Wall reflection issues for optical diagnostics in fusion devices

Simulation of Spectra Code (SOS) for ITER Active Beam Spectroscopy

The concept and structure of the Simulation of Spectra (SOS) code is described starting with an introduction to the physics background of the project and the development of a simulation tool enabling the modeling of charge-exchange recombination spectroscopy (CXRS) and associated passive background spectra observed in hot fusion plasmas. The generic structure of the code implies its general applicability to any fusion device, the development is indeed based on over two decades of spectroscopic observations and validation of derived plasma data. Four main types of active spectra are addressed in SOS. The first type represents thermal low-Z impurity ions and the associated spectral background. The second type of spectra represent slowing-down high energy ions created from either thermo-nuclear fusion reactions or ions from injected high energy neutral beams. Two other modules are dedicated to CXRS spectra representing bulk plasma ions (H+, D+, or T+) and beam emission spectroscopy (BES) or Motional Stark Effect (MSE) spectrum appearing in the same spectral range. The main part of the paper describes the physics background for the underlying emission processes: active and passive CXRS emission, continuum radiation, edge line emission, halo and plume effect, or finally the charge exchange (CX) cross-section effects on line shapes. The description is summarized by modeling the fast ions emissions, e.g., either of the α particles of the fusion reaction or of the beam ions itself.

A photoelastic-modulator-based motional Stark effect polarimeter for ITER that is insensitive to polarized broadband background reflections

A motional Stark effect polarimeter insensitive to polarized broadband light is proposed. Partially polarized background light is anticipated to be a significant source of systematic error for the ITER polarimeter. The proposed polarimeter is based on the standard dual photoelastic modulator approach, but with the introduction of a birefringent delay plate, it generates a sinusoidal spectral filter instead of the usual narrowband filter. The period of the filter is chosen to match the spacing of the orthogonally polarized Stark effect components, thereby increasing the effective signal level, but resulting in the destructive interference of the broadband polarized light. The theoretical response of the system to an ITER like spectrum is calculated and the broadband polarization tolerance is verified experimentally.

Resolving small signal measurements in experimental plasma environments using calibrated subtraction of noise signals

The performance of many diagnostic and control systems within fusion and other fields of research are often detrimentally affected by spurious noise signals. This is particularly true for those (such as radiation or particle detectors) working with very small signals. Common sources of radiated and conducted noise in experimental fusion environments include the plasma itself and instrumentation. The noise complicates data analysis, as illustrated by noise on signals measured with the heavy ion beam probe (HIBP) installed on the Madison Symmetric Torus. The noise is time-varying and often exceeds the secondary ion beam current (in contrast with previous applications). Analysis of the noise identifies the dominant source as photoelectric emission from the detectors induced by ultraviolet light from the plasma. This has led to the development of a calibrated subtraction technique, which largely removes the undesired temporal noise signals from data. The advantages of the technique for small signal measurement applications are demonstrated through improvements realized on HIBP fluctuation measurements.

Plasma tomographic reconstruction from tangentially viewing camera with background subtraction

Light reflections are one of the main and often underestimated issues of plasma emissivity reconstruction in visible light spectral range. Metallic and other specular components of tokamak generate systematic errors in the optical measurements that could lead to wrong interpretation of data. Our analysis is performed at data from the tokamak COMPASS. It is a D-shaped tokamak with specular metallic vessel and possibility of the H-mode plasma. Data from fast visible light camera were used for tomographic reconstruction with background reflections subtraction to study plasma boundary. In this article, we show that despite highly specular tokamak wall, it is possible to obtain a realistic reconstruction. The developed algorithm shows robust results despite of systematic errors in the optical measurements and calibration. The motivation is to obtain an independent estimate of the plasma boundary shape.