Nonlinear contribution of neutral beam injection in TCV EC - heated advanced tokamak scenarios

TCV (Tokamak à Configuration Variable) is a tokamak device capable of many different plasma shape s and positions, equipped with a flexible system of Electron Cyclotron (EC) antennas and a new Neutral Beam (NB) injector [1] . The auxiliary power from the beam can reach 1 MW and it is injected tangentially co - current , coupling mostly with ions . This heating system allows n e w insights on advanced tokamak scenarios in TCV which, up to now , have been performed only with EC heating (ECH) . These scenarios have high ? N , high non - inductive current fraction and a relevant energetic particle (EP) population fraction (?10 %) . An internal transport barrier can be generated by reversing the q - profile using EC current - drive (ECCD) [2] . In this work we show that the effect of the sum of the two heating sources (NBI and EC H ) in TCV high ? N plasmas is not linear, and interpretative modelling is carried out to understand the behaviour of the NB EP s when ECH is present . A statistical study on a set of experiments with both ECH and NBI is presented to show t he effect of NB injection (NBI) on plasma performance: ? N and the plasma stored energy do n o t increase linearly with NB power. Furthermore, the contribution to the total plasma current from ohmic transformer, bootstra p current and current drive are respectively estimated , showing that EC has a strong impact on Z eff , modifying therefore the plasma resistivity and the ohmic contribution to the current. This effect is taken into account when applying the Monte Carlo inter pretative NBI code NUBEAM. It results that with the combined application of ECRH and ECCD , the electron temperature and plasma equilibrium change significantly, impacting on the NB power deposition: CX and orbit losses tend to increase, reducing by 20% the power deposited on plasma species. Modelling suggests that the variation in T e changes the EP power redistribution among the species, transfe rring more power to the ions. The impact on EP s orbit given by ECCD equilibrium modification is performed with the Monte Carlo ASCOT code for NBI modelling , capable of solving the EP full gyro - motion.