Modified branching ratio method for absolute intensity calibration in VUV spectroscopy

Abstract

A new modified branching ratio (MBR) method for the absolute intensity calibration of a vacuum ultraviolet (VUV) spectrometer is presented. The spectrometer is equipped with a multichannel detector, consisting of an open microchannel plate coupled to a charge-coupled device (CCD), or with a single channel photomultiplier. This technique extends the number of calibration points available from those provided by the branching ratio (BR) calibration technique. The MBR method is a variation of the conventional BR method, where we relax the condition that the two spectral emissions, in the visible and VUV spectra, come from the same excited level, to include transitions from different sublevels of the same energy level. However, a critical study of the statistical equilibrium of sublevels from the same ion energy level was necessary. As a result, we have more than doubled the number of calibration points for our spectrometer used in tokamak plasma diagnostics. The appropriate identification of new spectral line pairs for absolute calibration here presented opens the path for future works in other devices with similar plasma conditions or impurities content.

VUV spectral line emission measurements in the TCABR tokamak

“…Figure 2 shows the detector spectral response versus wavelength. The wavelengths were calibrated by different methods [9,12], in order to cover a longer wavelength range. One can observe that wavelengths below 550Å and above 2300Å are much attenuated due to the detector sensitivity.…”

“…However, in VUV region, the Doppler broadening is smaller than in the visible region (due to its wavelength), the absolute calibration is much more arduous, and measurable spectral range are smaller compared to the visible region [9].…”

Calibration of Vacuum Ultraviolet spectroscopy diagnostic on J-TEXT tokamak

“…[10] could be measured in both visible and VUV range simultaneously. Thus, we introduced the collisional-radiative line ratio method (sometimes, it is also called modified branching ratio), which utilizes line pair with different upper energy level to implement the calibration [4,9,11,12]. To take into account the influence of electron temperature on the ratio, the impurity emission profile of each lines are estimated using the impurity transport code STRAHL with the atomic physics database Atomic Data and Analysis Structure (ADAS) [13][14][15].…”

Multichannel detector for ion temperature determination in vacuum ultraviolet spectrum

“…Using a 1200 grooves/ mm grating ͑dimensions 96ϫ 56 mm 2 ͒ the spectrometer covers the range 500-3000 Å and has 8.3 Å / mm dispersion. 8 The multichannel plate ͑MCP͒ ͑XSI Instruments͒ has a diameter of 40 mm and is coated by a CsI film to convert VUV photons into electrons. The MCP channels are separated by 12 m and have a 10 m diameter each.…”

“…So, tokamak discharges with very low power ͑and temperature lower than 15 eV͒ were chosen for this purpose. 8 The Doppler broadening ͑⌬ D ͒ is related to the ion temperature ͑T, in eV͒ by the equation…”