Hybrid Garfield++ simulations of GEM detectors for tokamak plasma radiation monitoring

Abstract

This work presents recent efforts in optimization of Gas Electron Multiplier (GEM) detector simulations developed so far. The mentioned triple-GEM based measurement systems were already tested at the WEST project and other devices recording soft X-ray (SXR) plasma radiation in the target range of 2-15 keV. Long term plans may envisage usage of gaseous detector based SXR diagnostics in DEMO reactor for real time plasma monitoring and control, as high neutron fluxes preclude application of semiconductor-based technologies. Conditions in fusion reactors as well as in currently operating devices necessitate simulating the interactions of high-energy photons and neutrons with the detector parts, including proper handling of primary ionization electrons paths, gas mixture fluorescence and collisions with detector materials. For this use case a full-detector simulation software was created based on the already existing interface and previous results obtained so far within the group. It combines Geant4 package for interactions with solid parts of the detector, Garfield++ for electron avalanching, Heed for X-ray interaction with gas, Gmsh and Elmer for meshing and electrostatics and introduces hybrid approach to the simulation of electron drift in different manner depending on the electron position with respect to multiplication stage. To optimize the computation time software employs precomputation algorithms in later stages of the electron cascades. The expected response profiles of the triple-GEM detector, exposed to 55Fe calibration source radiation, were simulated and verified by comparing them with experimental data.