER relevant design concept based on reflective optics only. The endoscope has an optimised observation in the near ultraviolet and in the blue spectral region to ensure the detection of the W I-emission line at 400.8nm. In parallel to the new optical design, a new type of ITER-like shutter system based on pneumatic techniques has been developed and integrated in the endoscope head. The new optical design includes options for an in situ calibration of the endoscope transmittance during the experimental campaign.