The Academy of Sciences of the Czech rebublic - ASCR


Thomson scattering diagnostics

Measured quantities:

Electron temperature, electron density

Spatial resolution:
core view: 9.8 - 14 mm (field of view 16 mm below midplane - 211 mm above midplane) 
edge view: 4 - 6 mm (field of view 195 mm - 284 mm above midplane)
Temporal resolution: 30 Hz with double probing energy
60 Hz
two measurements with variable time gap (1 μs – 17 ms) running at 30 Hz
Responsible person:

P. Bílková, M. Aftanas, P. Böhm

Collaboration:

Electron Kinetics Group, MAST Tokamak, Culham Centre for Fusion Energy, Abingdon, UK
Consorzio RFX, Padova, Italy
TEXTOR tokamak:

FOM-Institute for Plasma Physics Rijnhuizen, Nieuwegein, Netherlands

Institute for Energy Research – Plasma Physics, Forschungszentrum Jülich GmbH, Germany

Department of Control Engineering, Faculty of Electrical Engineering, Czech Technical University, Prague, Czech Republic

 

Diagnostic description:

 

  • Thomson scattering is a laser-aided diagnostic widely used in the field of high-temperature plasma diagnostics

  • Perform well localized measurements of the electron temperature and density in the plasma

  • Scattering of monochromatic electromagnetic radiation from hot electrons in a plasma leads to a spectral broadening of the scattering spectrum due to the Doppler effect

  • Electron temperature is determined from a spectral width of the Thomson scattered signal, electron density from scattered signal intensity

 

Experimental setup:

 

Scheme of Thomson scattering diagnostics

 

  • Two Nd:YAG lasers (1064 nm): 1.5 J energy, 7 ns pulse duration, 30 Hz repetition rate each

  • Two objectives for collecting the scattered light from both the core and the edge plasma regions. The light is then focused on bundles of optical fibres

  • Optical fibres to transfer the scattered light to polychromators:

    • 12 bundles of fibres for core plasma region

    • 16 bundles of optical fibres to for the edge plasma region

    • 1 special bundle for alignment of the diagnostic

  • Polychromators for spectral analysis of the scattered light

    • 28 polychromators analyzing the light from 56 spatial points in plasma (2 adjacent spatial points served by one polychromator) and 1 polychromator used for both alignment and the light spectral analysis

    • Spectral analysis done by a set of five spectral filters specifically designed for the range of temperatures of 10 eV – 5 keV

    • Detectors - Avalanche photodiodes possessing enhanced infrared sensitivity, developed specifically for the needs of this diagnostic

  • Fast analog-digital converters (ADCs) for digitization of analog signals coming from detectors

    • Signals (120 spectral channels) from polychromators are synchronously digitalized by fast and slow ADCs

    • The fast ADCs convert data with high throughput of 1 GSample/sec and 8 bit resolution

    • The slow digitizers: 16 bit ADC per channel with sampling rate of 500 kSample/sec

 

Spatial profiles measured by Thomson scattering for  shot No 6962.

 

Fig.1: Spatial profiles measured by Thomson scattering for shot No 6962. From the top: Electron temperature, electron pressure and electron density.