K. Kirov

In this work, we present a real-time algorithm designed to monitor the 14.1 MeV fusion neutron production rate generated by Deuterium–Tritium (DT) fusion reactions. The study was proposed for fusion–fission hybrid reactors; however, the approach has the potential to be applied to any reversed-field pinch (RFP)-based DT machines currently under consideration within the fusion community. The algorithm, based on experience from TFTR and JET DT operations, continuously tracks the 14.1 MeV fusion neutron emission rate. If this parameter decreases below a predefined threshold, for example due to fuel dilution, the onset of MHD instabilities, or temporary actuator unavailability, the algorithm can trigger an alarm. This enables plasma recovery when actuator capabilities allow it, or ensures safe plasma termination when recovery is not feasible. Alpha-particle confinement in RFP plasmas has been investigated using the ORBIT code for different RFP magnetic field topologies, while the algorithm performance has been assessed through dedicated test cases in preparation for its real-time implementation.

Among the burning plasma controllers for research fusion reactors, the dud detector will be of primary importance as it determines whether the plasma is performing well or if it is a dud. In the latter case, the discharge needs to be terminated to remain within tritium and neutron activation limits. To this scope, monitors which track the plasma performance will be integrated in the plasma control system. In this work, we present a novel dud detector that has been empirically identified based on deuterium–tritium (DT) campaigns carried out at JET and TFTR. This controller estimates a proxy of the neutron rate using a combination of the diamagnetic energy and the density peakedness. If the predicted neutron rate deviates from the real-time measurement, then the dud detector will trigger an alarm leading to a safe plasma termination if plasma recovery is not expected or, to actuator requests when adjustment is possible. This monitoring function can also be coupled with equilibrium solver and control-oriented models, such as RAPDENS, as proposed in the 15 MA plasma current, 5.3 T toroidal magnetic field baseline DT ITER scenario.

Since the installation of a beryllium–tungsten (Be/W) first wall on JET an intense scenario development programme has taken place to realize a high-current (3.5–4 MA) scenario, denominated JET baseline scenario, with q 95 ∼ 3 , β N ∼ 1.8 and capable of delivering high fusion performance ( P fus > 10 MW) for 5 s in the deuterium–tritium (D–T) campaigns conducted in 2021 and 2023. The JET baseline scenario was realized in D–T at 3.5 MA in 2021 (DTE2 campaign), but, despite achieving transitorily P fus ∼ 8 MW, it could not be sustained for more than 3 s. In this paper we will revisit and expand the analysis of the baseline scenario at 3.5 MA and present new results obtained in D–T at 3.0 MA in DTE3, where we were able to sustain the scenario for 5 s. We will also present recent modelling results showing how available semi-empirical and first-principles transport models can explain a variety of phenomena ranging from fusion performance, impurity transport and control of the fuelling mix. Many of these points are potentially relevant for bigger machines such as ITER. Finally, we will discuss open questions and improvements to our present modelling capabilities required to extrapolate the results to power plant devices.

The interplay between externally induced locked modes and the plasma toroidal rotation is studied in JET L-mode plasmas, through the analysis of shots in which the non-disruptive compass scan technique (Piron et al 2024 Nucl. Fusion 64 066029) was performed. In these experiments, an external magnetic field perturbation was applied up to the onset of a locked mode, with toroidal mode number n = 1, at constant plasma density. In Ohmic plasmas, during the execution of the compass scan, it is observed that the toroidal rotation brakes in a wide range of the core. In the case of neutral beam injection-heated shots, the rotation braking is significantly reduced. The experimental analyses have been interpreted by the RFXlocking code adapted to the JET tokamak (Zanca et al 2015 Nucl. Fusion 55 043020), which also allowed to investigate the roles of the electromagnetic force and of the neoclassical toroidal viscosity, in the rotation braking phenomenon. Plasma rotation has been analysed also during a density variation in the absence of external magnetic field perturbations, highlighting that a profile transition occurs in the core.

In JET-ILW, beam-target reactions contribute to a large fraction of the fusion power generated in Deuterium – Tritium (D-T) plasmas, with core ion temperatures of 10-12keV and large neutral-beam injection (NBI) power. Previous modelling done in preparation for the recent D-T campaigns in JET have shown that injecting D beam ions with energies of ~120keV in T-rich plasmas produces larger 14MeV fusion yield than in 50:50 D:T plasmas, but such scenario had never been tested in past D-T experiments. In addition, the simulations showed that fundamental ion-cyclotron resonance heating (ICRH) of the D ions can significantly boost the net fusion reactivity, since both the D-bulk ions and the fast D-beam ions are accelerated to energy ranges that are optimal for the D-T reactions to take place. In the last JET D-T campaigns (DTE2 and DTE3), dedicated experiments confirmed - for the first time - the improved fusion performance of T-rich plasmas with high D-NBI power and highlighted the key impact of fundamental D ICRH on the fusion performance. This new scenario led to the world-wide D-T fusion energy record ever achieved in a fusion device and allowed to sustain more than 12MW of fusion power averaged over 5s. The main results of these unprecedented experiments will be presented and the NBI+ICRF physics responsible for the high fusion performance achieved will be highlighted through numerical modelling.

The low density and high temperature at the pedestal in hybrid scenarios on JET provide a unique test of the neo-classical screening in the periphery of the plasma that has previously been observed on JET in such plasmas. Hybrid scenario plasmas have been further developed to aid in the study of temperature gradient screening of high-Z impurities from the core of the plasma via plasma current, toroidal field, gas fuelling, density and isotope variations. The steps required to adapt the pulses from deuterium to deuterium-tritium are discussed and provide some guidance for future experiments that may operate in deuteriumtritium. The optimisation of the scenario allowed for improved bolometry data compared to the Deuterium Tritium Experiment 2 (DTE2) campaign on JET, hence this work provides vital evidence of impurity screening in a high fusion power, deuterium-tritium plasma.

The understanding of the time-scales and associated transient behavior of fusion exhaust plasmas plays a crucial role in its dynamic modeling and its control. This work presents an overview of experimental investigations of the exhaust dynamics in TCV, MAST-U, ASDEX-Upgrade, WEST, DIII-D, and JET. From the presented experiments, a clear picture arises on properties of the exhaust dynamics across machines. Particularly, we observe that the scrape-off layer equilibrates on fast time-scales ( > 70 Hz) and that exhaust dynamics measured in response to gas valve modulations mostly behave smoothly and linearly, with similarities across devices, across scenarios (H-mode, L-mode), injected species, and injection locations. The measurements presented have formed the basis for systematic exhaust control on the considered devices. We now present this database for the essential validation of dynamic exhaust models for reactor design and control.

The spherical tokamak for energy production (STEP) is the UK’s flagship program to design a prototype fusion power plant capable of delivering net electricity to the grid. Due to geometric constraints in spherical tokamaks, STEP is being developed for fully non-inductive operation, relying primarily on microwave-based electron cyclotron (EC) and electron Bernstein wave systems for plasma heating and current drive (HCD). A key requirement for achieving this operation is the optimization of these systems to ensure high wall-plug efficiency and reliable current profile control. This paper presents a self-consistent modeling approach using the JINTRAC framework, where the JETTO transport code is coupled with the GRAY quasi-optical beam-tracing code. Previous scenario development relied on simplified, prescribed EC current drive (ECCD) profiles. In this work, these were replaced with the profiles derived from detailed parametric GRAY scans, optimized for launcher geometry, frequency, and polarization. These realistic ECCD profiles are integrated into a fully self-consistent JETTO/GRAY simulation, capturing the dynamic interplay between plasma transport, heating, and current evolution. The methodology developed in this study offers a more accurate representation of HCD dynamics and is applicable to future STEP scenario development.

This study presents results from particle transport modelling for D/T ratio control experiments conducted during the JET DTE3 campaign. TRANSP interpretative and JETTO predictive simulations for D and T densities were performed and their results are discussed. Despite using simplified models based on Bohm-gyroBohm transport, the simulations incorporate self-consistent sources and impurities and cover the full radial range. The simplified models effectively reproduced the evolution of electron density and neutron rates. However, the predicted D/T ratio evolution responded to control requests faster than what was experimentally observed, suggesting that the employed models possess certain limitations. Specific cases involving swapped gas injection species were also studied, highlighting the potential applicability of the proposed methodology in future experimental scenarios. TRANSP interpretative analysis indicates that a Real-Time (RT) scheme employing simplified quasi-neutrality and Z _eff estimations can be implemented with high degree of reliability. JETTO predictive analysis suggests that a simplified modelling approach for the behaviour of the future RT controllers of D/T mixture can be effective. Such an approach involves using measured temperatures, omitting explicit modelling of the SOL physics, and adopting simplified assumptions for the particle transport.

In the last experimental campaign of the JET tokamak in December 2023, long duration discharges in deuterium plasmas were performed to assess the sustainment of the plasma performance over many resistive time scales and to address plasma-wall interaction physics in a full metallic environment with the ITER-like wall (ILW), featuring a W divertor and a Be first wall [1,2]. Two types of long duration discharges were developed: (i) a 30s flat-top ELMy H-mode with combined 12-14MW neutral beam injection (NBI) and 2MW of ion-cyclotron resonance heating (ICRH) and (ii) a 60s long pulse with 4-6MW of NBI during the entire flat-top and 2MW of ICRH for about 40s. The pulses were stationary from the radiation point of view without sign of core impurity accumulation and featured regular type-I ELMs with roughly equilibrated ion and electron temperatures in most cases. The discharges required significant technical adjustments in many subsystems, ranging from plasma shape control, machine protection and diagnostic settings [3]. The experiments achieved the maximum energy ever injected in a single pulse in JET (EIN=450MJ) and challenged the operational domain of the inertially cooled divertor limits. An overview of the main properties of these discharges in terms of plasma stationarity, ELM behaviour, plasma-wall interaction and overall performance will be presented and the contribution of these unprecedented JET-ILW results to the multi-machine CICLOP (Coordination on International Challenges on Long duration OPeration) database [4] will be highlighted.