Exploitation of Avalanche Photodiodes for Thomson Scattering Diagnostics in Tokamaks

The COMPASS tokamak, originally from UKAEA, Culham laboratory, U.K., will be reinstalled at Institute of Plasma Physics, Prague. The scientific programme is focused mainly on the investigation of the edge plasma region with high temperature and density gradients called pedestal. Many innovations will be done and new diagnostics have to be constructed. One of the key diagnostics is an incoherent Thomson scattering for measurements of electron temperature Te and density ne with a high spatial resolution. Several construction options are considered. In this paper a possibility of using a Nd:YAG laser in combination with a detection system based on Avalanche Photodiodes is discussed. Introduction Thomson scattering is one of direct diagnostic methods for measuring the electron density ne and temperature Te of plasma. It became a standard tool for diagnosing high temperature plasmas in tokamaks. Theory of Thomson scattering (TS) was described first by J.J. Thomson at beginning of the 20 century. If the wave vector of incident electromagnetic radiation is much longer 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 [Thomson 1897, Thomson 1906, Barth 1998]. In many fusion devices, Thomson scattering diagnostic is used only for the central region of plasma column. Measurements at the plasma edge are much more demanding, because both Te and ne are lower by an order of magnitude, and consequently the intensity of the scattered light is low. The construction of TS systems is based on different lasers [Beurskens 1999], mainly on Neodym-Yag or ruby laser and combined with several detection systems. Avalanche photodiodes (APD) or ChargedCoupled Devices (CCD) are used for the detection of the scattered light. The Table 1 summarizes status of the TS diagnostics on European tokamaks. Table 1. Overview of TS on major European fusion devices. Facility Location LASER (energy and wavelength) Detection system ASDEX-Upgrade Germany, Garching Nd-Yag (1064nm, 2J) APD JET UK, Culham Nd-Yag (1064nm, 5J), Ruby (694nm, 3J) APD MAST UK, Culham Nd-Yag (1064nm, 1,2J) APD RFX Italy, Padova Nd-Yag, Ruby (694nm, 15J) APD TCV Switterland,Lausanne Nd-Yag (1064nm, 1,8J) APD TEXTOR Germany, Julich Nd-Yag, Ruby (694nm, 10-12J) CCD TJ-II Spain, Madrid Ruby (694nm, 10J) CCD WDS'07 Proceedings of Contributed Papers, Part II, 224–228, 2007. ISBN 978-80-7378-024-1 © MATFYZPRESS