Nuclear fusion could offer clean, abundant energy. However, managing the power exhausted from the core fusion plasma towards the reactor wall remains a major challenge. This is compounded in emerging compact reactor designs promising more cost-effective pathways towards commercial fusion energy. Alternative Divertor Configurations (ADCs) are a potential solution. In this work, we demonstrate exhaust control in ADCs, employing a novel method to diagnose the neutral gas buffer, which shields the target. Our work on the Mega Ampere Spherical Tokamak Upgrade shows that ADCs tackle key risks and uncertainties for fusion energy. Their highly reduced sensitivity to perturbations enables active exhaust control in otherwise unfeasible situations and facilitates an increased passive absorption of transients, which would otherwise damage the divertor. We observe a strong decoupling of each divertor from other reactor regions, enabling near-independent control of the divertors and core plasma. Our work showcases the real-world benefits of ADCs for effective heat load management in fusion power reactors.

Ion cyclotron resonance heating is a versatile heating method that has been demonstrated to be able to efficiently couple power directly to the ions via the fast magnetosonic wave. However, at temperatures relevant for reactor grade devices such as DEMO, electron damping becomes increasingly important. To reduce electron damping, it is possible to use an antenna with a power spectrum dominated by low parallel wavenumbers. Moreover, using an antenna with a unidirectional spectrum, such as a travelling wave array antenna, the parallel wavenumber can be downshifted by mounting the antenna in an elevated position relative to the equatorial plane. This downshift can potentially enhance ion heating as well as fast wave current drive efficiency. Thus, such a system could benefit ion heating during the ramp-up phase and be used for current drive during flat-top operation. To test this principle, both ion heating and current drive have been simulated in a DEMO-like plasma for a few different mounting positions of the antenna using the FEMIC code. We find that moving the antenna off the equatorial plane makes ion heating more efficient for all considered plasma temperatures at the expense of on-axis heating. Moreover, although current drive efficiency is enhanced, electron damping is reduced for lower mode numbers, thus reducing the driven current in this part of the spectrum.

Ion cyclotron resonance heating (ICRH) has the potential of providing efficient ion heating of reactor grade fusion plasmas especially during the start-up phase. In order to assess such heating scenarios, ICRH modelling is required. However, the physics is complex and certain elements are not universally taken into account in ICRH modelling. In this paper we discuss the importance of including Doppler shift displacements of resonance points away from the cold resonance (i.e. where ω = n Ω c ) in Fokker-Planck calculations of the distribution function of resonating ions. In particular, the resonant interaction time and the wave electric field varies with the local Doppler shifted resonance positions. The importance of accounting for these variations in Fokker-Planck modelling is investigated. Furthermore, it is shown how these effects can be included in a simplified Fokker-Planck treatment that is sufficiently quick for integrated modelling frameworks of fusion plasmas. Because 2D effects in velocity space play a crucial role in determining Doppler shifts, we employ a model of the anisotropy of the non-thermal distribution function. Simulation results show that taking the Doppler effects into account in Fokker-Planck modelling can have a significant impact on the distribution functions of fast ions and important quantities, such as the collisional power transfer to the background plasma. This is especially important in cases where the poloidal variation of the left-hand component of the wave electric field is strong.

The objective of thermonuclear fusion consists of producing electricity from the coalescence of light nuclei in high temperature plasmas. The most promising route to fusion envisages the confinement of such plasmas with magnetic fields, whose most studied configuration is the tokamak. Disruptions are catastrophic collapses affecting all tokamak devices and one of the main potential showstoppers on the route to a commercial reactor. In this work we report how, deploying innovative analysis methods on thousands of JET experiments covering the isotopic compositions from hydrogen to full tritium and including the major D-T campaign, the nature of the various forms of collapse is investigated in all phases of the discharges. An original approach to proximity detection has been developed, which allows determining both the probability of and the time interval remaining before an incoming disruption, with adaptive, from scratch, real time compatible techniques. The results indicate that physics based prediction and control tools can be developed, to deploy realistic strategies of disruption avoidance and prevention, meeting the requirements of the next generation of devices.

This paper presents a new Monte Carlo algorithm intended for use in orbit following Monte Carlo codes (OFMC) to describe resonant interaction of ions with Radio Frequency (RF) waves in axi-symmetric toroidal plasmas. The algorithm is based on a quasi-linear description of the wave–particle interaction and its effect on the distribution function of a resonating ion species. The algorithm outlined in the present paper utilises a two-step approach for the evaluation of the Monte Carlo operator that has better efficiency and a stronger convergence than the standard Euler–Maruyama scheme. The algorithm preserves the reciprocity of the diffusion process. Furthermore, it simplifies how the displacement of the resonance position, as a result of wave–particle interaction, is accounted for. Such displacements can have a noticeable effect on the deterministic part of the Monte Carlo operator. The fundamental nature of guiding centre displacements of resonant ions as a result of wave–particle interaction is reviewed.

The PION code has been integrated into the European Transport Solver (ETS) transport workflow, and we present the first application to model Ion Cyclotron Resonance Frequency (ICRF) heating scenarios in the next-step fusion reactor ITER. We present results of predictive, self-consistent and time-dependent simulations where the resonant ion concentration is varied to study its effects on the performance, with a special emphasis on the resulting bulk ion heating and thermal ion temperature. We focus on two ICRF heating schemes, i.e., fundamental H minority heating in a 4He plasma at 2.65 T/7.5 MA and a three-ion ICRF scheme consisting of fundamental 3He heating in a H-4He plasma at 3.3 T/ 8.8 MA. The H minority heating scenario is found to result in strong absorption by resonant H ions as compared to competing absorption mechanisms and dominant background electron heating for H concentrations up to 10%. The highest H absorption of ∼80% of the applied ICRF power and highest ion temperature of ∼15 keV are obtained with an H concentration of 10%. For the three-ion scheme in 85%:15% H:4He plasma, PION+ETS predicts 3He absorption in the range of 21–65% for 3He concentrations in the range of 0.01–0.20%, with the highest 3He absorption at a 3He concentration of 0.20%.

During ion cyclotron resonance heating (ICRH) in fusion plasmas the fast magnetosonic wave transports wave energy to the plasma core, where it is transferred to both electrons, thermal ions and fast ions. The modelling of these processes requires a self-consistent treatment of the wave propagation and absorption, as well as the acceleration of fast ions by the wave. Here, a new self-consistent model is presented based on the FEMIC full wave solver [1] and the FOPPLER Fokker-Planck solver [2]. The use of optimised commercial wave solvers in FEMIC and a reduced 1D Fokker-Planck model make the model relatively fast and therefore suitable for e.g. the use in a transport solver.The novelty of this model, compared to other codes with 1D Fokker-Planck models, is the consistent description of Doppler physics in the FEMIC and FOPPLER codes. This description is of particular interest for scenarios with strong absorption around the ion-ion hybrid layer, like in 3-ion scenarios [3] and certain minority scenarios. Here we will present modelling of such scenarios, quantifying the impact of the Doppler shift, as well as characterising the non-linear effects associated with the acceleration of fast ions.

This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.

References:[1] P. Vallejos et al., Nuclear Fusion 59, 076022 (2019)[2] L. Bähner et al., to be submitted (2024) [3] Y.O. Kazakov et al., Nuclear Fusion 55, 032001 (2015)

In the JET DTE2 campaign a new method was successfully tested to detect the heating of bulk electrons by alpha-particles, using the dynamic response of the electron temperature T e to the modulation of ion cyclotron resonance heating (ICRH). A fundamental deuterium (D) ICRH scheme was applied to a tritium-rich hybrid plasma with D-neutral beam injection (NBI). The modulation of the ion temperature T i and of the ICRH accelerated deuterons leads to modulated alpha-heating with a large delay with respect to other modulated electron heating terms. A significant phase delay of similar to 40 degrees is measured between central T e and T i, which can only be explained by alpha-particle heating. Integrated modelling using different models for ICRH absorption and ICRH/NBI interaction reproduces the effect qualitatively. Best agreement with experiment is obtained with the European Transport Solver/Heating and Current Drive workflow.

The Doppler-shifted resonance condition for high frequency Alfv & eacute;nic eigenmodes has been extensively studied on ASDEX Upgrade in the presence of one or a combination of two neutral beam injected (NBI) fast ion populations. In general, only centrally deposited NBI sources drive these modes, while off-axis sources globally stabilize the mode activity. For the case of a single central NBI source, the observed trend is: the highest frequency modes are driven by the lowest energy and lowest pitch angle NBI sources, in line with the expectation from the Doppler-shifted resonance condition. The expected mode frequencies are determined analytically from the two-fluid cold plasma dispersion relation and the most unstable frequency relation, while the mode growth rates are estimated using the fast ion slowing down distribution functions from the ASCOT code. The overall mode frequency trend in a source-to-source variation is tracked, although a systematic overestimate of similar to 1 MHz is observed. Possible causes of this overestimate include the finite size of the resonant fast ion drift orbit and non-linear effects such as mode sideband formation. Alternatively, the expected mode frequencies are determined by tracking the growth rate maxima trajectories, this method improves the agreement with the experimentally measured values. A combination of two central mode-driving NBI sources results in the suppression of the mode driven by the lowest energy and the lowest pitch angle NBI source. Computing the analytically expected mode frequency following the method outlined above, again, generally tracks the experimentally observed trend. The mode's Alfv & eacute;nic nature allows for a practical application to track the core hydrogen fraction by following the mode frequency changes in response to a varying ion mass density. Such application is demonstrated in a discharge where the average ion mass is varied from similar to 2m(p) to similar to 1.5m(p) (where m(p) is the proton mass) via a hydrogen puff in a deuterium plasma, in the presence of a strong mode activity. The expected mode frequency changes are computed from the existence of the resonance condition, and the values track the measured results with an offset of similar to 0.5 MHz. Overall, the results suggest an intriguing possibility to monitor and control the D-T ion fraction in the core of a fusion reactor in real time using a non-invasive diagnostic.