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We introduce detailed numerical modelling of the fast neutral particles inside the duct of a new neutral beam injector (NBI) recently installed at the COMPASS tokamak (major radius R 0 = 0.56 m , vessel midplane minor radius a = 0.23 m , toroidal field B t = 0.9â€"2.1  T ). This new NBI system is able to deliver 1 M W power to the plasma at nominal injection energy of 80 k e V . Collisions with the background neutrals inside the beam duct give birth to fast ions according to the density of the gas and tabulated cross-sections. The ion trajectories are then computed in the complete 3D magnetic field, showing the importance of the stray magnetic field and the magnitude of the field within the gap in between toroidal coils.During the experimental campaign dedicated to the new 1 MW NBI, the beam duct heating was measured by a row of thermocouples located on the top-half of the beam duct. The fast ions collisions with the duct wall cause a local temperature increase with a characteristic pattern. In COMPASS, the location of the fast ions power deposition measured experimentally is in qualitative agreement with modelling of ion losses when following them after the re-ionization process. We trace back the details of the orbits corresponding to deposition at the hot-spot inside the beam duct. Quantitative comparison between the experiment and the simulation shows that a larger than expected amount of neutral gas was inside the duct and this study will guide the design of the future NBI duct in COMPASS Upgrade. [ABSTRACT FROM AUTHOR]

The COMPASS upgrade tokamak (Panek et al 2017 Fusion Eng. Des. 123 11–16) will be a tokamak of major radius R0 = 0.894 m with the possibility to reach high field (Bt ∼ 5 T) and high current (Ip ∼ 2 MA). The machine should see its first plasma in 2023 and H-mode plasma will be obtained from 2025. The main auxiliary heating system used to access H-mode will be 4 MW of neutral beam injection (NBI) power. The NBI will have a nominal injection energy of 80 keV, a maximum injection radius Rtan = 0.65 m and will create a population of well-confined energetic D ions. In this contribution, our modelling studies the NBI deposition and losses when a significant edge background density of neutrals is assumed. We follow the fast ions in the 3D field generated by the 16 toroidal field (TF) coils using the upgraded EBdyna orbit solver (Jaulmes et al 2014 Nucl. Fusion 54 104013). We have implemented a Coulomb collision operator similar to that of NUBEAM (Goldston et al 1981 J. Comput. Phys. 43 61) and a charge-exchange operator that follows neutrals and allows for multiple re-ionizations. Detailed integrated modelling with the METIS code (Artaud et al 2018 Nucl. Fusion 58 105001) yields the pressure and current profiles for various sets of achievable engineering parameters. The FIESTA code (Cunningham 2013 Fusion Eng. Des. 88 3238–3247) calculates the equilibrium and a Biot–Savart solver is used to calculate the intensity of the perturbation induced by the TF coils. Initial distributions of the NBI born fast ions are obtained from the newly developed NUR code, based on Suzuki et al (1998 Plasma Phys. Control. Fusion 40 2097). We evolve the NBI ions during the complete thermalization process and we calculate the amount of NBI ions loss in the edge region due to neutralizations. Results indicate the NBI losses for various injection geometries, various engineering parameters and various assumptions on the magnitude of the background neutral densities. [ABSTRACT FROM AUTHOR]

Measurements using a recently installed edge fast-ion D-alpha (FIDA) diagnostic at the ASDEX Upgrade tokamak show a clear effect of edge localised modes (ELMs) on the passive FIDA signals. While a reduction in the passive FIDA emission is observed in the scrape-off layer (SOL) region, measurements close to the last closed flux surface show an increase in signals shortly after ELMs, followed by a decrease. The decrease provides a clear sign of fast-ion losses in the SOL, while the increase can be explained by an enhanced neutral density during ELMs inside the plasma. In addition, small ELMs are observed, which barely change the neutral density and plasma position but still cause significant changes in the passive FIDA signals. A comparison of the measurements with forward modelling shows that 60% to 80% of the fast ions are lost by ELMs outside the last closed flux surface. In addition, a 20% decrease of the fast-ion density in a range up to 4 cm within the last closed flux surface can be inferred. This range agrees well with the latest modelling results of ELMs using the non-linear MHD code JOREK and shows that less than 0.3% of all fast ions are lost by ELMs. [ABSTRACT FROM AUTHOR]

Here we present a new method that allows the determination of background neutral density profiles based on measurements from neutral particle analyzers (NPA). Bayesian optimization is used to obtain a reliable 5-parameter representation of the inferred profiles. The method has been benchmarked using forward modelling from FIDASIM against measured data. The systematic errors coming from assumptions made in the analysis were evaluated and they are lower than ≲5%. The new method has been tested using data from the ASDEX Upgrade tokamak. When using reconstructed neutral density profiles, good agreement is found between the measured NPA fluxes of neutralized fast ions with predictions based on TRANSP and FIDASIM. Moreover, a clear drop of neutral density is observed at the plasma boundary after edge localized mode (ELM) activity. As suggested by KN1D simulations, this drop is mainly due to an increase of scrape-off layer (SOL) ionization rate, resulting from higher SOL plasma densities and temperatures after the ELM crash. Moreover, the new method allows calculating the local plasma ionization source which will be important for future transport studies. [ABSTRACT FROM AUTHOR]