Development of phosphor scintillator-based detectors for soft x-ray and vacuum ultraviolet spectroscopy of magnetically confined fusion plasmas

Specialized soft x-ray and vacuum ultraviolet ~VUV! diagnostics used to monitor impurity emissions from fusion plasmas are often placed in a very challenging experimental environment. Detectors in these diagnostics must be simple; mechanically robust; immune to electromagnetic interference, energetic particles, and magnetic fields up to several tesla; ultra-high-vacuum compatible; and able to withstand bakeout temperatures up to 300 °C. The design and the photometric calibration of a detector consisting of a P45 phosphor (Y2O2S:Tb), two incoherent fiber-optic bundles coupled with a vacuum feedthrough fiber-optic faceplate, and a photomultiplier tube ~PMT! are reported. We have successfully operated the detectors of this type in novel soft x-ray and VUV diagnostics on several fusion plasma facilities. Measurements of the visible photon throughput of the silica/silica incoherent fiber-optic bundle, and the light loss associated with the coupling of the two fibers with the faceplate are presented. In addition, improved absolute measurements of the conversion efficiency of the P45/PMT photodetector based upon the use of a PMT with a bialkali photocathode instead of a multialkali one are presented for the soft x-ray and VUV range of photon wavelengths. The conversion efficiency is defined as the ratio of the photoelectrons ejected from the photocathode of a visible detector, which are excited by the scintillated photons that are emitted from the phosphor in a solid angle of 2p, to the number of soft x-ray photons incident on the phosphor. Sensitive electronic gain measurements of the PMT using the visible scintillated light from the P45 phosphor are compared with the gain measurements supplied by the manufacturer of the PMT, which were performed with a tungsten filament lamp operated at 2856 K. © 2003 American Institute of Physics. @DOI: 10.1063/1.1605493#