Vacuum coupling of photo multiplier tube with monochromator for improved monitoring of VUV emission from Aditya tokamak

UV sensitive solar blind photo multiplier tube (PMT) attached on the exit slit of the VUV monchromators through a vacuum interface of MgF₂ window are, generally, used for detection of emissions of wavelength larger than 2000 Å. However, using windows in front of PMTs always remains a concern from the point of view of detection of weak radiations and also for monitoring the radiations having wavelength lower than 2000 Å due to absorption by air. In the present paper, a technique is presented to monitor spectral emissions from Aditya tokamak plasma in vacuum ultraviolet (VUV) wavelength range down to 1150 Å using a solar blind UV sensitive photo multiplier tube, which is kept inside a vacuum cavity and directly coupled with exit slit of the monochromator and hence avoiding any window interface between monochromator and detector. This technique enables us to improve the detection of VUV radiation in Aditya tokamak, as the VUV photons reaching to detector increase in number due to the absence of any vacuum interface. The comparison of results obtained with this technique with other window based techniques, is also presented.