Multiphysics approach to plasma neutron source modelling at the JET tokamak

A novel multiphysics methodology for the computation of realistic plasma neutron sources has been developed. The method is based on state-of-the-art plasma transport and neutron spectrum calculations, coupled with a Monte Carlo neutron transport code, bridging the gap between plasma physics and neutronics. In the paper two JET neutronics tokamak models are used to demonstrate the application of the developed plasma neutron sources and validate them. Diagnostic data for the record JET D discharge 92436 are used as input for the TRANSP code, modelling neutron emission in two external plasma heating scenarios, namely using only neutral beam injection and a combination of the latter and ion cyclotron resonance heating. Neutron spectra, based on plasma transport results, are computed using the DRESS code. The developed PLANET code package is employed to generate plasma neutron source descriptions and couple them with the MCNP code. The effects of using the developed sources in neutron transport calculations on the response of JET neutron diagnostic systems is studied and compared to the results obtained with a generic plasma neutron source. It is shown that, although there are significant differences in the emissivity profiles, spectra shape and anisotropy between the neutron sources, the integral response of the time-resolved ex-vessel neutron detectors is largely insensitive to source changes, with major relative deviations of up to several percent. However it is calculated that, due to the broadening of neutron spectra as a consequence of external plasma heating, larger differences may occur in activation of materials which have threshold reactions located at DD neutron peak energies. The PLANET plasma neutron source computational methodology is demonstrated to be suitable for detailed neutron source effect studies on JET during DT experiments and can be applied to ITER analyses.

Generation and observation of fast deuterium ions and fusion-born alpha particles in JET D-He-3 plasmas with the 3-ion radio-frequency heating scenario

Javier García Hernández, Alberto Mariani, Henri Weisen

Dedicated experiments to generate energetic D ions and D-(3) He fusion-born alpha particles were performed at the Joint European Torus (JET) with the ITER-like wall (ILW). Using the 3-ion D-(D-NBI)-(3) He radio frequency (RF) heating scenario, deuterium ions from neutral beam injection (NBI) were accelerated in the core of mixed D-(3) He plasmas to higher energies with ion cyclotron resonance frequency (ICRF) waves, in turn leading to a core-localized source of alpha particles. The fast-ion distribution of RF-accelerated D-NBI ions was controlled by varying the ICRF and NBI power (P-ICRF approximate to 4-6 MW, P-NBI approximate to 3-20 MW), resulting in rather high D-D neutron (approximate to 1x10(16) s(-1)) and D-(3) He alpha rates (approximate to 2x10(16) s(-1)) at moderate input heating power. Theory and TRANSP analysis shows that large populations of co-passing MeV-range D ions were generated using the D-(D-NBI)-(3) He 3-ion ICRF scenario. This important result is corroborated by several experimental observations, in particular gamma-ray measurements. The developed experimental scenario at JET provides unique conditions for probing several aspects of future burning plasmas, such as the contribution from MeV range ions to global confinement, but without introducing tritium. Dominant fast-ion core electron heating with T-i approximate to T-e and a rich variety of fast-ion driven Alfven eigenmodes (AEs) were observed in these D-(3) He plasmas. The observed AE activities do not have a detrimental effect on the thermal confinement and, in some cases, may be driven by the fusion born alpha particles. A strong continuous increase in neutron rate was observed during long-period sawteeth (>1 s), accompanied by the observation of reversed shear AEs, which implies that a non monotonic q profile was systematically developed in these plasmas, sustained by the large fast-ion populations generated by the 3-ion ICRF scenario.

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

Marcelo Baquero Ruiz, Basil Duval, Alexander Karpushov, Antoine Pierre Emmanuel Alexis Merle, Dmytry Mykytchuk, Mario Podesta, Olivier Sauter, Lorenzo Stipani, Duccio Testa, Matteo Vallar

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 tokamak a configuration variable (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 (delta) 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 delta shows that the fast ion drifts are larger for delta < 0. Furthermore, the sawtooth-induced transport in this case is larger than delta > 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 fast ion D-alpha signals. Additional fast ion diffusion, however, improves the interpretation of the experimental data. For delta < 0, the amplitude of the perturbation better representing the experimental data is larger. Finally, an exploratory study for 50 keV particles (soon available in TCV) shows that the situation does not worsen for such particles.

IOP Publishing Ltd2022

Anomalous Diffusion of Fast Ions in the Presence of Fishbone Instabilities in Tokamaks

Alexandre Dominique Bovet

Fast ions, whether produced by fusion reactions, or by ionisation of neutral beam, are expected to play a major role in the heating of burning plasmas. Therefore, the study of the fast ion’s behaviour in tokamaks is important for the future burning plasma experiments such as ITER. Understanding the effect of neutral beam injection on the current profile is also important for current profile control and for achieving steady state scenarios where non-inductive current drive is necessary. Several plasma discharges have been carried out in the MAST tokamak to investigate to which extent the q-profile may be modified by neutral beam current drive (NBCD). Transp simulations of the beam deposition during steady state experiments, with off-axis NBCD, have been carried out. It has been found that an anomalous diffusion (with a diffusion coef- ficient of roughly Db ∼ 0.5 m2 /s) of the fast ion is needed to explain the significantly lower neutron rate measured than predicted by the Transp code using an assumption of classical beam deposition and collisional ther- malisation. Transp simulations show that this diffusion broadens the fast ion deposition profile and may help to avoid harmful instabilities [1]. This anomalous diffusion is suspected to be caused by fishbone instabilities, as the time of the largest discrepancy between simulated and measured neutron rates correlates well with the highest magnitude of fishbone activity. The aim of this work is to investigate, with simulations of the Hagis code, if the interaction between fast ion resulting from off-axis NBCD and fishbone instabilities may be responsible for the fast ion anomalous diffusion needed to explain the observed neutron rate.

2009

Generation and observation of fast deuterium ions and fusion-born alpha particles in JET D-He-3 plasmas with the 3-ion radio-frequency heating scenario

Javier García Hernández, Alberto Mariani, Henri Weisen

IOP PUBLISHING LTD2020