Multilayer mirror based soft x-ray spectrometer as a high temperature plasma diagnostic

A fully calibrated multilayer mirror (MLM) based soft x-ray spectrometer ha s been developed and installed on the Compact Helical System (CHS). This ML M based spectrometer is a new approach to devising a soft x-ray spectromete r with medium time and energy resolutions. The spectrometer mainly consists of a MLM as the dispersive element and a 20-channel (p-i-n) pin diode arra y for the detection of the soft x-rays. Both the MLM and the pin diodes hav e been calibrated at the KEK (High Energy Accelerator Research Organization ) Photon Factory for an energy range of 300-1200 eV. The reflectivity of th e mirror was found to increase from 3% (photon energy of 335 eV) to 25% (ph oton energy of 1050 eV). The pin diodes were found to have almost 100% effi ciency for creating electron-hole pairs in the calibrated energy range. The spectrometer was installed on the CHS with the aim of fast electron temper ature measurement and study of fast magneto-hydrodynamic (MHD) events occur ring in plasmas. The experiments show that in the energy range of measureme nt, the electron "temperature" determined from the slope of the soft x-ray spectrum is much lower than that measured by the Thomson scattering diagnos tic. Analysis showed that the soft x-ray spectrum is highly contaminated by impurity emission. Therefore it may be possible to measure electron temper ature with this diagnostic if we choose another energy range where we can m easure the continuous spectrum. The present time resolution of the system i s of the order similar to0.1 ms, which made it possible to study the behavi or of the plasma during fast MHD events. Modulations in the soft x-ray inte nsity were observed during MHD events in CHS plasmas. Analysis of these sho ws that these may be due to modulations in the temperature or the impurity concentration. (C) 2001 American Institute of Physics.