Absolute calibration of LIDAR Thomson scattering systems by rotational Raman scattering

Abstract

Absolute calibration of LIDAR Thomson scattering systems on large fusion devices may be achieved using rotational Raman scattering. The choice of calibrating gas molecule presents different options and design trade-offs and is likely to be strongly dependent on the laser wavelength selected. Raman scattering of hydrogenic molecules produces a very broad spectrum, however, with far fewer scattered photons than scattering from nitrogen or oxygen at the same gas pressure. Lower laser wavelengths have the advantage that the Raman cross section increases, sigma(Raman) proportional to 1/lambda(0)(4), but the disadvantage that the spectral width of the scattered spectrum decreases, Deltalambda(Raman) proportional to lambda(0)(2). This narrower spectrum makes measurement closer to the laser wavelength necessary. The design of the calibration technique presents a number of challenges. Some of these challenges are generic to all Thomson scattering systems. These include detecting a sufficient number of photoelectrons and designing filters that measure close to the laser wavelength while simultaneously achieving adequate blocking of the laser wavelength. An issue specific to LIDAR systems arises since the collection optics operates over a wide range of depth of field. This wide depth of field has the effect of changing the angle of light incident on the optical interference filter with plasma major radius. The angular distribution then determines the effective spectral transmission function of the interference filter and hence impacts on the accuracy of the absolute calibration. One method that can be used to increase absolute calibration accuracy is collecting both Stokes and anti-Stokes lines with optical filter transmission bands specifically designed to reduce systematic uncertainty.

Application of Thomson scattering to helicon plasma sources

“…The rotational anti-Stokes lines of nitrogen are routinely used for the absolute calibration, this technique is commonly used for TS in large fusion devices(LeBlanc 2008;Scannell et al 2010). The anti-Stokes part of the nitrogen spectrum is composed of narrow lines, mostly located in the region of the spectrum covered by filters F1 and F2, as shown (a) Location of the anti-Stokes Raman lines with respect to the filters' transmissivity (b) Raman calibration data obtained with filter 1 and filter 2, and linear fits of the experimental data.…”

Neutronic analysis of the ITER poloidal polarimeter

“…The LIDAR and ETS systems basically consist of a laser injection system and a light collection optical system. The MCNP model of the ETS system and its neutron shield are based on the latest designs [4]. However, the LIDAR system is currently under development.…”

“…Since the LIDAR system [4] and the Edge Thomson Scattering (ETS) system [5] are installed in the same EQ port plug, the measurement systems in this port plug are very complicated. In this work, a model for calculating the effects of the ETS and the LIDAR systems with those neutron shielding is made and combined into the EQ port plug together with the poloidal polarimeter system.…”

Electron Density Measurement by Raman Calibration of the HuanLiuqi-2A Thomson Scattering System

“…For nitrogen gas at T = 295 K, W = 463.11. The Raman wavelength of scattered line spectrum is given by [19],…”