Estimation of Plasma Light in Thomson Scattering System Designed for the COMPASS Tokamak

The COMPASS tokamak, originally from Culham laboratory, UKAEA, has been reinstalled at Institute of Plasma Physics, Prague. Many innovations will be done and new diagnostics have to be constructed. Electron temperature Te and density ne as key parameters of fusion plasma will be measured directly by the newly designed high spatial resolution incoherent Thomson scattering system. The scientific programme of reinstalled COMPASS is aimed at an edge plasma region, where these physical quantities have large gradients. Several construction options of TS are considered. In this paper these options are discussed from a point of view of data analysis and estimation of background radiation. Introduction Incoherent Thomson scattering (TS) is a direct diagnostic method for measurement the electron density ne and temperature Te of plasma. J. J. Thomson described the theory of Thomson scattering (TS) at the beginning of the 20 century [Thomson 1897, Thomson 1906]. Since the 1970's, TS has been widely used as a standard tool to detect high temperature plasmas inside tokamak reactors. If the wavelength of incident electromagnetic radiation is much shorter than the plasma Debye length, the radiation is scattered on free electrons. The scattered power is given by incoherent summation over the scattered power of the individual electrons. The scattering spectrum reflects the electron velocity distribution, from which the local electron temperature and density can be determined [Barth 1998]. A construction of the TS diagnostic for reinstalled COMPASS is demanding because of measurement of the whole plasma profile including a pedestal region (about COMPASS parameters see [Valovic 1997]). At plasma edge, both Te and ne are lower by an order of magnitude than in the centre, and consequently the intensity of the scattered light is low. Different lasers (mainly Nd:YAG or ruby laser) and detection systems (photomultipliers, avalanche photodiodes, CCD matrices) are considered for the construction of the TS system. The aim of current design studies at IPP Prague is to select the most suitable combination of the laser and of the detection system to be able to fulfil the scientific programme at the COMPASS tokamak.