Overview of Active Beam Spectroscopy developments for ITER

Abstract

Active Beam Spectroscopy diagnostics on ITER measure spectra of light emitted due to plasma interaction with the beams of neutral hydrogen isotopes injected into the plasma. They fall in roughly 3 groups: -Charge Exchange Recombination Spectroscopy (CXRS), analyses line emission of plasma ions that receive electrons from the neutral beam atoms. The intensity, Doppler width and Doppler shift reveal information on the plasma ion density (typically an impurity), the ion temperature and the bulk ion velocity, respectively. -Motional Stark Effect (MSE), analyses Stark split line emission of the beam atoms itself due to collision with the background plasma. The Stark split is a result of the Lorentz electric field experienced by the beam atoms due to their motion across the magnetic field. Both the polarization of and the wavelength separation of the emission lines reveal information about the local magnetic field that provides powerful constraints to the plasma equilibrium reconstruction. -Beam Emission Spectroscopy fluctuations (BES), provides a fast (~ several 100 kHz bandwidth) measurement of the (filtered) intensity of the above mentioned MSE emission. The intensity fluctuations are proportional to electron density fluctuations due to turbulence and MHD modes. On ITER, active beam spectroscopy is faced with several challenges that are less present in current day devices: dominating background emission, partly due to reflection on the metallic walls and potentially polarized, low signal due to strong neutral beam attenuation, deposition of coatings on first mirrors, compliance with nuclear regulations and remote handling, limited access for in-vessel calibration et cetera. Presented in this contribution are an overview of the main challenges and the current status in research, development and design of the ITER active beam spectroscopy systems. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization