Speaker
Description
A numerical tool modelling the excitation and evolution of electron avalanche ionization in the breakdown phase of start-up in tokamaks is presented. We estimate the energization efficiency of the nonlinear interaction between spatially localized Gaussian EC-fields propagating in vacuum with an ensemble of seed electrons. This process is coupled with the acceleration of electrons due to the induced loop voltage along the vacuum vessel, as well as the impact ionization and elastic collision events that lead to the abrupt increase of electron density during the avalanche process. Special care is taken to incorporate the effect of the toroidal magnetic field in the collision statistics [1]. Several numerical experiments are performed for configurations relevant to existing tokamaks [2] as well as ITER [3], dealing with all alternative start-up initiation procedures (ECRH pre-ionization [4], ECRH-assisted [5] and ohmic [6]). A simple analytical auxiliary tool based on the dynamics of avalanche evolution is developed in order to estimate the breakdown time as a function of the RF-field parameters, the loop voltage and the prefill pressure of the neutral gas.
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.
References:
[1] P. Ch. Papagiannis, etal. Proc. of the 48rd EPS Conf. on Plasma Physics, P5a.109 (2022)
[2] J. Stober etal., Nucl. Fusion, 51, 083031 (2011)
[3] P.C. de Vries and Y. Gribov, Nucl. Fusion, 59, 096043 (2019)
[4] J. Sinha etal. Nucl. Fusion 62, 6206601 (2022)
[5] D. Ricci, etal. Proc. of the 45th EPS Conf. on Plasma Physics, P4.1074 (2018)
[6] P.C. de Vries et al Nucl. Fusion 53, 053003 (2013)
