Modelling of sawtooth-induced fast ion transport in positive and negative triangularity in TCV

Abstract

Internal kinks are a common magneto hydro-dynamic (MHD) instability observed in tokamak operation when the q profile in the plasma core is close to unity. This MHD instability impacts both the transport of the bulk plasma (current, particle and energy transport) and minority species, such as fast ions. In TCV (R 0 /a = 0.88 m/0.25 m) the fast ion population is generated in the plasma by neutral beam tangential injection of energies up to 28 keV. TCV features 16 active shaping coils permitting a great flexibility in plasma shape, including negative triangularity (δ) configurations that show surprisingly high confinement. This study focuses on the transport of fast ions induced by sawteeth, by comparing two triangularity cases and simulation results with experimental data. Comparison of two equilibria with opposite δ shows that the fast ion drifts are larger for δ < 0. Furthermore, the sawtooth-induced transport in this case is larger than δ > 0 in similar conditions. Comparison with experimental data confirms the dominance of the modification of thermal kinetic profiles following the sawtooth crash in explaining drops in the neutron rates and FIDA signals. Additional fast ion diffusion, however, improves the interpretation of the experimental data. For δ < 0, the amplitude of the perturbation better representing the experimental data is larger. Finaly, an exploratory study for 50 keV particles (soon available in TCV) shows that the situation does not worsen for such particles.

Development of a reduced model for energetic particle transport by sawteeth in tokamaks

“…For instance, frequent sawteeth tend to result in a q = 1 surface located closer to the magnetic axis than 'monster' sawteeth, thus frequent sawteeth will result in a smaller amplitude. This expected behavior will be tested on other devices in future publications [51,52].…”

Time-helicity de-resonation (T-H) diagram for energy-selective mixing of charged particles during sawtooth crashes in tokamaks

“…On the applied modeling side, there has been recent work on computationally efficient reduced fast ion transport models that took into account mechanisms responsible for energy-and pitch-selectivity of a sawtooth crash in order to improve the quantitative agreement with experimental measurements [17][18][19][20][21][22]. The methods proposed in the present paper may also be utilized in such efforts.…”

Excitation of toroidal Alfvén eigenmodes with counter-current NBI in the TCV tokamak

“…The Tokamak á Configuration Variable (TCV) [3]) is a flexible device equipped with two tangential neutral beam (NB) injectors [4], both with a nominal injected power of ⩾1 MW. The study on the fast ion population injected by such beams and on the interaction with MHD modes has been explored in [5][6][7]. TCV is also equipped with electron cyclotron emission (ECE) diagnostic and short pulse reflectometry [8], which allow to measure the fine fluctuations of electron temperature and density and thus reconstruct the MHD mode spatial structure.…”