Costanza F Maggi

L–H transition experiments in JET with a Be/W wall (JET-ILW), with N ₂ injection into the divertor region, have revealed that the H-mode power threshold, P _L–H , increases with N ₂ injection rate in the high-density branch and modifies the density at which the power threshold is minimum, n _e,min . P _L–H values approaching those obtained in JET with a C wall are achieved for a sizeable increase in N concentration. At a given density in the high-density branch of P _L–H , both the total loss power ( P _loss ) and the power crossing the separatrix ( P _sep ) at the L–H transition increase by a factor of ∼1.5 at high N ₂ levels. At a given line-averaged density, while the edge density is similar in plasmas with and without N ₂ injection, the edge temperature at the L–H transition is higher in pulses with high N seeding levels and a larger shear of the edge radial electric field E _r is measured. At low N concentrations, little change in P _L–H is observed. Qualitative agreement is reported with the impact of Z _eff on L-mode edge turbulence drive (Bourdelle et al 2015 Nucl. Fusion 55 073015). The enhanced L-mode edge turbulence drive with N ₂ seeding at higher Z _eff implies that a higher P _L–H is expected as, indeed, has been measured in experiments. It is assumed that the mean equilibrium E _r × B shear is a key player in the reduction of L-mode edge turbulence, leading to the L–H transition. In this framework, the L–H transition observations in JET-Be/W + N ₂ are interpreted by an increase in L-mode edge plasma turbulence with increased N concentration and the need for a larger stabilizing E _r × B shear, and thus a higher P _sep , to trigger the L–H transition. For future fusion reactors having scenarios with extrinsic low-Z impurity seeding for mitigation of divertor heat load our results suggest the need to predict P _sep for H-mode access taking into account not only the high radiation fractions from the bulk plasma but also the impact of low- Z impurity concentration on P _loss and P _sep at the L–H transition.

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 ability to capture the isotope mass scaling of core confinement seen in experiment is validated with JETTO-TGLF for low power JET-ILW discharges across H, D and T. The cases analysed include Ohmic discharges spanning the linear and saturated Ohmic confinement regimes as well as a trio of L-modes. The TGLF saturation rules SAT1-SAT3 are seen to predict a similar isotope scaling across both the ITG- and TEM-dominated discharges simulated, despite for the latter case the inclusion of the TEM branch of SAT3. The models demonstrate good agreement with experiment for the scaling between D and T plasmas, however a discrepancy is observed for H in the ITG-dominated discharges of higher density, as well as a systematic overprediction of the confinement time on the order of 20% in most cases. A retuned version of the SAT3 model, which was fit to better recreate fluxes close to the transport threshold, is seen to improve the magnitude of confinement predictions across all shots owing to an increased transport stiffness. This retuning was not seen to influence the confinement isotope scaling however, and possible transport mechanisms responsible for the continued discrepancy of higher density Ohmic and L-mode discharges in H are discussed.

The dimensionally matched deuterium–tritium pulse pair under JET L-mode conditions showed 13%–16% improvement in the energy confinement time in favour of the tritium pulse. This favourable isotope scaling can be seen clearly in the effective diffusion coefficients throughout the radius. The isotope scaling originates dominantly from the electron heat transport channel and from the edge part of the plasma. The phase and amplitude profiles in response to the gas puff modulation robustly show that there is no room for a large isotope scaling in the particle transport channel in the core plasma at ρ tor ⩽ 0.95 . This can also be seen in the derived particle transport coefficients between the deuterium and tritium pulses. EDGE2D-EIRENE simulations found that the radial ionisation profiles are very similar between the dimensionally matched deuterium and tritium identity pulses. A similar deviation from the gyro-Bohm scaling, i.e. strong isotope scaling favouring tritium in heat transport was found with gyrokinetic simulations in the edge at ρ tor = 0.95 under the JET L-mode conditions. The strong edge isotope scaling favouring tritium is consistent with the experimental observation. This edge isotope effect ρ tor = 0.95 in the GENE simulations is also much larger than found in the core plasma ρ tor = 0.6 when comparing similarly deuterium and tritium simulation results. Both the experimental results and the GENE simulations suggest that the isotope mass scaling is a nonlinear function of the isotope mass itself, being significantly stronger between deuterium and tritium than between hydrogen and deuterium, at least in JET L-mode conditions.

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 empirical scaling for the H-mode power threshold in tokamaks has been revisited using a database with threshold data from machines with a metallic first wall as part of International Tokamak Physics Activity (ITPA) task TC-26. The database contains discharges from ASDEX Upgrade (W), JET (Be/W) and Alcator C-Mod (Mo). This was motivated by reports that in like-for-like discharges the power threshold was reduced by approximately 30% after the change from carbon based to metallic first wall materials in AUG [F. Ryter et al., Nuclear Fusion, 53(11):113003 (2013)] and JET [C.F. Maggi et al., Nuclear Fusion, 54(2):023007 (2014)]. The database contains L-H transition data for all hydrogen isotopes and mixtures, including T and DT from the recent JET campaigns. compared to the ITPA 2008 scaling [Y R Martin et al., Journal of Physics: Conference Series, 123(1):012033 (2008)], the metal wall scaling has a smaller magnetic field exponent but a larger density exponent. We present an additional parameter to capture the strong dependence of the L-H power threshold (approx. factor 2) on the magnetic configuration in the divertor on JET. The scaling recovers the approximate inverse isotope mass scaling of the threshold power. Alternative scalings involving the plasma current and poloidal magnetic field are explored. Despite the reduction in threshold observed earlier, the scalings based on the metal wall database do not necessarily extrapolate to a lower threshold for ITER compared to the ITPA 2008 scaling, especially at high density. The divertor configuration effect induces the largest uncertainty in the extrapolation.

Understanding the physics of the H-mode pedestal structure and dynamics and predicting its performance in future fusion reactors is key to reducing the uncertainties associated with the realization of burning plasma conditions. ITER and future fusion reactors will operate with mixed deuterium–tritium (D–T) plasmas. Perhaps the greatest challenge to date related to understanding the impact of hydrogen isotope mass on plasma transport lies in the processes governing the plasma edge region and H-mode pedestal. This paper, based on a review talk at the 19th H-mode workshop (Mito, Japan, September 2024), reviews recent progress in understanding the impact of isotope mass on pedestal structure and inter-ELM transport. Recent T and D–T experiments on JET have confirmed a strong mass dependence on the pedestal density and provided new and unique results. State of the art pedestal diagnostics were employed to measure the pedestal structure in D, D–T and T and the most advanced theoretical models were used to identify the physics mechanisms linked to the dependence of pedestal stability and inter-ELM transport on isotope mass. Different modelling and analysis workflows are consistent in finding that changes in pedestal stability and in inter-ELM transport with isotope mass are required to explain the isotope mass dependencies of pedestal density and temperature. The competing levels of turbulent and neoclassical transport with varying isotope mass has been quantified using the gyrokinetic code GENE for selected JET pedestals. However, progress in this area is hindered by the difficulty to obtain accurate measurements of the edge particle source. The isotope dependence of neutral fuelling in the H-mode pedestal region was examined quantitatively in DIII-D experiments using direct spectroscopic measurements of neutral hydrogen penetration in H vs D plasmas. Projecting to D–T H-modes in future reactors, which will be largely opaque to neutrals, the impact of isotope mass dependence of edge neutral fuelling is expected to be negligible, while the pedestal density structure will depend primarily on its transport properties, in turn affecting pedestal stability.

The development of a high current baseline scenario ( I p = 3.5 M A , q 95 ≈ 3.0 , β N < 2 ) in deuterium (D), tritium (T) and deuterium–tritium (D–T) for high D–T fusion performance at JET with Be/W wall is described. We show that a suitable scenario capable of delivering up to 10 MW of fusion power, depending on the auxiliary heating power available, was successfully developed in D. However, when translated to T and D–T, the same scenario could not be sustained for the target duration of 5 s due to the impossibility to achieve the stationary compound edge localized modes regime necessary to flush the tungsten (W) from the plasma and control the density. Nevertheless, a peak fusion power in the order of 8 MW, with 60% of the power coming from thermal fusion reactions, was obtained in D–T at 3.5 MA, with ≈ 30 MW of neutral beam injection heating and 3–4 MW of ion cyclotron resonance heating, in line with the predictions obtained with the JINTRAC integrated scenario modelling suite of codes equipped with the QuaLiKiZ transport model and based on the extrapolation of the performance of similar D plasmas.

Previous investigations on JET suggest half or less of plasma stored thermal energy Wth is radiated (frad,th≲0.5) using either massive gas injection (MGI) or shattered pellet injection (SPI) disruption mitigation. We investigate whether the apparent incomplete frad,th is explained by radiation peaking near the injection plume. High toroidal peaking throughout the pre-thermal quench is found in argon–deuterium MGI on JET, with typically >3× higher radiation near the injector than toroidally distant. Previously unexplained toroidal bolometry measurements in neon–deuterium SPI are reproduced with similar peaking using the Emis3D radiation analysis code. These observations align with results from Alcator C-Mod and KSTAR. This peaking is not captured by previous JET studies that found poor thermal mitigation. Two sets of neon–deuterium SPI and two sets of argon–deuterium MGI are analyzed using Emis3D. In SPI, frad,th rises from no-plume estimates of 0.31 and 0.66 to lower bounds of 0.84 and 0.92, respectively, and frad,th∼1 is possible. In MGI, the toroidal spread of the peaking feature is poorly constrained. frad,th up to 0.85 and 0.65 are possible using the largest possible spread, increasing from 0.42 and 0.28, although frad,th∼1 does not appear to be reached. Revised mitigation estimates on JET suggest a lower melt risk to the divertor in mitigated disruptions on ITER and SPARC than previously thought. However, peaking near injectors could increase flash melting risk on nearby plasma facing components.

This paper presents a study on the dependence of the ion temperature stiffness on the plasma main ion isotope mass in JET ITER-like wall and C wall discharges. To this aim, a database of H, D and T shots is analyzed, including new dedicated shots, comparing experiments with lower and higher power injected by the NBI system. In order to characterize the turbulence dependence on the isotope mass, three of these discharges (two in T and one in D) with same external heating scheme are studied in detail and interpreted with gyrokinetic linear and nonlinear simulations. The analysis is performed at fixed radius ρ tor = 0.33 , selected in order to maximize the electromagnetic stabilizing effects on turbulence, both from thermal and suprathermal particles. The experimental results show a clear ion temperature stiffness reduction when heavier isotopes are considered, thus moving from H to D to T, which is attributed to an increasing thermal electromagnetic stabilization with increasing main isotope mass.