Runaway electron mitigation by resonant and non-resonant magnetic perturbations in RFX-mod tokamak discharges

Understanding the mechanisms behind Runaway Electron (RE) generation and the ways RE formation can be prevented is of paramount importance for future fusion reactors. The injection of massive gas into the plasma is the main solution presently envisaged to avoid the RE avalanche. Also other strategies have been developed to enhance the RE decorrelation as the application of external magnetic perturbations (MP) [1]. The RFX-mod device [2], which can be run as a Tokamak, is equipped with an advanced system for the control of Magnetohydrodynamic modes based on 192 saddle coils independently fed and on a state of the art real-time hardware and software architecture [3]; for this reason, it is particularly suited to study the possible impact on RE de-confinement in response of applied magnetic perturbations with different modal numbers and amplitude. The presence and the amount of fast electrons in RFX-mod can be revealed and analyzed by several diagnostics like the soft-xray tomography, blind CCD cameras and in particular 2 scintillators, which allow the detection of hard-x-rays produced when the high energy electrons impact with the wall. This work presents the necessary conditions for mitigation and control of RE via MP at different density levels have been studied, thus contributing to the Tokamak scaling laws [4]. Recent experiments are promising: we find that RE losses are significantly enhanced in low-q plasmas when the applied 3D field resonates with the m=2/n=1 external kink; in this case the plasma response amplifies the external field and thus increases the MP decorrelation efficacy. Resonant field amplification effects may play a role in RE decorrelation also in larger tokamaks. The mechanisms generating RE and the effect of the MP on their confinement are interpreted by the ORBIT code [5], which integrates the guiding center motion equations of test particles and has been recently updated to a relativistic version. The role played by different magnetic equilibria, hence also of different resonant modes, on the energy of RE and on their loss rates is investigated. ORBIT simulations indicate that RE enhanced losses are associated to a raised level of stochasticity, the effect being more pronounced when the MP amplitude is higher and internally resonant. [1] Lehnen et al. PRL 100. 255003 (2008) [2] Sonato P et al 2003 Fusion Eng. Des. 66 161 [3] Marrelli L et al 2007 Plasma Phys. Control. Fusion 49 B359 [4] Granetz R.S. et al 2014 Phys. Plasmas 21 072506 [5] White R.B. et al.1984 Phys. Fluids 27 2455