Simulating the impact of charge exchange on beam ions in MAST-U

A model for simulating charge exchange (CX) of fast ions with background atoms in magnetically confined fusion plasmas has been implemented in the ASCOT orbit-following code. The model was verified by comparing simulated reaction mean free paths to analytical values across a range of fusion-relevant parameters. ASCOT was used to simulate beam ions slowing down in the presence of CX reactions in a MAST-U target scenario. ASCOT predicts the CX-induced loss of beam power to be 22%, which agrees to within 15% with the TRANSP prediction. Because of CX, plasma heating and current drive by beam ions are strongly reduced towards the edge. However, an overall lower but noticeable increase of up to 20% in current drive is predicted closer to the core. The simulated deposition of fast CX atoms on the wall is concentrated around the outer midplane, with estimated peak power loads of 70–80 kWm-2 on the central poloidal field coils (P5) and the vacuum vessel wall between them. This analysis demonstrates that ASCOT can be used to simulate fast ions in fusion plasmas where CX reactions play a significant role, e.g., in spherical tokamaks and stellarators.

Alfvén cascade eigenmodes above the TAE-frequency and localization of Alfvén modes in D- 3 He plasmas on JET

Various types of Alfvén Eigenmodes (AEs) have been destabilized by fast ions over a broad frequency range from ~80 kHz to ~700 kHz in a series of JET experiments in mixed D-3He plasmas heated with the three-ion ICRF scenario [M. Nocente et al., Nucl. Fusion 60, 124006 (2020)]. In this paper, we identify the radial localization of AEs using an X-mode reflectometer, a multiline interferometer and soft X-ray diagnostics. The analysis is focused on the most representative example of these measurements in JET pulse #95691, where two different types of Alfvén cascade (AC) eigenmodes were observed. These modes originate from the presence of a local minimum of the safety factor qmin. In addition to ACs with frequencies below the frequency of toroidal Alfvén eigenmodes (TAEs), ACs with frequencies above the TAE frequency were destabilized by energetic ions. Both low- (f ≈80-180 kHz) and high-frequency (f ≈ 330-450 kHz) ACs were localized in the central regions of the plasma. The characteristics of the high-frequency ACs are investigated in detail numerically using HELENA, CSCAS and MISHKA codes. The resonant conditions for the mode excitation are found to be determined by passing ions of rather high energy of several hundred keV and similar to those established in JT-60U with negative-ion-based NBI [M. Takechi et al., Phys. Plasmas 12, 082509 (2005)]. The computed radial mode structure is found to be consistent with the experimental measurements. In contrast to low-frequency ACs observed most often, the frequency of the high-frequency ACs decreases with time as the value of qmin decreases. This feature is in a qualitative agreement with the analytical model of the high-frequency ACs in [B.N. Breizman et al., Phys. Plasmas 10 3649 (2003)]. The high-frequency AC could be highly relevant for future ITER and fusion reactor plasmas dominated by ~ MeV energetic ions, including a significant population of passing fast ions.

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

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.

Neutron energy spectrum measurement using CLYC7-based compact neutron emission spectrometer in the Large Helical Device

Tangential compact neutron emission spectrometer (CNES) based on the Cs2LiYCl6:Ce with 7Li-enrichment (CLYC7) scintillator is newly installed in the Large Helical Device (LHD). Measurement of neutron energy spectrum was performed using CNES in tangential neutral beam (NB) heated deuterium plasma discharges. The Doppler shift of neutron energy according to the direction of tangential NB injection has been obtained. When the fast ions moving away from the CNES, lower shifted neutron energy is obtained, whereas the upper shifted neutron energy is obtained when the fast ions moving toward the CNES. The obtained neutron energy is almost consistent with the virgin deuterium-deuterium neutron energy evaluated by the simple two-body kinematic calculation.

Orbit Tomography of energetic particle distribution functions

Both fast ions and runaway electrons are described by distribution functions, the understanding of which are of critical importance for the success of future fusion devices such as ITER. Typically, energetic particle diagnostics are only sensitive to a limited subsection of the energetic particle phase-space which is often insufficient for model validation. However, previous publications show that multiple measurements of a single spatially localized volume can be used to reconstruct a distribution function of the energetic particle velocity-space by using the diagnostics' velocity-space weight functions, i.e. Velocity-space Tomography. In this work we use the recently formulated orbit weight functions to remove the restriction of spatially localized measurements and present Orbit Tomography, which is used to reconstruct the 3D phase-space distribution of all energetic particle orbits in the plasma. Through a transformation of the orbit distribution, the full energetic particle distribution function can be determined in the standard {energy,pitch,r,z}-space. We benchmark the technique by reconstructing the fast-ion distribution function of an MHD-quiescent DIII-D discharge using synthetic and experimental FIDA measurements. We also use the method to study the redistribution of fast ions during a sawtooth crash at ASDEX Upgrade using FIDA measurements. Finally, a comparison between the Orbit Tomography and Velocity-space Tomography is shown.

Analyzing Fast-ions Trajectories in a Nuclear Fusion Reactor through Its Poincaré-Island Size and Ripple Resonance

Fast-ions confinement is a prominent subject in developing nuclear fusion reactors due to its importance in sustaining the burning plasma and keeping energy production. However, confining them has proven to be difficult until now, and one of the reasons is that the inherent discrete magnetic field produces a magnetic ripple. A better understanding of fast-ions transport using appropriate numerical calculation tools needs to be developed to overcome such a challenge in the engineering aspect. This study revisited data collection of fast ion transport simulated under the ripple presence in a nuclear fusion device. The ion trajectories were followed using two orbit-following equation schemes, and the ripple-resonance island size in the Poincaré section was compared. The result showed that the island size obtained by each scheme was different when the particle resonates with a stronger ripple field and, proportionally, the diffusion coefficients are different. The physical meaning and consequence behind this discovery were discussed in this paper.