Design of Filters for COMPASS Thomson Scattering Diagnostics

Reinstalled tokamak COMPASS, originally from Culham Science Centre, is now operating at the Institute of Plasma Physics, Prague and many new optical diagnostics are being developed [Sestak et al, 2009]. For measurement of electron temperature and density profiles, high resolution incoherent Thomson scattering spectroscopy based on Nd:YAG laser and avalanche photodiodes is under development now. The paper outlines part of the design of this diagnostic proper choice of spectral filters. The design of filters is shown and its evaluation taking into account different angle of scattered light is introduced. Error bars of the electron temperature, density and pressure are determined with respect to assumed electron temperature and density profiles and to required high spatial resolution at the pedestal region. Introduction Thomson scattering (TS) is a direct diagnostic method for measuring the electron density ne and the electron temperature Te in plasma. It is a standard tool for diagnosing high temperature plasmas in tokamaks. Theory of TS was described first by J.J. Thomson [1897, 1906] at beginning of the 20th century. Tokamak COMPASS [Panek et al., 2006] is a divertor tokamak. It is the smallest tokamak (R = 0.56 m, a = 0.23 m) with ITER like geometry and a clear H-mode. Two (2 x 300 kW) neutral beam injectors (NBI) and (400 kW) lower hybrid heating (LHH) system will be installed to achieve ITER relevant plasma conditions. COMPASS is also equipped with a set of saddle coils for resonant perturbation techniques. The scientific programme is focused on pedestal physics and wave-plasma interaction studies. Also resonant perturbation of magnetic field will be studied. New TS system will contribute to this scientific programme by measuring the electron temperature and density radial profiles. Thomson scattering theory If the frequency π of incident wave is sufficiently low that hπ is much less than the rest energy of the charge mc this interaction is generally called Thomson scattering. The scattered power is given by incoherent summation over the scattered power of the individual electrons. Due to the Doppler effect the scattering spectrum reflects the electron velocity distribution (Figure 1), from which the local electron temperature and density can be determined [Barth, 1998]. Using this diagnostic for Te and ne measurement at the plasma edge is quite demanding due to low scattering cross-section and low ne, (Figure 2), consequently the intensity of the scattered light is low. TS for COMPASS The new COMPASS TS system will be based on Nd:YAG laser system (two 1.6 J and 30 Hz lasers) and the detection part consisting of polychromators equipped with spectral filters and avalanche photodiodes (APD) . Te and ne profiles (Figure 2) will be measured along a vertical chord for these plasma parameters: Te = 20 – 5000 eV, ne ≥ 5·10 m.Two sets of collecting optics, one for the core and one for the edge plasma with spatial resolution of 10 mm and 3 mm respectively will be used. The light will be focused on optical fibre bundles and will be bring to polychromators (Figure 3). 144 WDS'09 Proceedings of Contributed Papers, Part II, 144–147, 2009. ISBN 978-80-7378-102-6 © MATFYZPRESS