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

Abstract

A fully calibrated multilayer mirror (MLM) based soft x-ray spectrometer has been developed and installed on the Compact Helical System (CHS). This MLM based spectrometer is a new approach to devising a soft x-ray spectrometer 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 array for the detection of the soft x-rays. Both the MLM and the pin diodes have been calibrated at the KEK (High Energy Accelerator Research Organization) Photon Factory for an energy range of 300–1200 eV. The reflectivity of the mirror was found to increase from 3% (photon energy of 335 eV) to 25% (photon energy of 1050 eV). The pin diodes were found to have almost 100% efficiency for creating electron-hole pairs in the calibrated energy range. The spectrometer was installed on the CHS with the aim of fast electron temperature measurement and study of fast magneto-hydrodynamic (MHD) events occurring in plasmas. The experiments show that in the energy range of measurement, the electron “temperature” determined from the slope of the soft x-ray spectrum is much lower than that measured by the Thomson scattering diagnostic. Analysis showed that the soft x-ray spectrum is highly contaminated by impurity emission. Therefore it may be possible to measure electron temperature with this diagnostic if we choose another energy range where we can measure the continuous spectrum. The present time resolution of the system is of the order ∼0.1 ms, which made it possible to study the behavior of the plasma during fast MHD events. Modulations in the soft x-ray intensity were observed during MHD events in CHS plasmas. Analysis of these shows that these may be due to modulations in the temperature or the impurity concentration.